ACUS48 KWNS 140904
SWOD48
SPC AC 140903

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0303 AM CST Thu Jan 14 2021

Valid 171200Z - 221200Z

...DISCUSSION...
Day 4 (Sunday) offshore flow over the Gulf will maintain stable
conditions inland. By day 5 (Monday), winds will return to southerly
over the western Gulf resulting in the advection of a modifying Gulf
boundary layer supporting dewpoints in the 50s F across south
through eastern TX. A few showers may develop in the pre-frontal
warm advection regime late Monday night from northeast TX into OK
but instability will remain too marginal for severe storms. An upper
low will settle into the southwest U.S. by day 6 (Tuesday) with
downstream weak upper ridging across TX in vicinity of a cold front
that will move slowly south. While showers and a few thunderstorms
may develop along the front and possibly farther south in the warm
sector across southeast TX, no severe weather is expected due to the
very weak thermodynamic environment. By day 7 the cutoff upper low
is forecast to settle into northern Mexico, but models remain
inconsistent regarding the timing and future trajectory of this
feature, resulting in low predictability.

..Dial.. 01/14/2021
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ACUS48 KWNS 150855
SWOD48
SPC AC 150853

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0253 AM CST Fri Jan 15 2021

Valid 181200Z - 231200Z

...DISCUSSION...
Day 4 (Monday), southerly winds will return to the western Gulf and
south TX resulting in the northward advection of a modifying Gulf
boundary layer through south and east TX. A few showers may develop
along a southward-advancing cold front and in the pre-frontal warm
advection regime late Monday night from OK into northeast TX, but
very little instability will exist to support thunderstorms. Upper
low is forecast to be situated over southern CA by day 5 (Tuesday)
with downstream weak upper ridging across TX in vicinity of a cold
front that will move slowly south. While showers and thunderstorms
may develop along the front and possibly farther south in the warm
sector across southeast TX, no severe weather is expected due to the anticipated very weak thermodynamic environment. By day 6
(Wednesday) model consensus is that the cutoff upper low will settle
into the Baja area with a downstream upper ridge across the Southern
Plains. Showers and a few thunderstorms may persist in vicinity of
the stalled and weakening front from central TX into the lower MS
Valley, but the expected parameter space does not appear conducive
for severe storms.

..Dial.. 01/15/2021
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ACUS48 KWNS 160928
SWOD48
SPC AC 160927

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0327 AM CST Sat Jan 16 2021

Valid 191200Z - 241200Z

...DISCUSSION...
...Tuesday/Day 4 and Wednesday/Day 5...
Zonal mid-level flow is forecast to be in place form the southern
Plains eastward to the Eastern Seaboard on Tuesday and Wednesday. At
the surface, high pressure is forecast to be dominate across much of
the continental United States on Tuesday. The associated surface
high is forecast to move across the Southeast on Wednesday. This
will keep a dry airmass in place over much of the nation limiting
thunderstorm development through mid week.

...Thursday/Day 6 to Saturday/Day 8...
The medium-range models are forecasting an upper-level trough to
move from the Desert Southwest to the southern Plains from Thursday
into Friday. Moisture return is expected to take place ahead of the
system in parts of the southern Plains. Thunderstorm development
will be possible from Thursday night into Friday along northern
parts of the moist airmass, most likely from central Texas eastward
into the lower Mississippi Valley. However, instability is expected
to remain very weak which should limit any severe potential. On
Saturday, model forecasts develop a low-amplitude upper-level ridge
across the central U.S. At this time, instability is expected to be
minimal across most of the nation helping to limit thunderstorm
development.

..Broyles.. 01/16/2021
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ACUS48 KWNS 170949
SWOD48
SPC AC 170948

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0348 AM CST Sun Jan 17 2021

Valid 201200Z - 251200Z

...DISCUSSION...
...Wednesday/Day 4 and Thursday/Day 5...
The medium range models maintain west-northwesterly mid-level flow
across much of the U.S. on Wednesday as an upper-level low remains
south of the southern California coast. The low is forecast to
weaken and move northeastward across the Desert Southwest on
Thursday. Ahead of this system, an upper-level ridge is forecast to
be in place across the southern Plains with a moist airmass located
across the western Gulf Coast region. Thunderstorm development will
be possible along the northwestern edge of the moist airmass on
Wednesday and Thursday, mainly from the Texas Hill Country and Texas
Coastal Plain east-northeastward into the Arklatex and lower
Mississippi Valley. Instability is expected to be very weak limiting
any severe potential.

...Friday/Day 6 to Sunday/Day 8...
The upper-level trough is forecast dampen and move quickly
east-northeastward across the southern U.S. on Friday. Thunderstorm
development will be possible ahead of the trough from parts of the
southern Plains eastward into the central Gulf Coast region.
Instability is expected to be weak on Friday minimizing any severe
potential. Low-level moisture is forecast to gradually increase in
parts of Texas from Saturday into Sunday, where the models suggest
scattered thunderstorms will be possible. The greatest potential for
an isolated severe threat Saturday and Sunday would be from central
Texas northeastward into the Arklatex. However, uncertainty is
substantial at this extended range in the forecast period.

..Broyles.. 01/17/2021
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ACUS48 KWNS 180951
SWOD48
SPC AC 180950

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0350 AM CST Mon Jan 18 2021

Valid 211200Z - 261200Z

...DISCUSSION...
...Thursday/Day 4 and Friday/Day 5...
The medium-range models move a shortwave trough across the Desert
Southwest on Thursday as zonal mid-level flow remains from the
southern Plains to the Eastern Seaboard. Thunderstorm development
will be possible on Thursday across parts of the western and central
Gulf Coast region along the northern edge of a moist airmass.
Instability is expected to be too weak for severe thunderstorm
development. The shortwave trough is forecast to move into the Gulf
Coast States on Friday. Thunderstorms will again be possible in the
northern part of the moist airmass which should be located from the
Texas Coastal Plain eastward to the Florida Panhandle. Instability
is again forecast to be weak suggesting severe thunderstorm
potential will be minimal.

...Saturday/Day 6 to Monday/Day 8...
On Saturday, the medium-range models move an upper-level ridge
across the Mississippi Valley as southwest mid-level flow becomes
established from the Four Corners region eastward into the Great
Plains. The models are in reasonable agreement on Sunday moving an
upper-level trough across the Intermountain West. Moisture return is
forecast to take place ahead of this system in parts of the southern
Plains. Thunderstorm development will be possible during the day on
Sunday from Texas northward into Oklahoma along an axis of
instability. Although a severe threat can not be ruled out,
predictability remains low for Sunday. On Monday, the upper-level
trough is forecast to move into the Great Plains as a cold front
advances into the Arklatex. Thunderstorm development will be
possible along and ahead of the front across the lower to mid
Mississippi Valley during the day on Monday. If the forecasts
verify, a severe threat would be possible in areas that can heat up sufficiently ahead of the front. However, predictability is low this
far out in the forecast period.

..Broyles.. 01/18/2021
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ACUS48 KWNS 200932
SWOD48
SPC AC 200930

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0330 AM CST Wed Jan 20 2021

Valid 231200Z - 281200Z

...DISCUSSION...
Medium-range models are in reasonably good agreement with respect to
the synoptic-scale features across the U.S. through roughly Day 5
(Sunday). Beyond that, increasing divergence in solutions suggests insufficient predictability to make any meaningful assessment of
convective potential.

Within the Day 4-5 time frame, the primary feature of interest will
be the large trough situated over the West at the start of the
period. More specifically, a short-wave feature moving through the broader-scale cyclonic flow -- which is progged to lie near the
central California coast at the start of Day 4 -- will substantially
influence convective potential in the medium range.

Day 4 (Saturday), this short-wave feature is forecast to shift
southeastward, moving into the southwestern U.S./northern Baja
during the overnight hours. As this occurs, an increase in
southerly low-level flow is progged over the southern Plains. This
trend will continue into Day 5, as the upper system shifts eastward
and crosses northern Mexico/Arizona/New Mexico -- but while
weakening steadily as it progresses eastward. In response, rather
ill-defined cyclogenesis is expected to occur over the Texas
vicinity. As ascent increases and low-level moisture advection
continues, an increase in convective potential will occur across the
southern Plains and into the lower Mississippi Valley. However,
despite favorable shear that would otherwise support severe
potential, some likelihood for persistent/weak boundary layer
stability beneath low-level capping is a concern. Still, some
severe potential could evolve, possibly focused along a weak
east-to-west warm frontal zone across east Texas and the Arklatex
region later Sunday and into Monday. However, degree of risk
remains questionable at this time, such that an outlook area will
not be introduced at this time.

By Day 6 (Monday), the weakening upper system is progged by the GFS
to essentially dampen out with time, as it crosses Oklahoma and
Kansas, and shifts into Missouri. Meanwhile, the ECMWF maintains a
much more well-defined feature, that moves quickly across the Ozarks
and Mid Mississippi Valley by evening, and then into/across the
Carolinas and Virginia by the end of the period. Along with this
more pronounced upper feature, a corresponding/well-developed
surface cyclone is also progged to cross the southeastern quarter of
the country Monday, suggestive of at least some severe potential
spreading eastward across this region should the pattern evolve more
similar to the ECMWF solution.

At this time however, given the increasing model differences beyond
Day 5/Sunday, no severe-weather assessment will be attempted through
the remainder of the medium-range period.

..Goss.. 01/20/2021
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ACUS48 KWNS 210921
SWOD48
SPC AC 210919

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0319 AM CST Thu Jan 21 2021

Valid 241200Z - 291200Z

...DISCUSSION...
Medium-range models are exhibiting reasonable agreement Day 4
(Sunday) and into Day 5 (Monday), after which divergence in
solutions begins to increase markedly.

On Day 4 (Sunday), the western U.S. trough is progged to begin
ejecting -- and deamplifying as it does -- across the Desert
Southwest and southern Rockies, and finally into the southern
Plains, as it acquires negative tilt through latter stages of the
period.

As this occurs, height falls will overspread northern and central
Texas and Oklahoma, atop southerly low-level flow providing theta-e
advection into the region. While a capping inversion -- remnants of
prior anticyclonic flow/subsidence aloft -- will likely hinder
potential for surface-based convection through much of the day,
isolated to scattered storms may evolve by early evening across
parts of central and northern Texas, and possibly into southeastern
Oklahoma. While questions persist with this scenario, including
degree of surface-based instability which may be present, shear
profiles will be quite supportive of organized/rotating storms.
Therefore, though details remain sketchy, enough consistency exists
within tonight's model runs, and those from prior days, to warrant
inclusion of a conditional 15% risk area for severe storms.

By Monday, differences begin to become apparent with
evolution/progression of the weakening mid-level system as it shifts
eastward toward/into the Midwest/Ohio Valley. Likewise, similar
differences are manifest in the strength/advance of the associated
surface low. This uncertainty, combined with likelihood for weaker
instability with eastward extent, suggests lesser/decreasing severe
potential, and diminishing confidence -- both therefore mitigating
against risk area inclusion at this time.

Beyond Day 5, model differences -- suggestive of an inherent lack of predictability -- preclude any assessment of convective potential
through the remainder of the medium-range period.

..Goss.. 01/21/2021
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ACUS48 KWNS 221004
SWOD48
SPC AC 221002

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0402 AM CST Fri Jan 22 2021

Valid 251200Z - 301200Z

...DISCUSSION...
Medium-range models appear to be in reasonably good large-scale
agreement through most of the period. Of primary interest from a
convective perspective, will be two short-wave troughs expected to
move out of the southwestern U.S. and quickly across the south
central and then southeastern portions of the country.

The first of these features -- initially progged to lie over the
Oklahoma vicinity at the start of the Day 4 period (Monday morning)
will move quickly east northeastward into/across the lower
Missouri/mid Mississippi/Ohio Valleys into Day 5. An accompanying
surface low will likewise shift eastward across this same general
region, but at this time, it appears that available instability will
be quite limited, due to weak lapse rates in the surface to roughly
700mb layer. As such, any potential for severe weather appears low
-- below the 15% threshold for areal inclusion.

As this system weakens and shifts off the East Coast Day 5
(Tuesday), a second short-wave trough crossing the southwestern U.S.
is expected to reach the into the central and southern Plains late Tuesday/early Wednesday (Day 6). The system is then progged to
continue eastward to the Tennessee Valley late Day 6/early Day 7,
before moving off the Atlantic Coast prior to the start of Day 8.

However, a cool/stable boundary layer is forecast to spread
southward into the south central and southeastern U.S. in advance of
this feature, and thus -- with the primary surface baroclinic zone
progged to lie over the northern Gulf of Mexico, only a muted
surface response is expected in association with the advance of this
system. As such, conditions will likely remain thermodynamically
unfavorable for any appreciable severe weather risk through the
middle and latter stages of the period.

..Goss.. 01/22/2021
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ACUS48 KWNS 230944
SWOD48
SPC AC 230942

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0342 AM CST Sat Jan 23 2021

Valid 261200Z - 311200Z

...DISCUSSION...
Medium-range models are in reasonable agreement with respect to
evolution of large-scale features through Day 6, after which
divergence in solutions increases through the end of the period.

Even early on in the period, however, differences at smaller scales
-- particularly with an upper short-wave trough ejecting from the
Southwest across the southern tier of the U.S. during the first half
of the period -- complicate assessment of convective potential.

Models generally agree in the maintenance of persistent western U.S.
upper troughing, with the aforementioned short-wave feature progged
to eject eastward, crossing the Four Corners states and eventually
emerging into the central/southern Plains Day 4 (Tuesday). By Day 5 (Wednesday), as the feature shifts across the Mid and lower
Mississippi Valley and then the Mid South and central Gulf Coast
states, surface cyclogenesis is expected to occur. With the GFS stronger/sharper with the upper feature, stronger surface
cyclogenesis is depicted, with a deepening low over the Tennessee
Valley area by Wednesday evening, while the weaker ECMWF solution
places a much weaker low over the southern Alabama/southern Georgia
vicinity. In either case however, it appears likely that the
boundary layer will remain stable, as a strong baroclinic zone over
the northern Gulf of Mexico just ahead of this system will likely be
slow to retreat northward. Thus, severe weather is not expected
with this system.

As the system moves offshore Day 6 (Thursday), large-scale ridging
will prevail over much of the country, ahead of re-establishment of
western U.S. troughing. Therefore, severe weather is not expected
Thursday, and then beyond which pattern predictability diminishes
steadily through the end of the period.

..Goss.. 01/23/2021
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ACUS48 KWNS 241146
SWOD48
SPC AC 241144

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0544 AM CST Sun Jan 24 2021

Valid 271200Z - 011200Z

...DISCUSSION...
Latest runs of the medium-range models show fair model-to-model
agreement with respect to large-scale features into Day 6 (Friday
Jan. 29), though even larger-scale similarities between models fade
further into the second half of the period. This increasing
large-scale disagreement centers around timing of the ejection of a
short-wave trough across the southwestern U.S. Day 6, and then
into/across the central U.S. Day 7. Depending upon evolution of
this feature, convective potential could increase over the south
central and later the southeastern U.S. during the second half of
the period. However, predictability concerns in this time frame
preclude any further assessment of convective potential.

Prior to this, through the first half of the period, a short-wave
trough is forecast to move from the central/southern Plains region
at the beginning of the period (Wednesday Jan. 27), to the East
Coast states by the start of Day 5. Slight differences in the
strength of this feature between the GFS and ECMWF result in more
notable deviations in depiction of associated surface low intensity,
with the GFS more aggressive in terms of surface cyclone development
over the southern Appalachians area, and thus more bullish with
respect to convective potential. With the ECMWF showing a much more
muted frontal wave much farther south -- shifting from the Gulf
Coast to the southeast Georgia coast -- confidence remains too low
to render a more thorough assessment.

As this upper feature moves offshore Day 5, strong cyclogenesis is
progged over the Gulf Stream/western Atlantic, with cold high
pressure building southward in its wake, across the eastern half of
the country -- in tandem with large-scale ridging/anticyclonic flow
across most of the CONUS.

..Goss.. 01/24/2021
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ACUS48 KWNS 160946
SWOD48
SPC AC 160945

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0345 AM CST Tue Feb 16 2021

Valid 191200Z - 241200Z

...DISCUSSION...
A surface cold front should clear much of the East Coast early on
Day 4/Friday as a large-scale upper trough continues advancing
across the eastern CONUS. Any thunderstorm potential Friday will
likely be limited to parts of the central/southern FL Peninsula.
With enhanced southwesterly mid-level flow present over this region,
a few strong storms cannot be ruled out along the front as it
advances quickly southeastward through Friday evening. However, weak instability will probably limit an organized severe risk.
Thereafter, meager low-level moisture across the CONUS should
preclude any severe risk this upcoming weekend into early next week.

..Gleason.. 02/16/2021
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ACUS48 KWNS 170836
SWOD48
SPC AC 170834

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0234 AM CST Wed Feb 17 2021

Valid 201200Z - 251200Z

...DISCUSSION...
Multiple upper troughs should progress eastward across the CONUS
from this upcoming weekend into the middle of next week. Rich
low-level moisture is not expected to return northward from the Gulf
of Mexico ahead of any of these upper troughs. Therefore,
instability should remain minimal, and organized severe
thunderstorms are not expected.

..Gleason.. 02/17/2021
$$
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ACUS48 KWNS 180814
SWOD48
SPC AC 180812

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0212 AM CST Thu Feb 18 2021

Valid 211200Z - 261200Z

...DISCUSSION...
An upper trough should move across the central/eastern CONUS from
Day 4/Sunday into early next week. Low-level moisture ahead of this
system will likely remain insufficient for any organized severe
thunderstorm risk. There are some indications in medium-range
guidance that another upper trough may amplify over the western
CONUS around Day 6/Tuesday through the end of the period. If this
occurs, then low-level moisture return across the southern Plains
and lower MS Valley could potentially support a risk for organized
storms by Day 8/Thursday. However, there are large differences in
guidance regarding the timing and amplification of the potential
upper trough at this extended time frame, leading to low
predictability.

..Gleason.. 02/18/2021
$$
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ACUS48 KWNS 190922
SWOD48
SPC AC 190920

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0320 AM CST Fri Feb 19 2021

Valid 221200Z - 271200Z

...DISCUSSION...
Medium-range guidance indicates that initially suppressed mid-level
ridging, across the eastern mid-latitude Pacific into the U.S.
Pacific coast, may undergo considerable amplification next week,
roughly between 130-160 W longitude. It appears that this may build
toward the Alaskan Arctic Circle vicinity, as a deep mid-level low
(initially centered to the north of Alaska, Yukon and the Canadian
Northwest Territories) shifts a bit southward and eastward across
north central Canada. As this occurs, a mean west-northwesterly
mid-level flow regime, from the northeastern Pacific through the
U.S., may take on a much more substantive northerly component near
the Canadian/U.S. Pacific coast vicinity, into deepening troughing
across the Southwest by late next week.

Arctic surface air is expected to begin to surge southward to the
lee of the Canadian Rockies, but it may remain to the north of the
U.S. international border through at least this period. However,
models suggest that it will be preceded by a southward surge of more
modestly cold air across much of the central and eastern U.S. by mid
to late next week. With guidance indicating little potential for
significant surface cyclogenesis to the lee of the southern Rockies,
and boundary-layer moistening over the Gulf of Mexico slowed by
reinforcing subsidence and drying early next week, the risk for
severe thunderstorms appears low (i.e., less than 10 percent
probabilities) through at least next Friday.

..Kerr.. 02/19/2021
$$
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ACUS48 KWNS 200941
SWOD48
SPC AC 200940

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0340 AM CST Sat Feb 20 2021

Valid 231200Z - 281200Z

...DISCUSSION...
An elongated center of low mid-level heights over the Arctic, to the
north of Alaska, Yukon and the Canadian Northwest Territories by the
beginning of this period, is forecast to redevelop
east-southeastward into the northern Hudson Bay vicinity by late
next week. At the same time, it appears that mid-level ridging will
remain prominent over the mid-latitude eastern Pacific, with a
number of significant short wave troughs progressing within the
westerlies around its periphery, and inland of the Pacific coast.

It appears that this regime will contribute to the evolution of
broad mid-level troughing over much of interior North America this
coming work week, accompanied by at least gradual surface cooling
across much of the U.S. By late in the week, mid-level subtropical
ridging is also forecast to build across the Caribbean and
southeastern Gulf of Mexico into the Southeast, and associated
low-level warming may contribute to a strengthening low-level
baroclinic zone across parts of the Gulf into south Atlantic Coast
states.

This front could provide a focus for increasing thunderstorm
activity as early as Thursday, into next weekend. It is possible
that this may include some risk for severe weather. However, models
continue to suggest that any evolving waves along the front will
remain generally weak, and the extent of Gulf boundary-layer
moistening by this time remains unclear. Anything more than
relatively marginal and/or isolated severe weather potential
currently appears unlikely, but it is not out of the question that
this could change in subsequent outlooks for this period.

..Kerr.. 02/20/2021
$$
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ACUS48 KWNS 210939
SWOD48
SPC AC 210937

Day 4-8 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0337 AM CST Sun Feb 21 2021

Valid 241200Z - 011200Z

CORRECTED FOR TYPO

...DISCUSSION...
Medium-range model output suggests that blocking mid-level ridging
may become increasingly amplified across the mid-latitude Pacific
late this week and remain prominent through next weekend. At the
same time, it appears that a deep mid-level low at Arctic latitudes,
initially developing east-southeastward across far northern Canada,
may linger near or just north of Hudson Bay, while a subtropical
mid-level ridge builds across much of the Gulf of Mexico and
Southeast. Between these features, it appears that a branching
regime will gradually develop across the northeastern Pacific into
the western U.S. by late next weekend, with flow becoming
increasingly confluent across and east of the Great Plains.

Short wave developments within this regime are becoming more
unclear, and the pattern predictability appears to be decreasing.
It seems possible, though, that boundary-layer moistening over the
Gulf of Mexico could become increasingly supportive of severe
weather potential by late next weekend, pending short wave
developments across the southern Rockies into the southern Great
Plains.

..Kerr.. 02/21/2021
$$
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ACUS48 KWNS 220916
SWOD48
SPC AC 220915

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0315 AM CST Mon Feb 22 2021

Valid 251200Z - 021200Z

...DISCUSSION...
Blocking may linger within the large-scale pattern across the
southern mid-latitude and subtropical eastern Pacific late this week
into the beginning of next week. However, the latest medium-range
guidance indicates that prominent/amplified mid-level ridging,
forecast to evolve across the eastern Pacific during the middle to
latter portion of this week, will become increasingly suppressed by
a series of significant short wave perturbations emerging from the
upstream westerlies.

Mid-level flow inland of the Pacific coast appears likely to become increasingly split, with a number of embedded progressive short wave perturbations. The northern branch is currently forecast to
maintain a relatively low amplitude with a west-northwesterly
component, to the southwest of an elongated deep vortex centered
just north of Hudson Bay. Within the southern branch, it appears
that relatively amplified mean troughing will develop across the
Pacific Coast into the Great Plains.

At the same time, subtropical ridging may become increasingly
prominent across much of the Gulf of Mexico into the Southeast.

This regime may become increasingly conducive to significant surface cyclogenesis to the lee of the southern Rockies by next
Sunday/Monday, as boundary-layer moistening continues over the Gulf
of Mexico (particularly the southwestern Gulf). Associated
large-scale forcing for ascent and destabilization could support
increasing severe thunderstorm potential across parts of the
southern Great Plains into the middle/lower Mississippi Valley.
However, much will hinge on the shorter wavelength developments,
which are still fairly uncertain at this extended range. One
particular concern is the timing of the northern branch short waves,
and their associated southward surges of colder air to the lee of
the Rockies, relative to the emerging southern branch short waves,
which could displace and/or mitigate the risk for severe storms.

..Kerr.. 02/22/2021
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ACUS48 KWNS 230946
SWOD48
SPC AC 230945

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0345 AM CST Tue Feb 23 2021

Valid 261200Z - 031200Z

...DISCUSSION...
Medium-range models continue to suggest that, although blocking may
linger to some degree through much of this period, an initially
prominent mid-level high and amplified ridging across the eastern
Pacific will become suppressed by a series of short wave
perturbations emerging from the upstream Pacific. As this occurs,
it still appears that a more pronounced split flow will evolve
downstream, across much of the U.S. and southern Canada. This may
include mean mid-level troughing in the southern branch, across the
southern Pacific coast through the Southwest, from which one or two
significant perturbations may emerge late this weekend into early
next week. At the same time, subtropical ridging, centered over the
southern Gulf of Mexico, may undergo a period of amplification, and
a deep mid-level low near/north of Hudson Bay may maintain some
influence, before perhaps returning northward across the Arctic
Circle vicinity by the end of the period.

This pattern could include potential for significant surface
cyclogenesis to the lee of the southern Rockies and a substantive
return flow of moisture off the western Gulf of Mexico by early next
week, accompanied by at least some increase in risk for severe
weather. However, based on sizable differences evident among the
various models, and from run to run, the predictability of the short
wave developments throughout this period appears relatively low.

..Kerr.. 02/23/2021
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ACUS48 KWNS 240951
SWOD48
SPC AC 240950

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0350 AM CST Wed Feb 24 2021

Valid 271200Z - 041200Z

...DISCUSSION...
An initially prominent mid-level high and amplified ridging across
the eastern Pacific likely will become suppressed by a series of
short wave perturbations emerging from the upstream Pacific, late
this week into the weekend. This has been, perhaps, the main point
of consistency in the medium-range guidance over the past several
days. However, blocking may persist within the large-scale flow to
some degree, and the pattern predictability, in general, appears
relatively low. Short wave developments, in particular, have been
highly variable within the various model output.

For the period late this weekend into the middle of next week,
guidance appears to be trending toward a solution with decreasing
potential for significant surface cyclogenesis to the lee of the
southern Rockies. Low amplitude wave development might still be
possible along an evolving frontal zone across parts of the southern
Great Plains through the Gulf and south Atlantic Coast states, and
this could still be accompanied by at least some risk for severe
weather. However, the potential for the development of more than
relatively marginal conditions for severe thunderstorms currently
seems low.

..Kerr.. 02/24/2021
$$
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ACUS48 KWNS 250947
SWOD48
SPC AC 250946

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0346 AM CST Mon Jan 25 2021

Valid 281200Z - 021200Z

...DISCUSSION...
Latest runs of the medium-range models exhibit fair agreement
through the medium-range period, though differences in strength and
timing of the advance of smaller-scale features is evident
throughout the period.

In general however, the period will be characterized primarily by
the slow evolution/migration of a trough -- initially just off the
West Coast -- that is progged to traverse the entire U.S. and reside
near the East Coast at the end of Day 8.

As this system progresses, an associated surface cyclone/frontal
system is forecast to emerge into the Plains by Day 6 (Saturday),
and then shift steadily eastward until moving off the East Coast
late Day 7/Day 8.

Preceding the advance of the surface system however, strong surface cyclogenesis off the East Coast Day 4 (Thursday) will be followed by
a surge of cold continental air across the eastern U.S. and through
the Gulf of Mexico. This will likely lead to only a very
incompletely modified return of low-level air ahead of the advancing
surface system, and thus likelihood for very minimal instability.
As such, risk for severe weather appears low, through the period.

..Goss.. 01/25/2021
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ACUS48 KWNS 260956
SWOD48
SPC AC 260954

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0354 AM CST Tue Jan 26 2021

Valid 291200Z - 031200Z

...DISCUSSION...
Medium-range models are again exhibiting fair model-to-model
agreement through most of the medium range, in terms of larger-scale
features. The overall scenario remains that a trough over the
western U.S. at the start of the period will gradually evolve while
shifting slowly but steadily eastward, finally reaching/moving off
the East Coast late Day 7 into Day 8 (early next week).

This system will be accompanied by a fairly well developed surface
low/frontal system, first emerging into the Plains late Day 4/early
Day 5 (Saturday morning). While southerly low-level flow within the
warm sector of this cyclone will permit theta-e advection to occur
into the south-central U.S., the prior intrusion of cold continental
air through the Gulf of Mexico will be incompletely modified in
advance of this system. Thus, minimal instability is anticipated,
and thus, only weak -- and largely elevated -- convection is
expected. As the system progresses through the rest of the period,
an increasingly stable airmass with eastward extent will likely
preclude any severe risk through the period.

..Goss.. 01/26/2021
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ACUS48 KWNS 270943
SWOD48
SPC AC 270941

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0341 AM CST Wed Jan 27 2021

Valid 301200Z - 041200Z

...DISCUSSION...
Medium-range models continue to exhibit reasonable large-scale
agreement with respect to evolution of the upper pattern, through
roughly Day 7. Beyond that, differences quickly arise with handling
of a large trough crossing the West late in the period.

Prior to then, the pattern aloft will be characterized by the
progression of a trough -- initially situated over the southern
Rockies/central and southern High Plains. As this feature
evolves/shifts eastward with time, it will be accompanied by a
well-developed surface low/frontal system, that models suggest will
move from the High Plains early Day 4 (Saturday), to near/off the
East Coast by the end of Day 6 (Tuesday morning).

Despite the well-developed nature of this surface system, its warm
sector will likely remain notably lacking in instability, due to the
antecedent cP airmass intrusion through the Gulf of Mexico, and
persistence of ridging aloft across the western Atlantic and into
the Gulf. As such, though precipitation -- including
weak/low-topped convection -- will accompany passage of the system,
severe weather is not expected.

..Goss.. 01/27/2021
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ACUS48 KWNS 280952
SWOD48
SPC AC 280951

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0351 AM CST Thu Jan 28 2021

Valid 311200Z - 051200Z

...DISCUSSION...
Agreement/similarity in solutions amongst medium-range models only
appears reasonable through Day 5 (Monday), in the most recent runs,
as models continue to differ rather markedly with the
evolution/advance of an eastern Pacific trough forecast to move
onshore over the West late Day 5/early Day 6. These continued
differences suggest ongoing lack of predictability within this
evolving pattern, rendering any convective assessment largely
meaningless beyond Day 5.

Early in the period, an eastern U.S. upper trough will advance
slowly toward the Appalachians Day 4 (Sunday), and then across the
mountains early Monday, possibly then evolving into a closed low and subsequently drifting up the Northeast U.S. coastal vicinity.

At the surface, a low-pressure system will take a similar eastward
path across the eastern U.S., with a cold front -- trailing from an
Ohio Valley low -- forecast to cross the Appalachians Day 4, before
shifting offshore Day 5. However, a lack of warm-sector instability
is expected, due to an insufficiently warm/moist boundary layer.
Therefore, severe weather is not expected through Day 5.

..Goss.. 01/28/2021
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ACUS48 KWNS 290851
SWOD48
SPC AC 290849

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0249 AM CST Fri Jan 29 2021

Valid 011200Z - 061200Z

...DISCUSSION...
A deep upper trough is forecast to exit the East Coast on D4/Monday,
with upper ridging (and associated stable conditions) moving across
the Plains and eastern CONUS in its wake. Consequently,
thunderstorms potential will be very low.

A strong shortwave trough is expected to move over the West Coast
late D5/Tuesday or early D6/Wednesday. Guidance varies on the
evolution of this system, but some severe potential could manifest
if a slower, more southerly track is realized. However, model
differences and forecast range currently limit predictability,
leading to low forecast confidence.

..Mosier.. 01/29/2021
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ACUS48 KWNS 300911
SWOD48
SPC AC 300909

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0309 AM CST Sat Jan 30 2021

Valid 021200Z - 071200Z

...DISCUSSION...
Upper ridging is forecast to traverse the central CONUS on
D4/Tuesday, leading to continued stable conditions east of the
Rockies. Thereafter, a deep upper trough is expected to move across
the western CONUS and into the Plains. Overall evolution of this
upper trough remains uncertain although it does appear increasingly
likely that a series of shortwave troughs will move through the
larger upper trough, contributing to severe potential as well as
ushering in a very cold air mass.

Current guidance suggests any severe potential would be linked to
the lead shortwave trough, which will likely eject out across the
southern Plains late D6/Thursday and into the MS Valley on
D7/Friday. Currently, modest moisture return, minimal buoyancy, and
deep southwesterly (i.e. boundary-parallel) flow all suggest limited
severe potential. Even so, very strong forcing for ascent and
vertical shear will accompany this shortwave, and some severe storms
may materialize if mesoscale details (which are difficult to
ascertain at this forecast range) align.

..Mosier.. 01/30/2021
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ACUS48 KWNS 310950
SWOD48
SPC AC 310949

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0349 AM CST Sun Jan 31 2021

Valid 031200Z - 081200Z

...DISCUSSION...
Medium-range guidance is in relatively good agreement on the overall progression of the upper pattern into the weekend, but continues to
show variance of embedded shortwave troughs.

Expectation is for a deep upper trough to begin the period extended
from western Canada to off the southern CA coast. Several shortwaves
will likely be embedded within this larger trough, with guidance
offering various solutions of their evolution. The southernmost
shortwave is forecast to move across the Southwest/northern Mexico
on D5/Thursday and across the southern Plains on D6/Friday. Previous
model runs suggested this shortwave had the potential to interact
with modest moisture return across the southern Plains. However,
most recent runs have trended faster with a cold front on
D5/Thursday, which pushes through before the shortwave arrives.

Another shortwave trough is expected to follow quickly in the wake
of the first, rotating through the parent upper trough and across
the southern Plains across on D6/Friday or D7/Saturday.

Main forecast uncertainty is whether or not either one of these
shortwaves will be able to interact with any low-level moisture.
Recent runs trended away from more vigorous interaction, but, given
the strength of these systems, some severe potential could still
manifest and guidance will be monitored closely for upcoming
outlooks.

..Mosier.. 01/31/2021
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ACUS48 KWNS 010919
SWOD48
SPC AC 010917

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0317 AM CST Mon Feb 01 2021

Valid 041200Z - 091200Z

...DISCUSSION...
Recent medium-range guidance is showing better consistency into the
weekend. Current expectation is for an upper trough to extend from
the Canadian Prairie Provinces to off the southern CA coast early
D4/Thursday. This upper trough is forecast to move eastward on
D4/Thursday and D5/Friday. The northern and central portions of the
trough will be more progressive than the southern portion, which may
lead to bifurcation and the development of a upper low off the
northern
+ coast of the Baja Peninsula.

Surface low associated to the primary trough is expected to move
from the central Plains northeastward across the MS Valley and upper
Great Lakes and into Ontario by D5/Friday. Attendant cold front will
sweep eastward/southeast across the southern Plains and mid/lower MS
Valley on D4/Thursday and remainder of the eastern CONUS on
D5/Friday. Modest low-level moisture will precede this front, but
relatively cool and cloudy antecedent conditions are currently
expected to limit thunderstorm coverage. Slight changes in the
thermodynamics would result in a more favorable environment and
trends will be monitored closely in subsequent outlooks.

Another shortwave trough and attendant surge of cold air is forecast
to follow quickly in the wake of the first system this weekend.
Short period between this surge and the lead wave is expected to
limit moisture return and thunderstorm potential.

..Mosier.. 02/01/2021
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ACUS48 KWNS 020957
SWOD48
SPC AC 020955

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0355 AM CST Tue Feb 02 2021

Valid 051200Z - 101200Z

...DISCUSSION...
An expansive upper trough is expected to persist across much of the
CONUS through the weekend before the flow then trends less
amplified/more zonal. Latest medium-range guidance forecasts a pair
of strong cold fronts, one on D5/Saturday and D6/Sunday and the
other on D7/Monday and D8/Tuesday, will likely keep conditions
stable across the majority of the CONUS. The only exception is from
the Gulf Coast into FL, where these fronts may interact with modest
low-level moisture to produce thunderstorms.

While the most recent run of the guidance suggests limited potential
throughout extended period, moderate run-to-run variability exists. Additionally, given the active/progressive nature of the pattern and
presence of strong dynamics, minor changes in system evolution could
result in more thermodynamically favorable environments. As such,
trends will be monitored closely in subsequent outlooks.

..Mosier.. 02/02/2021
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ACUS48 KWNS 030919
SWOD48
SPC AC 030917

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0317 AM CST Wed Feb 03 2021

Valid 061200Z - 111200Z

...DISCUSSION...
At least broadly cyclonic flow aloft is forecast to persist across
the CONUS until the middle of next week. A series of shortwave
troughs are expected to traverse this parent upper troughing, the
first on D4/Saturday, followed by another on D5/Sunday, and
potentially a third on D6/Monday or D7/Tuesday. The first two waves
will be accompanied by surges of cold, continental air, acting to
reinforce the stable conditions expected to be in place. This will
limit any low-level moisture return and associated buoyancy,
preventing thunderstorms.

The only exception is across FL, where enough low-level moisture may
remain for isolated storms ahead of each frontal passage. Enhanced
mid/upper level southwesterly flow will likely be in place over FL
from D4/Saturday through D5/Sunday, resulting in the potential for a
few stronger storms as the surface low and attendant front progress
across the peninsula. After D5/Sunday, the stronger flow aloft is
forecast to retreat northward, limiting severe potential with the
next frontal passage.

..Mosier.. 02/03/2021
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ACUS48 KWNS 040905
SWOD48
SPC AC 040904

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0304 AM CST Thu Feb 04 2021

Valid 071200Z - 121200Z

...DISCUSSION...
Broad upper trough is expected to persist into the middle of next
week as a series of shortwave troughs continued to reinforce the
prevailing pattern. Additionally, a cold, polar air mass is expected
to move into the northern/central Plains and Upper Midwest on
D4/Sunday before continuing southward/southeastward into the
southern Plains and MS and OH Valleys on D5/Monday. High surface
pressure associated with this cold air mass will then dominate
sensible weather across the majority of the CONUS for the remainder
of the period.

The only exception is across FL, where a few thunderstorms are
possible on late D5/Monday into early D6/Tuesday as a shortwave
trough glances the region.

..Mosier.. 02/04/2021
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ACUS48 KWNS 060926
SWOD48
SPC AC 060924

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0324 AM CST Sat Feb 06 2021

Valid 091200Z - 141200Z

...DISCUSSION...
Run-to-run model consistency continues to be an issue in the medium
to extended time range. In the mean, upper troughing will persist
across much of the CONUS, with a series of embedded shortwave
troughs ejecting from the Rockies eastward through the period.
Surface high pressure will generally be maintained over the northern
Plains and much of the Midwest, resulting in a cold and dry air mass
over those regions for the next week or so. Guidance has wavered
with regard to how far south this arctic air mass intrusion will
progress and the timing of such. Some guidance suggests that a cold
front will dive southeast across the southern Plains and into the
Southeast late in the week, potentially leading to thunderstorms and
a low-end severe threat across the Gulf Coast states around Day
5/Wed-Day 6/Thu. However, this signal has not been consistent. As
such, confidence is low in any severe potential during the Day 4-8
period.

..Leitman.. 02/06/2021
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ACUS48 KWNS 070928
SWOD48
SPC AC 070926

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0326 AM CST Sun Feb 07 2021

Valid 101200Z - 151200Z

...DISCUSSION...
A series of shortwave upper troughs are expected to migrate through larger-scale troughing across the CONUS during the Day 4-8 period.
Some strong to severe storm potential may develop from the ArkLaTex
vicinity into the lower MS Valley on Days 4-5/Wed-Thu as the initial
shortwave ejects eastward from the southwest U.S. to the
Mid-Atlantic during that time frame. Southerly flow ahead of the
trough will allow moisture to surge northward from southern and
eastern TX through the Gulf coast states ahead of an arctic cold
front. This warm advection regime ahead of the front would likely
result in elevated thunderstorm activity to the cool side of the
warm front over the Gulf coast region, or undercut by the advancing
cold front. Medium range guidance run-to-run consistency continues
to be poor, and the timing of these features varies, resulting in
low predictability of severe potential.

Beyond Day 5/Thu, guidance suggests several surges of arctic air
will overspread much of the CONUS, resulting in cold and stable
conditions and limiting thunderstorm activity through the weekend.

..Leitman.. 02/07/2021
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ACUS48 KWNS 080909
SWOD48
SPC AC 080908

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0308 AM CST Mon Feb 08 2021

Valid 111200Z - 161200Z

...DISCUSSION...
A progressive upper pattern is forecast during the Day 4-8 period,
with a series of troughs migrating across the CONUS. The first upper
trough in this series will spread eastward across the Plains and
toward the Atlantic coast on Day 4/Thu. At the surface, an arctic
air mass will percolate south and east behind a cold front,
extending from the coastal Carolinas to the central Gulf coast
vicinity by Day 5/Fri morning. Thunderstorm potential will exist
across parts of the Southeast in the vicinity of the front on
Thursday and Friday, though most of this activity is expected to
remain elevated/to the cool side of the boundary, limiting severe
potential in the absence of stronger surface cyclogenesis.

Strong surface high pressure will continue to build south/southeast
across the Plains and Midwest, as a second upper trough digs across
the western states and shifts eastward over the Plains and Midwest
during the weekend. This will reinforce a bitterly cold air mass
across much of the U.S. through the end of the period, precluding
severe thunderstorm potential.

..Leitman.. 02/08/2021
$$
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ACUS48 KWNS 090921
SWOD48
SPC AC 090919

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0319 AM CST Tue Feb 09 2021

Valid 121200Z - 171200Z

...DISCUSSION...
Severe potential will be low during the Day 4-8 period. Strong
surface high pressure and an arctic airmass will persist across much
of the CONUS through at least Day 7/Monday. Thunderstorms are
possible across parts of GA/FL on Day 4/Friday as a cold front sags
southward and stalls over northern FL. Severe storms are not
expected with this activity given poor lapse rates and weak forcing.
Given the persistent arctic airmass over the eastern two-thirds of
the CONUS, thunderstorm activity will remain limited.

..Leitman.. 02/09/2021
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ACUS48 KWNS 100914
SWOD48
SPC AC 100913

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0313 AM CST Wed Feb 10 2021

Valid 131200Z - 181200Z

...DISCUSSION...
An arctic airmass will envelop much of the CONUS through at least
Day 6/Mon. A series of upper troughs will allow reinforcing shots of
cold air to spread southeast across the Plains and Midwest. While a
cold and dry airmass will settle across the Southeast U.S.,
southwesterly deep-layer flow will prevail, maintaining weak warm
advection across the region with a surface boundary extending across
the northern Gulf of Mexico and northern FL for much of the period.
This will allow for periods of showers and thunderstorms across
parts of the Southeast, but severe potential appears low.

The exception may be around Day 6/Mon or Day 7/Tue when a shortwave
trough is forecast by some guidance to develop eastward across
northern/central Mexico and into the Gulf, before lifting northeast
toward the Mid-Atlantic by the end of the period. If this occurs,
surface cyclogenesis across the lower MS/TN Valley vicinity could
result in a favorable warm sector overlapping with strong shear and
upper forcing across parts of the Southeast ahead of another strong
cold front. Medium-range guidance currently differs in timing and
intensity of these features, resulting in low predictability, but
severe probabilities could be needed in future outlooks if
confidence in this scenario increases.

..Leitman.. 02/10/2021
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ACUS48 KWNS 120926
SWOD48
SPC AC 120925

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0325 AM CST Fri Feb 12 2021

Valid 151200Z - 201200Z

...DISCUSSION...
An upper trough is forecast to develop eastward across parts of the central/eastern CONUS from Day 4/Monday into Day 5/Tuesday.
Low-level moisture should return northward across parts of the Gulf
Coast states and the Carolinas ahead of a northeastward-developing
surface low. A severe risk may develop across parts of these
regions, mainly from late Monday into Tuesday. However, confidence
in sufficient destabilization to support surface-based storms is not
great enough to include 15% severe probabilities for now.

Medium-range guidance is in relatively good agreement that another
large-scale upper trough will move across the central/eastern states
around the Day 6/Wednesday to Day 7/Thursday time frame. Some severe
risk may once again materialize across parts of the central Gulf
Coast states and perhaps the Carolinas as low-level moisture returns
northward across these areas. Regardless, there is far too much
uncertainty in the prospect for substantial boundary-layer
destabilization to include any severe probabilities at this extended
time frame.

..Gleason.. 02/12/2021
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ACUS48 KWNS 130956
SWOD48
SPC AC 130954

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0354 AM CST Sat Feb 13 2021

Valid 161200Z - 211200Z

...DISCUSSION...
Some severe threat may exist on Day 4/Tuesday across parts of the
eastern Carolinas and the central/southern FL Peninsula ahead of a
cold front. Strong deep-layer shear associated with a
northeastward-ejecting shortwave trough should be present across
these regions. However, forecast instability appears marginal to
support organized severe storms. Have therefore not included any 15%
severe probabilities for Day 4/Tuesday.

In the wake of this lead system, medium-range guidance is in fairly
good agreement that another upper trough should develop eastward
across the central/eastern CONUS in the Day 5/Wednesday to Day
6/Thursday time frame. Sufficient low-level moisture may return
northward across parts of the Southeast ahead of the upper trough to
support surface-based storms, mainly late Wednesday into Thursday.
An associated severe risk appears possible based on forecast
instability and shear. However, the overall severe threat appears
too marginal and uncertain at this extended time frame to introduce
15% severe probabilities. By late next week into the following
weekend, meager low-level moisture behind another frontal passage
should greatly limit the severe potential across the CONUS.

..Gleason.. 02/13/2021
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ACUS48 KWNS 140937
SWOD48
SPC AC 140936

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0336 AM CST Sun Feb 14 2021

Valid 171200Z - 221200Z

...DISCUSSION...
A large-scale upper trough is forecast to amplify over the Plains on
Day 4/Wednesday while moving slowly eastward. Substantial low-level
moisture confined to the Gulf of Mexico and FL Peninsula ahead of
this feature should return northward across parts of the Southeast
Wednesday night into Day 5/Thursday. An organized severe threat
could develop over parts of the central Gulf Coast states if
low-level moisture sufficient to support surface-based storms can
advance far enough inland. At this point, there is too much
uncertainty regarding the quality of low-level moisture and related boundary-layer instability to include 15% severe probabilities for
any part of the Southeast for Wednesday night or Thursday. From Day
6/Friday through next weekend, organized severe potential appears
nil across the CONUS, as low-level moisture behind a cold front
passage should be very limited.

..Gleason.. 02/14/2021
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ACUS48 KWNS 150954
SWOD48
SPC AC 150952

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0352 AM CST Mon Feb 15 2021

Valid 181200Z - 231200Z

...DISCUSSION...
A highly amplified upper trough should continue moving eastward from
the Plains across much of the MS/OH/TN Valleys and Southeast on Day
4/Thursday. A related surface low is forecast to develop
northeastward across the central Gulf Coast states and towards the
vicinity of SC by Thursday evening. Greater low-level moisture and a
warm sector should likewise develop northward over portions of the
FL Panhandle, southeastern AL, and southern/central GA through the
day. Rather strong low- and mid-level flow appears likely to
overspread these areas, and deep-layer shear will easily support
organized severe convection. Strong to severe storms may be ongoing
Thursday morning across the western FL Panhandle/southern AL
vicinity near the surface low. Current expectations are for these
storms to move quickly east-northeastward on Thursday across the
developing warm sector.

Both damaging winds with a possible squall line and a couple
tornadoes with any supercells either ahead of or embedded within the
line may occur given a favorable veering/strengthening low-level
wind profile and sufficiently unstable airmass. With a consistent
signal in guidance depicting this scenario, have included a 15%
severe area for Day 4/Thursday where surface-based storms appear
probable. Some severe risk may extend northeastward into parts of
the Carolinas Thursday evening/night. However, too much uncertainty
currently exists regarding the degree of boundary-layer instability
to extend the 15% severe probabilities any farther northeastward.
Once the cold front associated with the surface low clears the East
Coast, potentially late Thursday night or early Day 5/Friday, then
the severe risk across the CONUS appears negligible through the
remainder of the extended forecast period.

..Gleason.. 02/15/2021
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ACUS48 KWNS 100722
SWOD48
SPC AC 100720

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0120 AM CST Sun Jan 10 2021

Valid 131200Z - 181200Z

...DISCUSSION...
On Wednesday/D4, models are in agreement showing a de-amplifying
shortwave trough moving quickly across the Southeast, where a dry
air mass will remain at the surface. As such, very little elevated
instability is expected to support thunderstorms.

On Thursday/D5, a major trough amplification is forecast to occur as
powerful northwest flow aloft dives southeastward into the Plains,
resulting in a large-scale upper trough over the East with upper low
over the Great Lakes. Given the antecedent dry air across the Gulf
of Mexico, a brief window of southwest surface flow over the
southern Plains and lower MS Valley is unlikely to be enough for any
CAPE to develop.

Beyond Thursday/D5, thunderstorm chances will remain nearly zero as
a large surface ridge moves from the Plains across the Southeast and
over the Gulf of Mexico. Additional trough amplifications are
possible across the midsection of the country with a mean upper
ridge over the West Coast, maintaining the dry surface pattern.

..Jewell.. 01/10/2021
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ACUS48 KWNS 110755
SWOD48
SPC AC 110754

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0154 AM CST Mon Jan 11 2021

Valid 141200Z - 191200Z

...DISCUSSION...
An inactive convective pattern is likely to persist through
Monday/D8, with a large upper trough developing over the central and
eastern states during the Thursday/D4 to Saturday/D6 period. Little
low-level moisture return is forecast over land ahead of the initial
cold front, which will be near the TX Coast at 00Z Friday.

Additional surges of high pressure are forecast to occur over the
central and eastern states from Sunday/D7 to Monday/D8, as models
indicate another upper trough amplification over the Plains and
Great Lakes region. As such, severe weather is quite unlikely, with
little thunderstorm potential in general.

..Jewell.. 01/11/2021
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ACUS48 KWNS 130931
SWOD48
SPC AC 130930

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0330 AM CST Wed Jan 13 2021

Valid 161200Z - 211200Z

...DISCUSSION...
Model consensus is that a synoptic upper trough will move through
the eastern U.S. day 4 (Saturday) followed closely by an upstream
trough on day 5. This pattern will be accompanied by offshore winds
over the Gulf, maintaining stable conditions inland. A pattern
transition is forecast to occur by day 6 (Monday) as an upper trough
amplifies across the Great Basin region. Modifying Gulf moisture
should return through south TX, but instability will remain
insufficient for a severe threat. Moisture return should be more
substantial across south and southeast TX and possibly into southern
LA by day 7 with potential for greater destabilization. However, by
this time frame. predictability diminishes due to uncertainty
regarding the trajectory of the Great Basin upper trough.

..Dial.. 01/13/2021
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ACUS48 KWNS 250954
SWOD48
SPC AC 250953

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0353 AM CST Thu Feb 25 2021

Valid 281200Z - 051200Z

...DISCUSSION...
Blocking may persist to some degree within the evolving large-scale
pattern over the eastern Pacific late this weekend into early next
week, maintaining a split flow with embedded short wave developments characterized by a relatively low degree of predictability. In
general, though, by the middle of next week, it does appear that a
progression of short waves will contribute to the suppression of
fairly prominent initial subtropical ridging centered over the
southeastern Gulf of Mexico/northwestern Caribbean. There also
appears consensus that amplified large-scale troughing will evolve
across the eastern Pacific (mostly to the west of the Pacific
coast), with downstream large-scale ridging building inland of the
Pacific coast through the U.S. Great Plains and Canadian Prairies.

While developments east of the Rockies are more uncertain, a frontal
zone initially extending from near or south of the Texas Big Bend
into the Mid Atlantic Coast region may provide a focus for
continuing thunderstorm development, and perhaps at least some
severe weather potential. In the wake of a deepening surface
cyclone across the Great Lakes into Quebec on Sunday, this frontal
zone may shift southward into the south Atlantic and Gulf Coast
states by early next week. As it does, it is possible that
destabilization along and south of it could become sufficient to
support vigorous thunderstorms with the potential to produce
damaging wind gusts and perhaps a risk for tornadoes, in the
presence of strong deep-layer mean wind fields and shear.

As the frontal zone becomes quasi-stationary across the southern
tier of the U.S. during the early to middle portion of next week,
the extent of any continuing severe weather potential will largely
depend on the strength of subsequent surface wave development along
it. There has been, and continues to be, considerable variability
within medium range output concerning this.

..Kerr.. 02/25/2021
$$
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ACUS48 KWNS 260932
SWOD48
SPC AC 260930

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0330 AM CST Fri Feb 26 2021

Valid 011200Z - 061200Z

...DISCUSSION...
An upper low will become an open shortwave trough as it ejects
eastward from NM to the southern Plains and the Ohio Valley Days
4-5/Mon-Tue. A second upper shortwave will then migrate from the
southern Rockies and merge with a larger-scale upper trough over the
central U.S. on Day 6/Wed. These features will induce surface low
development over coastal TX late Mon/early Tue. The weak surface low
will lift east/northeast toward the Mid-South on Tuesday and the
Mid-Atlantic vicinity late Wednesday. This will bring a cold front southeastward across the southeastern states through Day 7 Thu. Some
low-end severe potential could develop near the cold front, mainly
Day 5/Tue into Day 7/Thu near the central Gulf Coast vicinity. Rich
Gulf moisture will be in place ahead of the front, and moderate
shear will exist as the shortwave troughs lift northeast of the
region. However, surface cyclogenesis will remain weak, and
widespread precipitation and cloudiness will likely inhibit
destabilization. At this time, the overall severe threat appears too low/uncertain to include probabilities.

..Leitman.. 02/26/2021
$$
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ACUS48 KWNS 270922
SWOD48
SPC AC 270920

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0320 AM CST Sat Feb 27 2021

Valid 021200Z - 071200Z

...DISCUSSION...
A series of upper shortwave troughs will migrate across the southern
tier of the U.S. during the Day 4-8 period. The first in this
series, located over the southern/central Plains on Day 4/Tue will
shift eastward to the Carolinas/Mid-Atlantic vicinity through Day
5/Wed. At the surface, a weak low will develop near coastal LA on
Tuesday, shifting east/northeast across the southern states and
offshore the Carolinas by Wednesday evening. This will bring a cold
front across the southeastern U.S. and widespread showers and
thunderstorms are expected. Much of this activity is expected to
remain elevated, with instability limited and shear relatively
modest. High pressure will settle across the eastern half of the
U.S. Day 6/Thu, with the surface cold front continuing southward
into the northern Gulf and FL Straits vicinity.

Late in the period, a second upper shortwave trough/closed low is
expected to develop eastward across the southern Plains and Gulf
coast vicinity around Day 7/Fri-Day 8/Sat. Guidance varies quite a
bit in the evolution of this system, with some medium range guidance
digging the trough fairly far south into the Gulf. This could limit
northward moisture return, keeping any severe potential offshore
from the central Gulf coast. However, this could bring an increase
in severe potential to the FL Peninsula over the weekend. Given
large spread, uncertainty is too great to include severe
probabilities at this time.

..Leitman.. 02/27/2021
$$
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ACUS48 KWNS 280928
SWOD48
SPC AC 280926

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0326 AM CST Sun Feb 28 2021

Valid 031200Z - 081200Z

...DISCUSSION...
Run-to-run consistency among medium-range guidance has been poor the
past day or two. In general, the lends to low confidence in any
given solution through the extended period. Forecast guidance
continues to show a couple of shortwave troughs migrating across the
southern tier of the CONUS during the period. One upper shortwave
trough over the southeastern states will quickly shift offshore on
Day 4/Wed, bringing a cold front across FL. Thereafter, high
pressure will persist east of the Rockies through at least Day
5/Thu. By the weekend, another shortwave is then forecast to track
eastward from the Plains to the southeastern U.S. At this time, poor
Gulf return flow ahead of this system is forecast to limit severe
potential.

..Leitman.. 02/28/2021
$$
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ACUS48 KWNS 010933
SWOD48
SPC AC 010932

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0332 AM CST Mon Mar 01 2021

Valid 041200Z - 091200Z

...DISCUSSION...
Medium-range guidance is in somewhat better agreement compared to
the past couple of days. Overall, severe potential appears low
during the Day 4-8 period. A shortwave upper trough over the western
U.S. will migrate eastward to the southeastern states through Day
5/Fri. As this occurs, a weak surface low is forecast to develop
over central TX and weak Gulf return flow will overspread parts of
coastal TX. Some low-end severe potential could develop ahead of low
and attendant cold front, but moisture return will likely be shallow
and ill-timed with this system. After Friday, surface high pressure
will dominate east of the Rockies, limiting any northward moisture
return through the end of the period.

..Leitman.. 03/01/2021
$$
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ACUS48 KWNS 020928
SWOD48
SPC AC 020926

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0326 AM CST Tue Mar 02 2021

Valid 051200Z - 101200Z

...DISCUSSION...
Severe potential will remain low for much of the Day 4-8 period.
Persistent surface high pressure east of the Rockies, and a cold
front dropping southward across the much of the Gulf of Mexico will
result in dry and stable conditions. Toward the end of the period,
an upper trough is forecast to develop southward along the Pacific
coast and shift eastward to the Plains. While forecast guidance
varies in the exact evolution and timing of this feature, associated
strong surface cyclogenesis could result in a deeper northward
transport of Gulf moisture and possible strong-to-severe
thunderstorm development around Day 8/Tue or just beyond the
forecast period from the southern Plains toward the MS Valley.

..Leitman.. 03/02/2021
$$
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ACUS48 KWNS 020928
SWOD48
SPC AC 020926

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0326 AM CST Tue Mar 02 2021

Valid 051200Z - 101200Z

...DISCUSSION...
Severe potential will remain low for much of the Day 4-8 period.
Persistent surface high pressure east of the Rockies, and a cold
front dropping southward across the much of the Gulf of Mexico will
result in dry and stable conditions. Toward the end of the period,
an upper trough is forecast to develop southward along the Pacific
coast and shift eastward to the Plains. While forecast guidance
varies in the exact evolution and timing of this feature, associated
strong surface cyclogenesis could result in a deeper northward
transport of Gulf moisture and possible strong-to-severe
thunderstorm development around Day 8/Tue or just beyond the
forecast period from the southern Plains toward the MS Valley.

..Leitman.. 03/02/2021
$$
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ACUS48 KWNS 030931
SWOD48
SPC AC 030930

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0330 AM CST Wed Mar 03 2021

Valid 061200Z - 111200Z

...DISCUSSION...
Medium-range guidance continues to indicate that severe potential
will be low through at least Day 6/Mon. Strong surface high pressure
will persist east of the Rockies during this time. Offshore
trajectories will keep Gulf moisture well offshore and dry/stable
conditions will prevail. Around Day 7/Tuesday, an upper trough will
eject across the western U.S. to the central/southern Plains. Strong
surface low development is forecast over the northern/central Plains
in response. Strong southerly low level flow will allow northward
Gulf moisture return, ahead of a surface cold front. Some strong-to-
severe thunderstorm risk could develop on Day 7/Tue or Day 8/Wed
across parts of the southern/central Plains to the Mid/Lower MS
Valley. However, uncertainty remains high with large spread among
various guidance in timing of the upper trough, surface high and
moisture return, precluding severe probabilities.

..Leitman.. 03/03/2021
$$
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ACUS48 KWNS 040925
SWOD48
SPC AC 040923

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0323 AM CST Thu Mar 04 2021

Valid 071200Z - 121200Z

...DISCUSSION...
Surface high pressure over much of the southern and eastern U.S.
will maintain generally dry and stable conditions east of the
Rockies through Day 5/Monday. By Day 6/Tuesday, an upper trough will
move across the western states and strong surface cyclogenesis is
forecast over the northern/central Plains. This will allow strong
southerly low-level flow to begin northward transport of Gulf
moisture across the southern Plains. A series of upper shortwave
impulses will eject northeast across the Plains to the Great Lakes
on Tuesday into Day 8/Thursday, maintaining southerly return flow
over the southern Plains to the lower MS Valley. This could result
in multiple days of increasing severe potential toward the end of
the forecast period for portions of the central/southern Plains to
the Middle/Lower MS Valley. Medium-range guidance shows this general
signal for increasing severe potential, but timing of the upper
shortwave impulses, surface cyclogenesis and northward moisture
transport continue to vary, resulting in low predictability and
precluding probabilities at this time.

..Leitman.. 03/04/2021
$$
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ACUS48 KWNS 051002
SWOD48
SPC AC 051000

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0400 AM CST Fri Mar 05 2021

Valid 081200Z - 131200Z

...DISCUSSION...
...Monday/Day 4 and Tuesday/Day 5...
The medium-range models move a shortwave ridge eastward from the
Great Plains to the Eastern Seaboard on Monday and Tuesday, keeping west-southwesterly mid-level flow in place across much of the
central part of the nation. At the surface on Monday, high pressure
is forecast to be in place from the Southeast into the Appalachians.
The high is forecast to move eastward on Tuesday, allowing low-level
moisture to return northward into the southern Plains. The models do
not show much convective potential on Tuesday. However, a few
thunderstorms related to warm advection will be possible near an
axis of low-level moisture from east Texas into Oklahoma Tuesday
evening. An isolated severe threat can not be ruled out in parts of
the southern Plains. Weak instability should keep any threat
marginal.

...Wednesday/Day 6 to Friday/Day 8...
On Wednesday, the models maintain a broad belt of strong mid-level
flow from the south-central U.S. into the northeastern states.
Low-level moisture is forecast to be maximized across eastern
portions of the southern Plains where isolated thunderstorm
development will be possible. Additional storms may develop further
to the north along a cold front in the central Plains. An isolated
severe threat may develop in parts of the southern and central
Plains, but weak instability may keep any threat marginal.

On Thursday, the models continue low-level moisture advection across
the southern Plains and Mississippi Valley. The warm-advection
region could result in thunderstorm development the southern Plains northeastward into parts of the lower Missouri Valley. The greatest
coverage of thunderstorm activity could occur in parts of the
central Plains along the northwestern edge of the low-level jet.
This convection could have a severe threat, but instability again is
forecast to be weak. This may keep any severe threat isolated.

On Friday, the models move an axis of maximized low-level moisture
into the lower Mississippi Valley as a cold front advances
southeastward. Thunderstorm development could occur along the cold
from parts of the Arklatex northeastward into the mid Mississippi
Valley. Although instability is forecast to be weak, an isolated
severe threat can not be ruled out.

Overall, the severe potential in the Day 4 to 8 period is expected
to remain isolated. Uncertainty is more substantial during the
latter half of the period. For this reason along with instability
that is forecast to be weak, will not add a threat area at this
time.

..Broyles.. 03/05/2021
$$
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ACUS48 KWNS 061002
SWOD48
SPC AC 061000

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0400 AM CST Sat Mar 06 2021

Valid 091200Z - 141200Z

...DISCUSSION...
...Tuesday/Day 4 and Wednesday/Day 5...
The medium-range models are in good agreement to start the Day 4 to
8 period. On Tuesday, the models show west-southwesterly mid-level
flow across much of the continental United States. Low-level
moisture advection is forecast from east Texas northward across
Oklahoma and Kansas. Although convective potential appears on the
low side, isolated thunderstorms can not be ruled out Tuesday night
across parts of the lower Missouri Valley within warm advection.

On Wednesday, the models continue to have west-southwest mid-level
flow in place from the Desert Southwest northeastward to the Great
Lakes. At the surface, a cold front is forecast to move into the
central Plains and upper Mississippi Valley. Thunderstorms would be
possible along and ahead of the front Wednesday afternoon and
evening. Weak instability is forecast near the front suggesting that
any severe threat would be isolated. Differences in the model
solutions suggest uncertainty on Wednesday is substantial.

...Thursday/Day 6 to Saturday/Day 8...
On Thursday, the models maintain southwest mid-level flow across the south-central and eastern U.S. At the surface, a cold front is
forecast to advance southeastward into northern portions of the
southern Plains and into the mid Mississippi Valley. Surface
dewpoints are only forecast to reach the 50s F near the front
suggesting that instability will remain relatively weak Thursday
afternoon. In spite of this, strong deep-layer shear is forecast
along the front, which could be enough to support organized
thunderstorm development. The greatest chance for strong storms
would be from northeastern Oklahoma into central Missouri and
central Illinois Thursday afternoon and evening. However,
uncertainty is substantial due to instability concerns and timing
issues with the front.

On Friday and Saturday, model solutions strongly diverge. For
example, the ECMWF keeps an upper-level low across the western U.S.
while the GFS moves the upper-level low eastward into the High
Plains. THE GFS ensemble has a large spread and shows both the
faster and slower solution. The ECMWF deterministic solution is much
less aggressive concerning a potential severe threat, keeping a
moist airmass located along the western Gulf Coast. The GFS
deterministic solution is more aggressive, advecting a moist airmass
into the southern Plains and Arklatex. Thunderstorms, with some
severe, would be possible in parts of the southern and central
Plains eastward into the mid Mississippi Valley both Friday and
Saturday. At this time, am favoring the slower ECMWF solution which
keeps the upper-level low further west. Due to model differences,
uncertainty is at high for Friday and Saturday.

..Broyles.. 03/06/2021
$$
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ACUS48 KWNS 070959
SWOD48
SPC AC 070958

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CST Sun Mar 07 2021

Valid 101200Z - 151200Z

...DISCUSSION...
...Wednesday/Day 4 and Thursday/Day 5...
The medium-range models start out the day 4 to 8 period in decent
agreement. For Wednesday, the various solutions have an upper-level
low near the coast of California and show southwest mid-level flow
from the Desert Southwest northeastward across much of the central
and eastern U.S. At the surface, a cold front is forecast to be
located in the central Plains and Upper Midwest. Thunderstorms will
be possible along the front from late Wednesday afternoon into the
overnight period. The most likely corridor for convective
development would along and ahead of the front, from northern
sections of the southern Plains northeastward into the mid to upper
Mississippi Valley. With surface dewpoints forecast to be in the 50s
F ahead of the front, instability should remain weak. However,
deep-layer shear is forecast to strong, making a severe threat
possible in spite of the weak instability. The potential for severe
storms could be maximized in Oklahoma and eastern Kansas Wednesday
night, where low-level flow and moisture are forecast to be the
greatest.

Southwest-mid-level flow is forecast to be maintained on Thursday
from the south-central U.S. into to the northeastern states. At the
surface by late Thursday afternoon, a cold front is forecast to be
located from northern sections of the southern Plains northeastward
into the southern Great Lakes. The ECMWF, GFS, UKMET and Canadian
models are reasonably close with the front position at 00Z/Friday.
The models suggest that convection will develop along the front from
late Thursday afternoon and persist through the evening and into the
overnight period. The strongest instability is forecast in the
southern Plains while the strongest deep-layer shear is forecast
much further northeast into the mid Mississippi Valley and southern
Great Lakes. In addition, many of the GEFS members have a shortwave
ridge located in the south-central U.S. Thursday night. Although a
severe threat could develop Thursday evening along and ahead of much
of the front, the factors previously mentioned make the forecast
magnitude of any severe threat highly uncertain.

...Friday/Day 6 to Sunday/Day 8...
On Friday, the medium-range models diverge sharply. Model solutions
vary on the position and strength of the upper-level low across the
western U.S. This will impact the forecast further to the east
across the Great Plains and lower to mid Mississippi Valley. The
models do show a moist airmass in place across the southern Plains
and lower Mississippi Valley. A cold front is forecast from Oklahoma
eastward into the mid Mississippi Valley but some solutions have the
front further to the south than others, such as the ECMWF. Severe
thunderstorm development could occur near the front Friday afternoon
and evening. The strongest instability would be in the southern
Plains suggesting the severe threat would be maximized from north
Texas into Oklahoma. A threat area may need to be added in later
outlooks, once the models show run-to-run consistency and certainty
increases.

On Saturday and Sunday, the models sharply diverge on the
upper-level pattern. Some solutions move the upper-level eastward
into the central states while others weaken the upper-level low.
There is a large spread in GEFS members as well. If the upper-level
low does move eastward into the Great Plains, a substantial severe
threat would be possible ahead of the system Saturday afternoon and
evening. The severe threat could redevelop further to the east along
and ahead of a cold front on Sunday in the Mississippi Valley. This
scenario will be dependent upon the upper-level low moving eastward
into the central U.S. If the upper-level gradually weakens, as the
ECMWF solution suggests, the severe threat in the south-central and southeastern U.S. would be much more localized. For the reasons
previously stated, predictability is low for Saturday and Sunday.

..Broyles.. 03/07/2021
$$
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ACUS48 KWNS 081001
SWOD48
SPC AC 080959

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0359 AM CST Mon Mar 08 2021

Valid 111200Z - 161200Z

...DISCUSSION...
...Thursday/Day 4 to Saturday/Day 6...
The medium-range models begin the Day 4 to 8 period in good
agreement. Model solutions show southwest mid-level flow from the
south-central U.S. to the Northeast. An upper-level low is forecast
over southern California. At the surface, a cold front is forecast
to advance southeastward across the southern Plains, Ozarks and mid
Mississippi Valley. Thunderstorm development will be possible along
parts of the front during the afternoon and evening. Although
instability is forecast to be weak ahead of the front Thursday
afternoon, strong deep-layer shear should be adequate for at least
isolated severe storms. The severe threat should extend from
northwest Texas northeastward across the Ozarks and into the lower
Ohio Valley. Hail and wind damage will be the primary threats.

On Friday, the models continue to be in good agreement. The
solutions move the upper-level low into the Desert Southwest and
have an upper-level ridge in the south-central states. The cold
front is forecast to move slowly southward and may stall from the
southern Plains eastward into the mid Mississippi Valley during the
day on Friday. Model forecasts suggest that thunderstorm development
will be possible along and near the front during the afternoon and
evening. The greatest potential for severe storms would be from
Oklahoma eastward into Arkansas, where low-level moisture, deep
layer shear and instability are forecast to be maximized.

Although there is some spread among the GEFS members, on Saturday
the deterministic solutions remain in relatively good agreement. The upper-level low is forecast to move into the Four Corners region as
a mid-level jet moves into the southern Plains. Ahead of the
approaching system, strong moisture advection is forecast in the
southern Plains where a corridor of 60+ surface dewpoints appears
likely by afternoon. Thunderstorms should be ongoing across parts of
the southern Plains and Ozarks throughout the day. An increase in
thunderstorm coverage may take place in the late afternoon and
evening as instability maximizes across the region. Model forecasts
show a potential for moderate instability, widespread large-scale
ascent and strong deep-layer shear from the southern Plains eastward
into the Ozarks. All severe threat hazards will be possible, but the
magnitude and spatial extent of the severe threat will depend upon
moisture return and the eastward timing of the system.

...Sunday/Day 7 and Monday/Day 8...
On Sunday and Monday, the upper-level low is forecast to move
eastward across the Great Plains. At the surface, a cold front is
forecast to move quickly eastward from the Great Plains into the
Ohio and Tennessee Valleys. Severe thunderstorms will be possible
along parts of the front both on Sunday and on Monday. At this time,
models suggest that the greatest severe threat will be located in
eastern parts of the southern Plains and in the Ozarks on Sunday.
The severe threat should shift eastward into the Ohio and Tennessee
Valleys on Monday. However, uncertainty concerning the timing of the
system is substantial at this extended range in the forecast period.

..Broyles.. 03/08/2021
$$
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ACUS48 KWNS 090957
SWOD48
SPC AC 090956

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0356 AM CST Tue Mar 09 2021

Valid 121200Z - 171200Z

...DISCUSSION...
...Friday/Day 4 and Saturday/Day 5...
The medium-range models are in decent agreement on Friday, with an
upper-level low in the Desert Southwest. Southwest mid-level flow is
forecast over the southern Rockies and southern Plains. Elevated
thunderstorms may be ongoing at the start of the period in parts of
Oklahoma, Kansas and Missouri. The southwestern U.S. upper-level
trough is forecast to move eastward during the day. In response, a
moist airmass should advect northward into parts of Oklahoma where
60+ surface dewpoints will be possible. A dryline is forecast to
take shape by afternoon across far western Oklahoma and west Texas.
To the east of the dryline, thunderstorms may develop during the
late afternoon and evening along a corridor of instability. Strong
deep-layer shear is forecast which may result in a potential for
supercells. Large hail and wind damage would be the primary threats.
Further to the east from northeastern Oklahoma into southern
Missouri, thunderstorms may also develop during the late afternoon
and evening. Although deep-layer shear may support an isolated
severe threat, the threat may remain marginal due to weak
instability.

On Saturday, the models continue to be in reasonable agreement, with
the upper-level low moving across the Four Corners area. A strong
low-level jet is forecast to develop in the southern Plains as the
exit region a 80 to 100 kt mid-level jet overspreads the southern
High Plains. In response, deep-layer shear will be favorable for
supercells with large hail. The current thinking is that a band of
strong to severe storms will develop from western Oklahoma southward
into northwest Texas Saturday afternoon. This band of storms could
continue to be severe, moving into central Oklahoma and north Texas
during the mid to late evening. All three hazards can not be ruled
out Saturday afternoon and evening.

...Sunday/Day 6 to Tuesday/Day 8...
The models continue to be in reasonably good agreement on Sunday and
there is not too much spread among GEFS members. An upper-level low
is forecast to move into the central High Plains as the mid-level
jet moves across the southern Plains. To the east of the jet, a
squall-line may develop and move into the Ozarks and Arklatex Sunday
afternoon. This squall-line may remain intact across the lower to
mid Mississippi Valley during the overnight period.

On Monday and Tuesday, model solutions diverge. Although a cold
front is expected to move across the Ohio/Tennessee Valleys and Gulf
Coast states, substantial uncertainty exists concerning the
upper-level level pattern. Strong thunderstorms will be possible
along and near the front but uncertainty is high. This uncertainty
continues into Tuesday due to very large differences among model
solutions.

..Broyles.. 03/09/2021
$$
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ACUS48 KWNS 100930
SWOD48
SPC AC 100929

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0329 AM CST Wed Mar 10 2021

Valid 131200Z - 181200Z

...DISCUSSION...
...Saturday/Day 4 and Sunday/Day 5...
An upper-level low is forecast to move across the Four Corners
region on Saturday as a 75 to 90 Kt mid-level jet moves into the
southern High Plains. At the surface, a low is forecast to rapidly
deepen across southeast Colorado as moisture advection continues
across the southern Plains. By afternoon, an axis of maximized
low-level moisture should be located from north Texas into western
Oklahoma and southern Kansas. The models show a corridor of
instability by late afternoon along the western edge of the moist
airmass. Thunderstorms are expected to develop within this corridor
of instability and move eastward across northwest Texas, the far
eastern Texas Panhandle and western Oklahoma during the late
afternoon and early evening. Squall line development may take place
during the evening, affecting north-central Texas and central
Oklahoma. Eastern Oklahoma and parts of northeast Texas could be
impacted during the overnight period. All hazards will be possible
across the threat area.

On Sunday, a disorganized line of storms may be ongoing in the
southern Plains at the start of the period. As surface temperatures
warm through the day, the line is expected to reorganize. Model
consensus places this line over eastern Oklahoma and northeast Texas
around midday. The squall-line is forecast to move across the
southern Ozarks and Arklatex regions during the afternoon. Surface
dewpoints ahead of the squall-line are forecast to be near 60 F.
This combined with strong deep-layer shear and enhanced large-scale
ascent will likely be favorable for severe storms. Although all
three severe hazards will be possible, the wind damage threat may be
the greatest of the three.

...Monday/Day 6 to Wednesday/Day 8...
Model solutions diverge sharply on Monday. Some models weaken the
central U.S upper-level low while others continue to move the system
eastward. The models also differ on the surface pattern, with some
solutions keeping a moist airmass located from the central Gulf
Coast states into the Tennessee Valley. If the more aggressive
solutions are reasonably close to verifying, then a severe threat
would be possible near the moist axis Monday afternoon, from middle
and eastern Tennessee southward into Mississippi and Alabama.

The large spread among solutions continues into Tuesday and
Wednesday. The most aggressive solutions develop an upper-level low
in the central states on Tuesday and move the system
north-northeastward into the upper Mississippi Valley on Wednesday.
Under this scenario, a severe threat would be possible in the
Arklatex on Tuesday afternoon and evening. On Wednesday, a severe
threat would be possible further to the east in the Ohio and
Tennessee Valleys. However, uncertainty is high toward the end of
the Day 4 to 8 period.

..Broyles.. 03/10/2021
$$
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ACUS48 KWNS 111000
SWOD48
SPC AC 110958

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CST Thu Mar 11 2021

Valid 141200Z - 191200Z

...DISCUSSION...
...Sunday/Day 4 and Monday/Day 5...
The medium-range models move an upper-level low across the central
Plains on Sunday. At the surface, a low is forecast to deepen across
the southern High Plains as a cold front advances quickly eastward
into the Ozarks and Arklatex. A squall-line appears likely to
develop just ahead of the front, moving eastward into the lower to
mid Mississippi Valley by early evening. Model forecasts continue to
show weak instability ahead of the squall-line. This should keep any
severe threat somewhat isolated, except in areas that develop
locally greater instability. Wind damage and hail will be possible
along the stronger parts of the squall-line. An isolated tornado
threat will also be possible.

On Monday, the models are in much better agreement. The upper-level
low is forecast to move into the mid Missouri Valley as a cold front
advances quickly eastward into the Tennessee Valley and central Gulf
Coast states. A remnant of the squall-line may be ongoing ahead of
the front in the morning. This line may re-intensify by midday as
instability increases. The current thinking is that an isolated wind
damage threat will develop during the afternoon from middle and
eastern Tennessee southward into Alabama.

...Tuesday/Day 6 to Thursday/Day 8...
Model spread increases on Tuesday as the pattern becomes less
amplified. An upper-level trough is forecast to move into the
southern High Plains with some moisture return occurring in the
southern Plains. The moist air mass is forecast to extend eastward
from east Texas across much of the Gulf Coast states. Thunderstorms
will be possible along the northern edge of this moist air mass
throughout much of the day. The greatest instability is forecast
across east Texas and Louisiana, where a severe threat could
develop. However, the models differ on the northward extent of the
moist air mass. The GFS is more aggressive with moisture return,
suggesting the severe threat could be as far north as the Red River.
However, uncertainty is high concerning this scenario.

On Wednesday, the model solutions sharply diverge. The GEFS also
shows considerable spread among the members. Among the deterministic
models, the GFS continues to be an aggressive solution, moving the
upper-level trough across the Great Plains. An associated cold front
is forecast to move into the Ohio and Tennessee Valleys. Severe
thunderstorms would be possible on the southern part of the front
from Tennessee southward to the Gulf Coast. In contrast, the ECMWF
confines the moist airmass to the coastal areas of the Southeast,
which would be less favorable for severe storms. A cool and dry air
mass is forecast on Thursday across much of the nation. This could
limit the severe potential.

..Broyles.. 03/11/2021
$$
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ACUS48 KWNS 131000
SWOD48
SPC AC 130958

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CST Sat Mar 13 2021

Valid 161200Z - 211200Z

...DISCUSSION...
Plenty of spread/differences continue between the GFS and ECMWF for
the medium-range period -- even as early as Days 4-5. While the
broad/overall picture remains fairly clear that a notable increase
in severe threat will occur perhaps as early as late Tuesday
afternoon (Day 4) near a southern Plains dryline, and then
continuing Day 5/Wednesday (lower Mississippi and possibly the lower
Ohio Valleys east across the central Gulf Coast states/Mid South),
and potentially Day 6/Thursday (southern Appalachians/Carolinas).

The differences amongst the models are centered around evolution of
the next upper system moving out of the western U.S. at the start of
the period. The ECMWF depicts a closed low -- strengthening with
time as it ejects, and thus the feature advances more slowly than
the GFS, which depicts a more progressive, open wave. This has
substantial implications for timing/location of surface features
through the first few days of the period -- with these differences
becoming great enough by Day 6 that no meaningful assessment of
areas of possible risk can be confidently highlighted.

With the ECMWF's pattern evolution very similar to the EC's ensemble
mean, and also similar to the UKMET solution, will lean a bit toward
a slower/less progressive evolution and thus a slightly more
westward forecast. Day 4 (Tuesday), some conditional risk for
mainly hail may exist as far west as a northwestern Texas/western
Oklahoma dryline during the early evening, with some hail risk
spreading across parts of Oklahoma and North Texas overnight.
However, with this risk more conditional, will opt not to include a
risk area for Tuesday at this time.

By Day 5/Wednesday, a more substantially unstable environment,
encompassing a large area, is likely, as a moist Gulf airmass
streams northward, beneath rather steep lapse rates advecting
eastward. With an eastward moving surface cyclone and attendant
warm and cold fronts focusing ascent, and the likelihood for broad
warm advection/QG ascent within the expanding warm sector, scattered
severe storms appear likely even given timing/location differences
of surface features amongst the models. While a more concentrated
area or areas of severe weather may evolve Wednesday, those details
-- and any associated outlook highlights -- will require discernment
in subsequent outlooks nearer the event. For now, a large/broad 15%
risk area will be introduced, representing a large envelope of
all-hazards severe potential.

Day 6, model differences become substantial enough that -- while
severe weather risk may continue, no areas will be highlighted due
to the predictability concerns.

..Goss.. 03/13/2021
$$
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ACUS48 KWNS 140909
SWOD48
SPC AC 140908

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0408 AM CDT Sun Mar 14 2021

Valid 171200Z - 221200Z

...DISCUSSION...
Medium-range models have come into somewhat better agreement with
respect to synoptic features through the early half of the period,
though smaller-scale differences persist. Still, confidence has
increased that a fairly widespread severe weather event will occur
Day 4 (Wednesday) across the lower Mississippi Valley/central Gulf
Coast region and northward into at least parts of the Tennessee
Valley, and then continuing into Day 5 (Thursday) east of the
Appalachians. In the wake of this severe-weather episode, high
pressure/stable air should spread across the U.S. east of the
Rockies, yielding at least a couple of days of quieter weather with
respect to convective potential.

On Wednesday, convection will likely be ongoing across the eastern Texas/Arkansas/Louisiana areas, with some severe risk possible
during the morning hours. By afternoon, an increase in coverage and
intensity is expected as closed upper system shifts out of the
Plains toward the Mississippi Valley. With a rather
large/destabilizing warm sector and ample shear across a broad
region, a rather large 15% area will be maintained, representative
of an all-hazards severe risk. Corridors of greater risk --
including possibly more concentrated tornado potential -- may evolve
during the afternoon, possibly near the northward-moving warm front.
While the ECMWF is farther south with this west-to-east front
(central MS/AL) during the afternoon than the GFS (southern TN),
will nonetheless introduce a 30% risk area -- encompassing a range
of possible warm-frontal locations -- given likelihood for
widespread severe weather that warrants greater probability at this
time.

The severe risk may diminish some into the evening/overnight across
the southern Appalachians area, but should increase somewhat
Thursday east of the mountains, possibly as far north as southern
Virginia and extending southward into north Florida. Timing
differences in progression of the surface front increase between the
GFS and ECMWF with time, so when the threat finally subsides -- as
storms move offshore -- remains uncertain. However, this should
certainly occur prior to the start of Day 6 (Friday), resulting in
what appears likely to be at least a couple of days of substantially
decreased convective potential.

..Goss.. 03/14/2021
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ACUS48 KWNS 150842
SWOD48
SPC AC 150841

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0341 AM CDT Mon Mar 15 2021

Valid 181200Z - 231200Z

...DISCUSSION...
Medium-range models are in reasonable agreement through roughly Day
6 (Saturday Mar. 20), sufficient to offer a reasonable assessment of
convective potential over the CONUS. After Day 6, differences
between the ECMWF and GFS become more obvious, as the next major
upper trough advances across the interior West. With a large area
of strong surface high pressure in place east of the Rockies prior
in advance of this next system, details in terms of timing and
strength -- and related evolution of the surface pattern -- will be instrumental with respect to moisture return/destabilization
potential when the system eventually emerges into the Plains. Thus
-- with models quite different in evolving the pattern through the
last half of the medium-range period, no convective assessment will
be attempted.

In the mean time, storms -- and associated severe potential -- which
should be ongoing at the start of Day 4 (Thursday) will spread east
of the Appalachians, possibly affecting areas as far north as
southern Virginia and as far south as northern Florida. Timing
differences exist between the GFS and ECMWF, with the slower ECMWF
more favorable for severe weather given the greater degree for
afternoon heating/destabilization across the Carolinas vicinity
ahead of the front, and associated convective band. In any case, a
moist and favorably sheared environment is expected, which should
prove supportive for appreciable severe potential.

As the front moves offshore overnight, and then across the western
Atlantic through the day Friday (Day 5), a large area of high
pressure will expand to encompass most of the U.S. east of the
Rockies. This high should largely remain in place Day 6 (Saturday),
suggesting a couple of days of quiescent conditions with respect to severe-weather potential. The next increase in risk could occur
with the aforementioned western system, but a more detailed
assessment will be delayed, pending better alignment of model
solutions.

..Goss.. 03/15/2021
$$
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ACUS48 KWNS 160859
SWOD48
SPC AC 160857

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0357 AM CDT Tue Mar 16 2021

Valid 191200Z - 241200Z

...DISCUSSION...
Medium-range models appear to be in reasonably good agreement
through Day 6 (Sunday Mar. 21). During this early half of the
period, models show a cold front having moved offshore by the start
of Day 4 (Friday) with the exception of far south Florida. In the
wake of this front, strong surface high pressure is depicted, which
persists over the eastern half of the country through Day 6.

Beyond Day 6, a trough moving across the interior West will begin to
emerge into the Plains. However, models exhibit at least some
differences with respect to the strength of this feature, and the
associated evolution of the surface pattern. In any case, presence
of ridging across the Gulf of Mexico into Day 6-7 and potentially
beyond, suggests incomplete/modest moisture return into the southern
Plains, and thus limited instability would be expected -- which
would appear to be unsupportive of any substantial severe weather
episodes through the end of the period.

..Goss.. 03/16/2021
$$
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ACUS48 KWNS 170857
SWOD48
SPC AC 170856

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0356 AM CDT Wed Mar 17 2021

Valid 201200Z - 251200Z

...DISCUSSION...
Latest runs of the medium-range models appear to be in reasonable
agreement through Day 6 (Monday Mar. 22), depicting low severe
convective potential through the first half of the period as
ridging/high pressure generally prevails east of the Rockies Days
4-5. By Day 6, the advance of the next southwestern U.S. trough
toward/into the central and southern Plains is indicated.
Intensity/timing differences between the models with respect to this
feature continue, thus correspondingly affecting surface development
details into the southern Plains during the daytime hours. However,
it appears in any case that only modest/incompletely modified
moisture will be available to return northward into the southern
Plains ahead of this feature. As such, severe weather risk appears
low overall, at this time.

On the heels of this feature, another upper system digging southward
across the West on Day 7 may turn eastward toward the Plains Day 8.
However, model differences are too great with respect to their
handling of this system, to warrant any meaningful attempt to
ascertain potential for severe convection across the south-central
U.S. at this time.

..Goss.. 03/17/2021
$$
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ACUS48 KWNS 180836
SWOD48
SPC AC 180834

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0334 AM CDT Thu Mar 18 2021

Valid 211200Z - 261200Z

...DISCUSSION...
Medium-range models are in general agreement on the large scale,
that a western U.S. trough will evolve through the period, with
several smaller-scale perturbations ejecting eastward across the
central and eastern U.S. through the period. However, details
regarding location, progression, and intensity of these
smaller-scale troughs are different within the different model
solutions. This casts considerable uncertainty with respect to the medium-range convective forecast -- uncertainty which increases
steadily with time through Day 8.

Even with the advance of the first of these smaller-scale
disturbances -- expected to dig across California into the Southwest
Day 4/Sunday, and then move into the Plains Day 5 -- differences are substantial, and are reflected notably in evolution of the surface
pattern east of the Rockies. With that said, it continues to appear
that quality of moisture return ahead of this system should remain
insufficient for appreciable severe risk through Day 5.

Beyond Day 5, this initial feature is expected to shift
northeastward and weaken, in response to the next feature digging
across the West. However, degree of spread within the model
solutions from Day 6 onward suggests very minimal predictability,
and thus no convective assessment will be rendered at this time.

..Goss.. 03/18/2021
$$
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ACUS48 KWNS 190855
SWOD48
SPC AC 190854

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0354 AM CDT Fri Mar 19 2021

Valid 221200Z - 271200Z

...DISCUSSION...
Medium-range models appear to be in good agreement into Day 6
(Wednesday), after which differences emerge amongst model solutions.

Day 4 (Monday), a rather vigorous southern-stream short-wave trough
is forecast to shift out of the southern Rockies and into the
southern Plains. This will induce surface cyclogenesis, with the
developing low shifting from the southern High Plains across
Oklahoma/Kansas through the afternoon and evening. However,
returning low-level air from the Gulf of Mexico in response to
increasing southerly low-level flow will be incompletely modified,
owing to presence of surface high pressure -- and associated
cool/subsident airmass -- residing over the Gulf in advance of this
system. As such, minimal CAPE is expected at best, and thus no more
than minimal severe risk would be expected, as a result.

As the upper system and associated surface low turn northward Day 5,
moving toward the upper Mississippi Valley/Upper Great Lakes region
in a bit of a Fujiwara-like manner in advance of the next western
system, a trailing surface cold front should settle just off the
Texas coast, over the far northwestern Gulf of Mexico.

By Day 6, as the next, western system crosses the Southwest and
moves into the southern Plains, surface wave formation will likely
occur along the remnant front. The GFS depicts this cyclogenesis
near or just off the Texas coast, while the ECMWF is much farther
offshore with the surface boundary, and thus with the frontal wave
development. While the more optimistic scenario in terms of onshore
convective potential would suggest possible, lower-end severe risk,
this small-scale uncertainty between the models precludes any areal
risk delineation.

Beyond Day 6, models begin more substantial deviation in solutions,
with the advance of this upper system, and the related evolution of
the associated surface pattern. As such, a decrease in confidence/predictability prevails through the end of the period.

..Goss.. 03/19/2021
$$
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ACUS48 KWNS 200850
SWOD48
SPC AC 200849

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0349 AM CDT Sat Mar 20 2021

Valid 231200Z - 281200Z

...DISCUSSION...
A compact upper low/shortwave trough over the south-central U.S. on
Day 4/Tue will lift north/northeast toward the Upper Midwest/Great
Lakes vicinity as another shortwave trough digs southward across the southwestern U.S./northern Mexico. A surface low will lift
north/northeast in tandem with the upper system, while a cold front
extends south/southwest from the mid-MS Valley into the Edwards
Plateau vicinity in TX. Some strong to severe convection could
develop in the warm sector ahead of the front from the lower MS
Valley into the Upper Texas Coast vicinity, but instability is
forecast to remain modest while stronger ascent will be lifting
northeast of the region. This should limit overall magnitude of
severe potential, precluding probabilities at this time.

Some severe potential could persist near the TX coast eastward along
the central Gulf coast on Day 5/Wed. However, a surface low is
forecast to develop south of the TX coast over the western Gulf.
This could maintain a warm front just offshore from the central Gulf
coast, resulting in mainly elevated convection is a warm advection
regime ahead of the next upper shortwave trough. Severe potential
will largely be dependent on the position of this warm front.
Medium-range guidance varies on the position of this boundary,
resulting in too much uncertainty to include severe probabilities.

The low over the western Gulf should develop northeastward sometime
on Day 6/Thu, as an upper trough over the Plains ejects eastward.
Strong deep-layer southwesterly flow will spread across the
southeastern U.S., while increasing southerly low level flow brings
richer Gulf moisture further inland ahead of an eastward-advancing
cold front. This could bring another bout of severe potential to
much of the South, but the timing of the upper trough, strengthen of
the surface low and quality of the thermodynamic environment remain
uncertain.

By the end of the period, another trough will deepen over the
western U.S. and eject over the southern Plains. While this pattern
would typically favor another round of severe potential, the cold
frontal passage through the southern U.S. around Day 6/Thu could
limit moisture across the Plains into portions of the lower MS
Valley, and forecast confidence at this time frame is low.

..Leitman.. 03/20/2021
$$
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ACUS48 KWNS 210837
SWOD48
SPC AC 210835

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0335 AM CDT Sun Mar 21 2021

Valid 241200Z - 291200Z

...DISCUSSION...
Medium-range guidance is fairly consistent in depicting a
progressive upper-level pattern during the Day 4-8 period. A mean
upper-level trough will persist across much of the CONUS, as several
shortwave troughs embedded in larger-scale flow focus potential for
heavy rainfall and perhaps some severe potential.

Day 5/Thu could see a broader area of severe potential across parts
of Texas into the Deep South. A compact shortwave trough is forecast
to pivot east/northeast across the Lower MS Valley, though the
timing and intensity of this feature still varies considerably
between the GFS and ECMWF. The attendant surface features likely
still show large spread in timing and location. The GFS shows a 1000
mb surface low over southern OH 00z Fri, while the ECMWF has a 993
mb low over northwest AR. This clearly has implications on the
quality of the warm sector and the location/timing of convection.
Nevertheless, trends will be monitored for increasing severe
potential on Day 5/Thu across the Lower MS/TN Valley/Central Gulf
Coast vicinity.

How the surface pattern evolves on Thursday will have big
implications for any continued severe threat into Day 6/Fri across
portions of the Southeast and Mid-Atlantic and confidence is low
during this period. Model spread continues to vary considerably into
the weekend, through some severe potential could persist/redevelop
across the central Gulf coast states, as an upper ridge shifts
eastward across the West and the larger-scale upper trough that
persisted through much of the forecast period finally ejects
eastward. However, uncertainty remains too high to include severe
probabilities at this time.

..Leitman.. 03/21/2021
$$
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ACUS48 KWNS 230846
SWOD48
SPC AC 230845

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0345 AM CDT Tue Mar 23 2021

Valid 261200Z - 311200Z

...DISCUSSION...
Day 4 (Friday) - Deeper forcing for ascent attending the shortwave
trough moving through the Northeast States will become increasingly
removed from the moist warm sector farther south. Though a few
strong storms cannot be ruled out along trailing portions of the
front from the Mid Atlantic into the Carolinas, confidence in a more
robust severe threat is low at this time.

Day 5 (Saturday) - An upper low is forecast to cutoff over the
Southwest States with weak, very low amplitude shortwave ridging
along the Gulf Coast states. A warm front will return inland through
the lower MS Valley during the day accompanied by a moist boundary
layer and destabilization. The most likely scenario is for storms to
develop during the evening or overnight along and north of warm
front as the low-level jet strengthens. Strong effective shear will
exist in this regime and a few severe storms with large hail are
possible. Severe probabilities may be introduced for this scenario
in later updates.

Day 6 (Sunday) - A cold front will surge southeast in response to an
amplifying northern-stream shortwave trough. At least modest
instability is expected in pre-frontal warm sector where storms may
develop through the Southeast States. If models continue to show
consistency with this scenario, severe probabilities may be
introduced on the next update.

..Dial.. 03/23/2021
$$
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ACUS48 KWNS 240904
SWOD48
SPC AC 240902

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0402 AM CDT Wed Mar 24 2021

Valid 271200Z - 011200Z

...DISCUSSION...
Split-flow upper pattern is expected to evolve early in the day 4-8
period with a closed upper low likely to become situated over the
Southwest States day 4, while a more progressive northern-stream
trough moves through the northern Plains, upper MS Valley and Great
Lakes region. Considerable model differences arise with handling of
both the northern and southern streams, contributing to low
predictability with this update.

Day 4 (Saturday) - Thunderstorms will likely be ongoing north of a
warm front from the lower MS Valley into the TN Valley early in the
day and could pose some risk for hail. As a northern-stream trough
moves through the Great Lakes/Ohio Valley, a cold front will
accelerate southeast later Saturday night. While some risk for
severe storms will exist with activity developing along the cold
front from the lower MS Valley into the Southeast States Saturday
evening and overnight, confidence in a more robust threat is low at
this time.

Day 5 (Sunday) - Thunderstorms will likely continue developing along
the cold front through the Southeast States and the lower MS Valley
and may pose some severe threat. However, low-level winds will
probably remain weak, and activity will become increasingly removed
from the stronger winds aloft, lowering confidence in a more robust
threat. Some moisture may also be advected westward through
southwest TX as the trailing portion of the front stalls and begins
to lift north as a warm front. Severe threat in this region will be
dependent on when the closed upper low over the Southwest States
moves east into the Southern High Plains, but models currently
differ considerably regarding this feature.

Days 6 through 7 - A more substantial severe threat might evolve
from Texas (day 6) and farther east into the lower MS and TN Valley
regions (day 7). However, models differ considerably with regard to
the intensity and timing of when the Southwestern U.S.
southern-stream closed low finally begins to accelerate east.

..Dial.. 03/24/2021
$$
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ACUS48 KWNS 250906
SWOD48
SPC AC 250904

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0404 AM CDT Thu Mar 25 2021

Valid 281200Z - 021200Z

...DISCUSSION...
Day 4 (Sunday) - Some threat for severe storms is expected Monday
mainly across a portion of the Carolinas into far southeast VA.
Low-level moisture with low 60s F dewpoints will have returned
through the warm sector contributing to modest instability. Storms
are expected to develop along a cold front associated with a
northern-stream shortwave trough during the afternoon and early
evening. Activity will be embedded within strong vertical shear
supportive of a few organized severe storms.

Other strong storms may continue farther southwest along the front
into a portion of the Southeast States, but confidence in a more
robust severe risk in this region is not particularly high.

Day 5 (Monday) - The cold front is expected to have moved offshore
and the southwest portion of this boundary should stall over the
northern Gulf. ECMWF suggests the front will retreat northward
allowing a small warm sector to move inland across LA and MS.
However, this will depend on the speed of ejection and amplitude of
the upper low that will close off over the Southwestern States. The
ECMWF has trended farther south with this feature the previous few
runs.

Day 6 (Tuesday-Wednesday) - a broad moist warm sector will likely
advect inland through central and eastern TX into the lower MS
Valley and Southeast. This will occur in advance of the next
northern-stream upper trough and its associated cold front. While
severe storms are probable with this scenario, considerable model
differences still exist with respect to the amplitude and speed of
the upper trough, so will maintain low predictability this update.

..Dial.. 03/25/2021
##
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ACUS48 KWNS 260849
SWOD48
SPC AC 260847

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0347 AM CDT Fri Mar 26 2021

Valid 291200Z - 031200Z

...DISCUSSION...
Day 4 (Monday) - Primary model differences arise early in the period
regarding the evolution of the shortwave trough now moving southeast
toward the Great Basin region. Some models maintain phasing with the
northern stream while other solutions maintain a separate southern
stream trough that continues through the Gulf Coast states Monday.
In either case, a dominant northern-stream trough will drive a cold
front into the northern Gulf with limited moisture return inland.

Day 5 (Tuesday) - Richer moisture will begin returning through TX
and into the Gulf Coast states as the front retreats northward in
response to increasing southerly winds ahead of a northern-stream
trough that will amplify across the Plains. Thunderstorms will
likely initiate along the accompanying, southeast-advancing cold
front and continue through east TX into the lower MS and TN Valley
regions. The overall setup appears ana-frontal, and at this time
does not appear conducive for a robust severe threat.

Day 6 (Wednesday) - Thunderstorms will continue developing along the
cold front as it moves through the Southeast States during the day
and into the Carolinas and Middle Atlantic later in the day and into
the evening. However, confidence in a more substantial severe threat
remains low at this time.

Days 7 and 8 appear to have low potential for severe storms as the
front moves offshore with building surface high pressure and stable
conditions inland.

..Dial.. 03/26/2021
$$
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ACUS48 KWNS 270840
SWOD48
SPC AC 270839

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0339 AM CDT Sat Mar 27 2021

Valid 301200Z - 041200Z

...DISCUSSION...
...Tuesday/Day 4 to Wednesday/Day 5...
The models are in fairly good agreement early in the Day 4 to 8
period. An upper-level trough is forecast to move eastward across
the southern and central Rockies on Tuesday and across the Great
Plains on Wednesday. Moisture advection is forecast across parts of
the Southeast ahead of a cold front moving southeastward through the
central U.S. Thunderstorm development will be possible in the
central Gulf Coast states on Tuesday, along a corridor of maximized
low-level moisture. Thunderstorm development will again be possible
in the central Gulf Coast states on Wednesday as the cold front
approaches. Additional storms may form ahead of the front on
Wednesday from Georgia into the Carolinas. An isolated severe threat
may develop in these areas but predictability is too low to warrant
adding any 15 percent contours.

...Thursday/Day 6 to Saturday/Day 8...
From Thursday to Saturday, an upper-level trough is forecast to move
eastward across the eastern U.S. and into the western Atlantic. A
northwest flow pattern is forecast to develop in the wake of the
upper-level trough across much of the eastern half of the nation.
This will help usher a cool and dry airmass into the central and
southern U.S. late in the week, minimizing the convective potential.

..Broyles.. 03/27/2021
$$
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ACUS48 KWNS 280843
SWOD48
SPC AC 280842

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0342 AM CDT Sun Mar 28 2021

Valid 311200Z - 051200Z

...DISCUSSION...
...Wednesday/Day 4 to Friday/Day 6...
The medium-range models begin the Day 4 to 8 period in good
agreement, moving an upper-level trough across the Mississippi
Valley on Wednesday. A cold front is forecast to advance
southeastward across the Southeast, where isolated strong to severe thunderstorms will be possible. Model forecasts keep instability
weak ahead of the front suggesting any threat could be marginal and
localized. The cold front is forecast to move quickly southeastward
across Florida on Thursday as a cool and dry high pressure system
influences the eastern half of the nation. This is expected to
minimize the convective potential for both Thursday and Friday.

...Saturday/Day 7 and Sunday/Day 8...
On Saturday, the models develop an upper-level ridge across the
western U.S. and suggest that moisture could increase across south
and central Texas. Isolated thunderstorm development would be
possible on the northern edge of the moist airmass from the Texas
Hill Country to the middle Texas Coast. Weak instability will
probably minimize any severe threat. On Sunday, model solutions
diverge. Some solutions return moisture northward into the southern
and central Plains while others keep the moist airmass near the
Texas Coast. This large spread makes predictability low concerning
the severe potential on Sunday.

..Broyles.. 03/28/2021
$$
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ACUS48 KWNS 290836
SWOD48
SPC AC 290834

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0334 AM CDT Mon Mar 29 2021

Valid 011200Z - 061200Z

...DISCUSSION...
...Thursday/Day 4 to Saturday/Day 6...
The medium-range models forecast a large area of surface high
pressure to move slowly eastward across the central and eastern U.S.
from Thursday to Saturday. This dry and cool airmass will prevent
moisture return from taking place across the southern U.S.,
minimizing thunderstorm potential across the continental United
States late this week.

...Sunday/Day 7 to Monday/Day 8...
On Sunday, the models suggest that low-level moisture could return
northward into parts of the southern and central Plains. Surface
dewpoints are forecast to increase into the 50s F from Texas
northward to Kansas. This could result in weak destabilization in
parts of the Great Plains during the afternoon on Sunday and on
Monday. Although thunderstorm development could occur each
afternoon, an upper-level ridge in the central U.S. is forecast to
keep convective coverage isolated late in the period. Weak
instability would keep any severe threat marginal.

..Broyles.. 03/29/2021
$$
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ACUS48 KWNS 310901
SWOD48
SPC AC 310859

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0359 AM CDT Wed Mar 31 2021

Valid 031200Z - 081200Z

...DISCUSSION...
Medium-range models continue to show a mostly quiet upper-air
pattern for severe potential in the extended period. A mean
mid-level ridge will reside over the High Plains this weekend
through the early part of next week. Model spread increases towards
the end of the period and the early stages of low-level moisture
return into the Great Plains will concurrently occur---resulting in
a predictability-too-low highlight.

..Smith.. 03/31/2021
$$
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ACUS48 KWNS 010831
SWOD48
SPC AC 010829

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0329 AM CDT Thu Apr 01 2021

Valid 041200Z - 091200Z

...DISCUSSION...
A progressive upper-air pattern is forecast during the extended
period. Initially, a mid-level ridge is forecast over the central
U.S. during the Sunday-Monday period as a mid-level trough moves
into the western U.S. Models begin to diverge considerably by
Tuesday with the subsequent evolution of the western U.S. trough as
it moves eastward into the Great Plains. Thunderstorm chances will
likely begin to increase across parts of the central U.S. by
Tuesday/Wednesday as low-level moisture begins to increase in the southern-central Great Plains and lower MS Valley. Predictability
concerns preclude specific mention of a potential spatial area at
this time until model agreement improves.

..Smith.. 04/01/2021
$$
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ACUS48 KWNS 020829
SWOD48
SPC AC 020827

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0327 AM CDT Fri Apr 02 2021

Valid 051200Z - 101200Z

...DISCUSSION...
Medium-range model guidance is in good agreement in showing a
mid-level low/trough over the Interior West on Monday (day 4). As
the western U.S. disturbance moves eastward into the central U.S.
during the Tuesday-Wednesday timeframe, the risk for thunderstorms
will increase for portions of the central states. Considerable
uncertainty exists for both thunderstorms and any potential severe
risk as this mid-level feature interacts with northward-advecting
Gulf moisture. By the latter part of the extended period, large
model spread is evident.

..Smith.. 04/02/2021
$$
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ACUS48 KWNS 030846
SWOD48
SPC AC 030844

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0344 AM CDT Sat Apr 03 2021

Valid 061200Z - 111200Z

...DISCUSSION...
Medium-range model guidance is converging towards a scenario of a
western U.S. trough evolving into a closed mid-level low over the
central U.S. during the early half of the extended period. Severe
potential will likely focus on Tuesday across KS and possibly OK
near a dryline with storms developing eastward overnight.
Uncertainty remains regarding moisture quality and capping concerns
but hail/wind are the main threats with this activity. Higher
confidence exists for organized severe storms on Wednesday from the
Ozarks eastward to the MS Valley as high momentum flow overspreads a
potential moderately unstable boundary layer centered on Arkansas.
All hazards are possible. A separate bi-modal area of concern could
develop farther north but there is low confidence in this scenario.
Uncertainty increases by Thursday owing in part to model spread and
potential consequential effects of appreciable convective
overturning for areas farther east/southeast on Thursday (day 6).
Model variability increases during the latter part of the extended
period.

..Smith.. 04/03/2021
$$
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ACUS48 KWNS 040837
SWOD48
SPC AC 040836

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0336 AM CDT Sun Apr 04 2021

Valid 071200Z - 121200Z

...DISCUSSION...
Medium-range models continue to show a mid-level low moving slowly
over the Midwest through the end of the work week. Confidence in
severe thunderstorm potential is currently greatest on Wednesday
across parts of the lower MS Valley. Substantial uncertainty exists
regarding the impact of thunderstorms and an effective boundary for
subsequent severe potential on Thursday. The area of potential
severe would possibly reside across the lower MS Valley into parts
of the TN Valley but confidence is low regarding details for this
possible scenario. Models vary considerably by next weekend with
low confidence in the overall forecast upper-air pattern.

..Smith.. 04/04/2021
$$
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ACUS48 KWNS 050823
SWOD48
SPC AC 050822

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0322 AM CDT Mon Apr 05 2021

Valid 081200Z - 131200Z

...DISCUSSION...
Medium-range models agree in showing a mid-level low over the
Midwest on Thursday with this feature possibly becoming absorbed
within a larger-scale evolving trough over the north-central Great
Plains by Friday. Severe thunderstorm activity/potential will
likely focus over parts of the South during the early half of the
extended period. Boundary placement and potential upstream
lower-latitude disturbances traversing across the south-central U.S.
into the Southeast are predictability concerns at this time. Model
spread increases towards the end of the period as the upper-air
pattern becomes less clear.

..Smith.. 04/05/2021
$$
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ACUS48 KWNS 060803
SWOD48
SPC AC 060801

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0301 AM CDT Tue Apr 06 2021

Valid 091200Z - 141200Z

...DISCUSSION...
Medium-range models show multiple disturbances moving eastward
across the MS Valley during the early part of the extended period.
There is substantial uncertainty regarding the evolution of a
forecast surface low east of the MS River during this timeframe. It
appears severe-weather potential will probably focus across parts of
the lower MS Valley into the TN Valley. During the latter part of
the extended period, a mid-level disturbance will move from the
north-central Rockies east into the Great Lakes/MS Valley. The
current trend is for a moist/unstable airmass to be relegated to the
Gulf Coast and displaced from the system early next week.
Therefore, a potential-too-low highlight may be more appropriate
despite increasing model variability.

..Smith.. 04/06/2021
$$
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ACUS48 KWNS 080836
SWOD48
SPC AC 080834

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0334 AM CDT Thu Apr 08 2021

Valid 111200Z - 161200Z

...DISCUSSION...
Medium-range models indicate a mid-level low over the southern Great
Lakes will weaken on Sunday (day 4) as an attendant trough moves
across the Mid-Atlantic states and Carolinas. If appreciable
instability can develop over parts of the Mid-Atlantic states, some
risk for strong thunderstorms could develop. This scenario is
characterized by relatively high uncertainty at this time. The
potential for severe thunderstorms will likely be lowest on Monday
as high pressure over the northern Gulf Coast states limits
destabilization. Models indicate southwesterly mid-level flow will
become established across the southern High Plains downstream of a
potential amplifying trough over the western U.S. Predictability is
too low during the latter half of the extended period.

..Smith.. 04/08/2021
$$
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ACUS48 KWNS 100804
SWOD48
SPC AC 100803

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0303 AM CDT Sat Apr 10 2021

Valid 131200Z - 181200Z

...DISCUSSION...
Severe potential appears limited through at least Day 5/Wed. Nearly
zonal flow will stretch from the central/southern Rockies through
the southern two-thirds of the U.S. during this time. Surface high
pressure over the Plains and Midwest will persist, and Gulf moisture
will remain limited to southern TX, eastward toward the central Gulf
Coast vicinity. Any appreciable instability will remain offshore as north/northeasterly low level trajectories extend into the northern
Gulf.

By Day 6/Thu or Day 7/Fri, some guidance develops a western U.S.
upper low and attendant trough eastward into the Plains while
weakening. However, model spread is quite large regarding the
evolution of the western trough and forecast confidence is low. Some
severe potential could develop over parts of the southern Plains
during this time if the upper trough does indeed shift eastward,
allowing for moist return flow ahead of a developing lee cyclone and
dryline. However, confidence in this scenario is too low to
introduce severe probabilities at this time.

..Leitman.. 04/10/2021
$
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ACUS48 KWNS 110830
SWOD48
SPC AC 110828

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0328 AM CDT Sun Apr 11 2021

Valid 141200Z - 191200Z

...DISCUSSION...
Overall severe potential appears limited during the Day 4-8 period.
Low amplitude westerly deep-layer flow will continue across the
southern tier of the CONUS for much of the period. Surface high
pressure will persist over the Plains and Midwest through Day 5/Thu.
A cold front will push southward across FL and into the northern
Gulf of Mexico and far south TX during this time, resulting in a
dearth of boundary-layer moisture across much of the Plains and
Southeast. On Day 6/Fri, a weak upper shortwave impulse is forecast
to migrate from the southern/central Rockies into the
central/southern Plains. A weak lee low is forecast to develop over
the southern High Plains and modest Gulf moisture will spread
northward across central TX ahead of a dryline. Some isolated severe
potential could develop Friday afternoon/evening along the dryline.
Another cold front is then expected to drop southeastward across the
Plains and southeastern U.S., maintaining northerly low level
trajectories across the Gulf of Mexico through the weekend.

..Leitman.. 04/11/2021
$$
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ACUS48 KWNS 130835
SWOD48
SPC AC 130834

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0334 AM CDT Tue Apr 13 2021

Valid 161200Z - 211200Z

...DISCUSSION...
Overall severe potential appears low during the Day 4-8 period. A
series of shortwave upper troughs will migrate across the CONUS
through early next week. A cold front will dive south/southeast
across the central/southern Plains on Day 4/Friday and some elevated thunderstorms will be possible across parts of TX. The front will
continue to push southward across the southeastern U.S. and into the
Gulf of Mexico into Saturday. Previous frontal intrusions into the
Gulf prior to the Day 4/5 period will result in limited moisture and instability, limiting severe potential across the central Gulf Coast
region on Saturday ahead of the cold front. Surface high pressure
will then persist across much of the CONUS east of the Rockies
through the end of the period, with Gulf moisture remaining
offshore, providing little opportunity for severe convection.

..Leitman.. 04/13/2021
$$
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ACUS48 KWNS 140845
SWOD48
SPC AC 140843

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0343 AM CDT Wed Apr 14 2021

Valid 171200Z - 221200Z

...DISCUSSION...
Severe potential will remain low during the Day 4-8 period. Several
upper shortwave trough migrating through larger-scale flow will
maintain general broad, upper troughing over the CONUS. At the
surface, a cold front will drop southeast across the Gulf of Mexico
on Day 4/Sat. Thunderstorms will be possible across FL on Saturday
and into Day 5/Sun, but heavy rain will be more of a concern than
severe potential given elevated convection and training
precipitation. High pressure will settle behind the cold front and
persist for much of the period east of the Rockies. This will cut
off any Gulf return flow, and a dearth of boundary-layer moisture
will result in stable conditions.

..Leitman.. 04/14/2021
$$
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ACUS48 KWNS 160831
SWOD48
SPC AC 160830

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0330 AM CDT Fri Apr 16 2021

Valid 191200Z - 241200Z

...DISCUSSION...
Rich low-level moisture will likely remain limited to parts of the
FL Peninsula along/south of a front from Day 4/Monday through at
least the middle of next week. Appreciable thunderstorm chances
should also remain confined to this region. But, organized severe
potential appears low through Day 6/Wednesday, as low-level
convergence along the front should remain weak and any storms that
form may remain generally elevated to the north of the front. By
late next week, an upper trough may eject from the western CONUS
across the southern Plains. Increasing low-level moisture may be
realized over part of the southern Plains by late Day 7/Thursday
into Day 8/Friday. It remains unclear how amplified the upper trough
will be, the speed of its eastward progress, and the quality of
low-level moisture return. Although predictability remains low at
this extended time frame, some increase in severe potential across
the south-central CONUS may be realized around Day 8/Friday.

..Gleason.. 04/16/2021
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ACUS48 KWNS 170838
SWOD48
SPC AC 170836

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0336 AM CDT Sat Apr 17 2021

Valid 201200Z - 251200Z

...DISCUSSION...
As large-scale upper troughing amplifies over the central CONUS on
Day 4/Tuesday, a low-amplitude shortwave trough should advance
eastward across FL and into the western Atlantic. A surface front
across the FL Peninsula may advance northward through the day, and
some potential for isolated strong to severe storms may exist
Tuesday along/south of this boundary. Still, the low-amplitude
nature of the upper trough and weak mass response at the surface
suggest any severe threat will probably remain limited.

Medium-range guidance is in reasonably good agreement that further amplification of an upper trough/low should occur as it moves
eastward across the OH Valley and into the Mid-Atlantic/Northeast on
Day 5/Wednesday. Strong low/mid-level flow will be present over
these regions as the upper trough/low passes, and a modest increase
in low-level moisture may occur Wednesday ahead of a surface cold
front. At this point, the potential for enough instability to
develop to support an organized severe threat appears too uncertain
to include any severe probabilities across the Mid-Atlantic.

Farther west, a shortwave trough may advance from the Southwest
across the southern Plains and Southeast from Day 6/Thursday through
Day 8/Saturday. There is fairly large spread in deterministic
guidance regarding the evolution and ejection of this feature. If a
more progressive solution occurs, as suggested by some guidance,
then substantial low-level moisture return may occur from late
Thursday into Friday across parts of the southern Plains. Some
severe risk could materialize across this region Friday as a
dryline/cold front sweep eastward. This possible severe risk may
then continue across the lower MS Valley and portions of the
Southeast on Saturday. Regardless, given the considerable
uncertainty regarding the progression of the shortwave trough, and
its impact on the degree of low-level moisture return, it appears
premature to include any severe probabilities for this scenario at
this time.

..Gleason.. 04/17/2021
$$
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ACUS48 KWNS 180851
SWOD48
SPC AC 180850

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0350 AM CDT Sun Apr 18 2021

Valid 211200Z - 261200Z

...DISCUSSION...
Low/mid-level winds will likely strengthen across the Mid-Atlantic
and Northeast on Day 4/Wednesday as an upper trough moves eastward
across these regions. There is still some uncertainty regarding how
much low-level moisture will be present ahead of a cold front. If
enough boundary-layer instability develops by Wednesday afternoon,
then strong/damaging winds appear possible with any storms that can
develop along or ahead of the quickly moving front. At this point,
confidence in organized severe convection producing a swath of
damaging winds remains too low to introduce 15% severe
probabilities. Still, some severe risk appears possible from roughly
eastern NC to southern NY, and this region may need to be included
in low severe probabilities in a subsequent outlook.

Low-level moisture return should begin to occur on Day 5/Thursday
across parts of the southern Plains as another upper trough develops
eastward across the Southwest and northern Mexico. There are still
substantial differences in medium-range guidance regarding the
evolution of this upper trough as it ejects over the southern Plains
on Day 6/Friday. Depending the on quality and northward extent of
the low-level moisture return, enough instability may be present to
the east of cold front and dryline to support organized severe
storms Friday across part of the southern Plains. If a more
progressive and amplified upper trough develops eastward as
indicated by some deterministic guidance, then a severe risk may
also occur across parts of the Southeast next weekend. Regardless,
far too much uncertainty remains regarding the timing/amplitude of
the upper trough and placement of related surface features to
include any severe probabilities at this extended time frame.

..Gleason.. 04/18/2021
$$
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ACUS48 KWNS 190856
SWOD48
SPC AC 190854

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0354 AM CDT Mon Apr 19 2021

Valid 221200Z - 271200Z

...DISCUSSION...
A shortwave trough should move eastward across the Southwest on Day
4/Thursday. Low-level moisture return appears likely across parts of
the southern Plains into Day 5/Friday, as lee cyclogenesis occurs
across the southern High Plains ahead of the approaching upper
trough. Run-to-run consistency over the past several runs of the
deterministic ECMWF lends confidence that the shortwave trough
should eject eastward over the southern Plains on Friday. Increasing
low-level moisture to the east of a dryline/cold front and steep
mid-level lapse rates should support at least moderate instability
in a corridor across parts of OK and TX by Friday afternoon. Strong
mid-level flow and related deep-layer shear appear more than
sufficient for organized severe storms, including supercells. As
mid-level height falls/ascent overspread the warm sector, robust
convective development should occur, with at least isolated severe
storms potentially spreading eastward towards southeast TX Friday evening/night. Confidence in this scenario occurring has increased
enough to include a 15% severe area for Friday.

Some severe risk may continue across parts of the lower MS Valley
and Southeast late Friday night into this upcoming weekend, but
details regarding the eastward progression of the upper trough and
subsequent convective evolution remain unclear. If a severe threat
does develop, then it may focus along/near parts of the central Gulf
Coast on Day 6/Saturday where low-level moisture return is forecast
to be greatest. Regardless, confidence is not high enough to include
a 15% severe area at this time. Depending on how quickly a surface
cold front moves eastward, a lingering severe risk may persist
across some portion of the Southeast into Day 7/Sunday. Finally,
medium-range guidance suggests that a large-scale upper trough may
develop eastward across the western states and towards the High
Plains early next week. Gradually increasing low-level moisture
return across the Plains may eventually lead to some severe risk,
but predictability is quite low at this extended time frame.

..Gleason.. 04/19/2021
$$
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ACUS48 KWNS 200900
SWOD48
SPC AC 200858

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CDT Tue Apr 20 2021

Valid 231200Z - 281200Z

...DISCUSSION...
Low-level moisture return appears likely across parts of the
southern Plains on Day 4/Friday as lee cyclogenesis occurs across
the southern High Plains ahead of an ejecting upper trough.
Medium-range guidance is now in reasonably good agreement that this
shortwave trough will eject eastward over the southern Plains on
Friday. Increasing low-level moisture to the east of a dryline/cold
front and steep mid-level lapse rates should support at least
moderate instability in a corridor across parts of OK and TX by
Friday afternoon. Strong mid-level flow and related deep-layer shear
appear favorable for organized severe storms, including supercells.
The low-level moisture should be somewhat greater in TX compared to
OK. As mid-level height falls/ascent overspread the warm sector,
robust convective development appears likely, with at least isolated
severe storms posing a threat for all severe hazards potentially
spreading eastward towards east/southeast TX and parts of the lower
MS Valley Friday evening/night in a strong low-level warm advection
regime.

It now appears probable that a surface warm front will lift
northward across parts of the central Gulf Coast vicinity on Day
5/Saturday as the shortwave trough continues eastward across the
Southeast. A strong low-level jet may tend to shift eastward quickly
during the day across this region, with low-level flow veering more
to westerly in its wake. Still, increasing low-level moisture south
of the front, a favorable/strengthening wind profile through mid
levels, and some eastward advection of an EML supporting potentially
moderate instability all suggest a threat for organized severe
storms. A 15% area has been introduced for Saturday from parts of
southern LA/MS into southern AL/GA and the FL Panhandle where the
best potential for surface-based storms exists. At least some severe
threat may continue farther east towards coastal GA/SC late
Saturday, but confidence was not high enough to include these areas
in the 15% delineation.

Deterministic medium-range guidance is in surprisingly good
agreement in depicting another upper trough ejecting from the
western CONUS across the Plains around Day 7/Monday into Day
8/Tuesday. Substantial low-level moisture return may occur ahead of
this feature across a part of the southern Plains. Assuming
sufficient low-level moisture does indeed materialize, then the
presence of steep mid-level lapse rates and favorable timing of the
ejecting upper trough suggest an organized severe risk may develop
on Tuesday across some portion of OK/TX. A 15% severe area may need
to be introduced for next Tuesday as soon as the next outlook update
if run-to-run model consistency continues.

..Gleason.. 04/20/2021
$$
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ACUS48 KWNS 210902
SWOD48
SPC AC 210900

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0400 AM CDT Wed Apr 21 2021

Valid 241200Z - 291200Z

...DISCUSSION...
There is good agreement in medium-range guidance that an upper
trough will continue eastward across the lower MS Valley and
Southeast on Day 4/Saturday. A warm front should lift northward
through the day across parts of central AL/GA and into SC as a
surface low develops eastward across the lower OH Valley to the
central Appalachians. There is some concern that a strong low-level
jet will shift quickly eastwards across these areas, possibly
outrunning the better low-level moisture return. Boundary-layer
winds veered to nearly westerly should occur as the low-level jet
core develops eastward. Still, the increasing low-level moisture,
strong deep-layer shear, and some eastward advection of steepened
mid-level lapse rates should overlap along/south of the warm front.
Any thunderstorms that can form in this regime and remain surface
based will pose a threat for all severe hazards. Additional severe
convection may develop Saturday afternoon along an eastward-moving
cold front. Convection will likely be ongoing Saturday morning given
the strong ascent/isentropic lift associated with the low-level jet,
and these morning thunderstorms may slow the northward progress of
the warm front. Regardless, given latest trends in guidance
suggesting a farther north position of the warm front by Saturday
afternoon, have expanded the 15% severe area to include more of
central AL/GA and southern SC.

The upper trough should continue eastward into the western Atlantic
on Day 5/Sunday, with rich low-level moisture confined to parts of
the FL Peninsula. Given minimal large-scale ascent forecast over
this region, overall severe potential ahead of a cold front appears
too low to include any severe probabilities. Medium-range guidance
remains in fairly good agreement that another large-scale upper
trough will advance eastward from the western CONUS to the Plains in
the Day 6/Monday to Day 7/Tuesday time frame. The past several runs
of the deterministic ECMWF show a very favorable setup for organized
severe thunderstorms capable of producing all severe hazards across
parts of central TX into OK and southern KS, with moderate to strong instability forecast to overlap rich low-level moisture in the
presence of strong deep-layer shear. There has been a consistent
enough signal in guidance suggesting a substantial severe threat may
develop across these areas next Tuesday to introduce 15% severe
probabilities. Greater probabilities may be needed if current model
trends continue. Finally, some severe threat may continue on Day
8/Wednesday across parts of the lower/mid MS Valley as the upper
trough moves eastward. But, there is too much uncertainty regarding
the evolution of convection on Tuesday and the quality of low-level
moisture return with eastward extent to include a 15% severe area
for next Wednesday.

..Gleason.. 04/21/2021
$$
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ACUS48 KWNS 220859
SWOD48
SPC AC 220858

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CDT Thu Apr 22 2021

Valid 251200Z - 301200Z

...DISCUSSION...
An upper trough is forecast to move off the East Coast early on Day
4/Sunday, with rich low-level moisture and boundary-layer
instability confined to parts of the FL Peninsula to the south of a
cold front. Both low and mid-level flow should veer to westerly
behind the upper trough passage. This should limit low-level
convergence along the surface front, and organized severe
thunderstorm potential appears low on Day 4/Sunday. Latest
medium-range guidance remains in remarkably good agreement with the
progression of an upper trough over the western CONUS from Day
4/Sunday into Day 5/Monday. If this upper trough evolves as
forecast, then lee cyclogenesis across the southern/central High
Plains should occur early next week. Rich Gulf moisture should
advect northward across the southern/central Plains to the east of a
sharpening dryline from Day 5/Monday into Day 6/Tuesday.

The run-to-run consistency of the ECMWF continues, with the latest
run indicating the development of moderate to strong instability
east of the dryline by Tuesday afternoon across central TX into
western/central OK and parts of KS. As the upper trough ejects
across the Plains, an enhanced mid-level jet should overspread the
dryline and warm sector. Large-scale ascent associated with the
upper trough and low-level convergence along the dryline appear
sufficient to initiate convection by Tuesday afternoon. If this
occurs, then an organized severe thunderstorm threat will develop,
with all severe hazards possible. The 15% severe area for Tuesday
has been expanded to include most of the length of the dryline where
severe thunderstorms appear possible. Expect adjustments to this
area in the coming days, including the potential for greater severe probabilities, as additional model guidance comes in.

For Day 7/Wednesday and Day 8/Thursday, confidence is not high
enough to introduce 15% severe probabilities. Depending on Tuesday's
convection and the quality of low-level moisture return ahead of the eastward-moving upper trough/low, some severe threat may exist on
Day 7/Wednesday across parts of the lower/mid MS Valley. But,
predictability is low at this extended time frame.

..Gleason.. 04/22/2021
$$
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ACUS48 KWNS 230840
SWOD48
SPC AC 230838

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0338 AM CDT Fri Apr 23 2021

Valid 261200Z - 011200Z

...DISCUSSION...
Robust moisture return is still anticipated early next week across
the southern Plains and Mid/Lower MS Valley as a deep upper trough
progresses across the Southwest and into the southern/central Plains
and Lower/Mid MS Valley. Speed of this eastward progression varies
by model, with the GFS representing the most progressive solution,
the Canadian the least progressive, and the ECMWF between these two.

Even with these differences, confidence remains high that severe
thunderstorms will impact the central and southern Plains on
D5/Tuesday. A lead shortwave trough is expected to eject out of the
southern High Plains into the central Plains, aiding in cyclogenesis
and sharpening of a dryline from far southwest KS
south-southwestward through southwest TX. Vertically veering wind
profiles, steep mid-level lapse rates, and warm/moist surface
conditions continue to suggest the potential for supercells capable
of all severe hazards.

Additional severe threat may materialize on D6/Wednesday, but it
will depend on both the extent/location of thunderstorm activity on
D5/Tuesday as well as how progressive the upper trough is.
Currently, portions of east/southeast TX into the Lower MS Valley
appear most favorable for a severe risk. However, uncertainty is too
high to outlook any areas with this forecast.

Some severe risk also appears possible across the Southeast on
D7/Thursday, but the previously mentioned uncertainties continue to
limit predictability.

..Mosier.. 04/23/2021
$$
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ACUS48 KWNS 240857
SWOD48
SPC AC 240856

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0356 AM CDT Sat Apr 24 2021

Valid 271200Z - 021200Z

...DISCUSSION...
Guidance has continued to trend towards a less progressive, deeper
upper trough over the western CONUS D4/Tuesday before gradually
moving eastward on D5/Wednesday and D6/Thursday. Strong mid-level
flow will extend throughout the base of this upper trough and into
the southern/central Plains on D4/Tuesday. A shortwave trough is
expected to move within this southwesterly flow aloft through the
southern High Plains and into the central Plains, interacting with
the warm and moist air mass returning over the southern/central
Plains. Forecast trajectory of this shortwave (from central NM into
eastern CO/western KS) displaces it from the bulk of the warm
sector, but it should still result in enough ascent to foster storm development, particularly as the dryline retreats across the TX
Panhandle. Strongly sheared and moderately unstable environment
coupled with this modest forcing will likely result in supercells
capable of all severe hazards during the late afternoon. Thereafter,
a strengthening low-level jet and resulting increase in isentropic
ascent will likely foster widespread thunderstorm development and
from southwest TX through western/central OK into south-central KS,
with the potential for upscale growth into an organized convective
line.

Additional severe threat may materialize on D5/Wednesday, but it
will depend largely on the convective evolution the preceding day.
Also, given that the upper trough will likely remain over the Southwest/northern Mexico, line-parallel character of the mid/upper
flow suggest a largely anafrontal, linear convective configuration.
As such, no areas will be outlooked on D5/Wednesday with this
forecast.

From D6/Thursday through D8/Saturday, current progs of a slow-moving
upper low traversing the southern Plains suggest a limited severe
potential. In contrast, materialization of a more progressive system
would suggest increased severe potential across the Southeast and
Mid-Atlantic.

..Mosier.. 04/24/2021
$$
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ACUS48 KWNS 250853
SWOD48
SPC AC 250851

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0351 AM CDT Sun Apr 25 2021

Valid 281200Z - 031200Z

...DISCUSSION...
A positively titled, southern-stream upper trough is forecast to
gradually move eastward across the Southwest on D4/Wednesday and
southern High Plains/southern Plains on D5/Thursday. Some severe
potential may exists across the southern Plains on D4/Wednesday as
this upper trough interacts with a moist antecedent air mass.
However, overall severe potential will depend largely on the
evolution of Tuesday night's thunderstorms and associated surface
boundaries. As such, predictability is limited at this forecast
range.

Guidance suggests the upper trough will gradually slow its forward
progress and trend towards an elongated upper low over northern
Mexico and Far West TX on D5/Thursday and D6/Friday. This evolution
would result in widespread cloudiness and precipitation, limiting destabilization and resultant severe potential.

A shortwave trough is expected to move through the northern stream
from the Upper Midwest to the Lower Great Lakes/Mid OH Valley on
D4/Thursday and through the Northeast/northern Mid-Atlantic on
D5/Friday. Showers and thunderstorms are possible ahead of this
shortwave and its attendant cold front, but modest instability is
currently expected to preclude a widespread severe-thunderstorm
risk.

..Mosier.. 04/25/2021
$$
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ACUS48 KWNS 260900
SWOD48
SPC AC 260858

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CDT Mon Apr 26 2021

Valid 291200Z - 041200Z

...DISCUSSION...
Northern-stream shortwave trough is forecast to move through the
mid/upper MS Valley and Upper Great Lakes on D4/Thursday before
continuing eastward and eventually offshore on D5/Friday. Evolution
of a southern-stream shortwave trough initially phased with the
northern-stream wave is a bit more uncertain, with some guidance
suggesting a quick eastward progression and other guidance favoring
a more displaced, slower eastward progression.

In either case, a cold front attendant to the northern-stream
shortwave will move across the eastern CONUS on D4/Thursday and
D5/Friday, with some potential for the southern portion of the front
to linger over the Southeast until D6/Saturday. Thunderstorms are
possible along this front as it moves eastward, some of which may be
severe. However, overall coverage and timing are uncertain and the
limited predictability precludes delineating any areas.

..Mosier.. 04/26/2021
$$
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ACUS48 KWNS 270846
SWOD48
SPC AC 270845

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0345 AM CDT Tue Apr 27 2021

Valid 301200Z - 051200Z

...DISCUSSION...
A cold front will likely move off the East Coast on D4/Friday while
high pressure moving across the eastern CONUS in its wake on
D5/Saturday.

A southern-stream upper low will likely be centered over the TX Big
Bend early D4/Friday. Little movement of this system is anticipated
D4/Friday before it becomes more progressive (but still relatively
slow) on D5/Saturday and D6/Sunday. Upper flow accompanying this
system is expected to be weak, limiting any potential severe threat.

Guidance is beginning to show the development of more western CONUS
troughing early next week. Robust moisture advection is currently
forecast across the southern/central Plains and MS Valley at the
same time, resulting in a pattern that appears favorable for severe thunderstorms. Predictability is too low to outlook any areas with
this forecast.

..Mosier.. 04/27/2021
$$
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ACUS48 KWNS 280845
SWOD48
SPC AC 280844

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0344 AM CDT Wed Apr 28 2021

Valid 011200Z - 061200Z

...DISCUSSION...
Upper low expected to drift slowly across Mexico on D4/Saturday is
forecast to open up and become more progressive on D5/Sunday, likely
reaching the Lower MS Valley by early D6/Monday. Air mass ahead of
this shortwave will be moist, but poor lapse rates should temper
instability, limiting the overall severe potential.

Another shortwave trough will likely move across the Southwest and
into the southern/central Plains early next week, preceded by strong
moisture advection. Strength of the shortwave and its attendant
surface low as well as overall timing remain uncertain, precluding
the delineation of any areas with this outlook. However, the overall
pattern is favorable for a severe risk across the southern/central
Plains early next week and then eastward into the MS Valley
mid-week.

..Mosier.. 04/28/2021
$$
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ACUS48 KWNS 290901
SWOD48
SPC AC 290900

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0400 AM CDT Thu Apr 29 2021

Valid 021200Z - 071200Z

...DISCUSSION...
A southern-stream shortwave trough is currently forecast to extend
from central OK to off the southeast TX coast early D4/Sunday.
Medium-range guidance has struggled with the evolution of this
system, particularly with how fast it moves eastward/northeastward.
Most recent guidance trends this system towards a negative tilt on
D3/Saturday before it moves northeastward through the Mid-South and
TN Valley on D4/Sunday. This evolution extends strong mid-level flow accompanying the system across a moist air mass over the Lower MS
Valley. Resulting long, vertically veering hodographs support the
potential for severe storms if enough heating/destabilization can be
realized. Despite preceding variability within the guidance, most
recent deterministic and ensemble guidance suggest this scenario is
probable enough to introduce a 15% delineation.

This shortwave is then expected to continue northeastward through
the OH Valley and Northeast on D5/Mon while another shortwave trough
moves through the Four Corners/Southwest and into the southern
Plains. Moist and unstable air mass will precede this shortwave
across the southern Plains, with ensemble guidance suggesting a high probability of at least 65 deg F surface dewpoints east of the
dryline from central OK into central TX. Moderate to strong
mid-level flow atop this low-level moisture will result in an
environment that supports severe thunderstorms.

Some severe potential may persist into the Southeast on D6/Tuesday,
but variability with guidance limits predictability past D5/Monday.

..Mosier.. 04/29/2021
$$
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ACUS48 KWNS 301315
SWOD48
SPC AC 301313

Day 4-8 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0813 AM CDT Fri Apr 30 2021

Valid 031200Z - 081200Z

CORRECTED FOR ADDITION OF LABEL ON DAY 6 GRAPHIC

...DISCUSSION...
Models suggest that a southern branch of split mid-latitude
westerlies may gradually become less prominent next week. It
appears that a couple of perturbations emanating from the northern
mid-latitude Pacific, and at least another pivoting south/southeast
of the Hudson Bay vicinity, may eventually contribute to mid/upper
trough amplification east of the Mississippi Valley by late next
week, as upstream flow in the western U.S. becomes more zonal.

Before this proceeds, it appears that one significant perturbation
may emerge from initially amplified troughing across the Four
Corners states early next week, and contribute to an organized
severe weather risk across parts of the southern Great Plains into
Ozark Plateau on Monday. This now seems most likely to become
focused across central/eastern Oklahoma into southwestern
Missouri/northwestern Arkansas, where forcing for ascent ahead of a
developing surface low may contribute to the evolution of an
organized convective system in the presence of moderate to strong
instability by Monday evening.

Thereafter, model spread and the uncertain influence of prior
convection result in more uncertainty, but there appears at least
some potential that an organized severe weather threat could develop
eastward with the mid/upper impulse, across the Mid South and
Tennessee Valley into the Mid Atlantic region Tuesday into
Wednesday.

..Kerr.. 04/30/2021
$$
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ACUS48 KWNS 010900
SWOD48
SPC AC 010858

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CDT Sat May 01 2021

Valid 041200Z - 091200Z

...DISCUSSION...
There may be some consolidation of remnant mid-level troughing
emerging from the Southwest with troughing digging south of the
Canadian/U.S. border, east of the Mississippi Valley, to the south
of a developing broad low centered near James Bay by mid week. This
may be accompanied by further strengthening of southwesterly
deep-layer mean wind fields to seasonably strong levels, across
parts of the Mid South/lower Mississippi Valley through the
Allegheny and Cumberland Plateaus on Tuesday. Depending on how
convection evolves Monday through Monday night, there may be a
lingering corridor of moderate to strong potential instability,
associated with seasonably moist low-level air beneath a remnant
plume of elevated mixed-layer air, to support organized severe
thunderstorm development. This could shift eastward and
southeastward into parts of the Mid Atlantic and Southeast on
Wednesday, but probably with diminishing instability, and low
predictability of synoptic and sub-synoptic developments maintaining
severe probabilities at less than 15 percent.

Thereafter, it still appears that the evolution of amplified
mid-level troughing east of the Mississippi through the Atlantic
Seaboard will suppress, if not preclude, severe thunderstorm
potential through at least the remainder of the work week and,
perhaps, into next weekend.

..Kerr.. 05/01/2021
$$
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ACUS48 KWNS 020857
SWOD48
SPC AC 020856

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0356 AM CDT Sun May 02 2021

Valid 051200Z - 101200Z

...DISCUSSION...
Although it appears that convective instability and CAPE will be
considerably less than preceding days, there may be some lingering
severe weather potential ahead of the eastward advancing cold front,
across parts of the mid and south Atlantic Seaboard on Wednesday.
Thereafter, at least one significant short wave impulse digging to
the east of initially amplified mid-level ridging across the Pacific
Northwest, and another digging around the western and southern
periphery of a mid-level low initially near/south of Hudson Bay, may
contribute to further amplification of large-scale troughing east of
the Mississippi Valley. It appears that this will contribute to
substantive low/mid-level drying and stabilization as far south as
much of the northern and eastern Gulf of Mexico, and the Florida
Peninsula, late this coming work week into next weekend.

By next weekend, it does appear that large-scale mid-level troughing
will develop inland of the Pacific coast through much of the
Intermountain West and Rockies, accompanied by the development of
surface troughing and low-level moisture return to the lee of the
southern Rockies. While the magnitude of this moisture return may
be initially limited, lower/mid tropospheric lapse rates likely will
become steep. This environment could become conducive to at least
some risk for severe storms, from the lee of the Colorado Front
range into adjacent portions of the central and southern Great
Plains.

..Kerr.. 05/02/2021
$$
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ACUS48 KWNS 030846
SWOD48
SPC AC 030844

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0344 AM CDT Mon May 03 2021

Valid 061200Z - 111200Z

...DISCUSSION...
Models continue to indicate that there will be pronounced lower/mid tropospheric drying as far south as the northern/eastern Gulf of
Mexico and Florida Peninsula late this work week, in the wake of
consolidating digging short waves digging through the base of
evolving larger-scale troughing east of the Mississippi Valley.
This will contribute to a period of diminished convective potential
into next weekend, before the large-scale pattern undergoes
transition.

In the mean, ensemble output indicates that large-scale troughing
will develop inland of the Pacific coast, through the Southwest, and
persist into at least early next week. Corresponding downstream
ridging likely will develop across eastern portions of the central
and southern Great Plains into the Mid Atlantic region, to the north
of an increasing subtropical high centered over the southern Gulf of
Mexico.

Increasing low-level moisture return beneath steepening mid-level
lapse rates probably will contribute to increasing convective
potential. This is expected to become focused along a developing
dryline across western Texas and Oklahoma, and a quasi-stationary
frontal zone near the northern periphery of warmer and more strongly
capping mid-level air associated with the subtropical ridge, from
the central Great Plains into the Ohio Valley. However, the extent
of any developing severe weather risk and location on any given day
remains unclear.

..Kerr.. 05/03/2021
$$
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ACUS48 KWNS 040854
SWOD48
SPC AC 040852

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0352 AM CDT Tue May 04 2021

Valid 071200Z - 121200Z

...DISCUSSION...
Models indicate that blocking mid-level ridging may linger, but it
may redevelop east of the Labrador Sea into the southern Greenland
vicinity during this period. Coincidingly, a deep mid-level closed
low appears likely to redevelop eastward, from near James Bay into
areas near/north of the Canadian Maritmes, with initially amplified
large-scale troughing to its south de-amplifying and while shifting northeastward out of the eastern U.S. As this occurs, a belt of
westerlies emanating from the northern mid-latitude Pacific may
become more prominent across and east of the Rockies.

Latest medium-range guidance now appears much lower in amplitude
with mid-level troughing developing across the Intermountain West
this weekend, before progressing east of the Rockies, as well as
with ridging building north of a subtropical high center developing
near the Yucatan peninsula. However, in association with the
progression of the troughing, there remains at least some potential
for severe thunderstorm development this coming weekend into early
next week across parts of the central and southern Great Plains into
the lower Mississippi Valley and Gulf Coast states.

This may be focused near the intersection of a developing frontal
cyclone and dryline across the central Great Plains on Saturday,
when lapse rates will be steep, but CAPE may still be a bit modest. Boundary-layer moisture return off the Gulf may become supportive of
more substantive CAPE on Sunday, when the focus for strongest
convection begins to become a bit more unclear. It is possible that
this may become confined to portions of central Texas into the Red
River vicinity, before spreading into portions of the western and
central Gulf states on Monday, ahead of the southward advancing
front. Due to sizable uncertainties, severe weather probabilities
are maintained at below 15 percent.

..Kerr.. 05/04/2021
$$
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ACUS48 KWNS 050858
SWOD48
SPC AC 050857

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0357 AM CDT Wed May 05 2021

Valid 081200Z - 131200Z

...DISCUSSION...
The medium-range models continue to indicate that a blocking
mid-level high may shift/redevelop east of the Labrador Sea, across
and perhaps just east of southern Greenland through this period.
Upstream, developments remain more unclear, but a mid-level high
developing this weekend across the Canadian Prairies, within the
northern branch of a splitting flow, may be relatively short-lived.
It appears that mid-level troughing developing within the southern
branch, inland of the Pacific coast through the southern Rockies,
probably will progress east of the Rockies as a series of emerging smaller-scale perturbations this weekend into early next week.

In association with the first emerging short wave impulse, a
developing cyclone, frontal zone and dryline across the central
Great Plains may become a focus for organized severe storm
development on Saturday. Beneath steep mid-level lapse rates, it
appears that low-level moisture return from the Gulf of Mexico may
become sufficient to contribute to moderate CAPE by late afternoon.
In the presence of strong deep-layer shear, a few supercells are
possible along the dryline and near a warm front across parts of
central Kansas, before convection grows upscale into an organizing
cluster Saturday evening. This may be accompanied by the risk for
large hail and a couple of tornadoes initially, before strong wind
gusts become the more prominent hazard.

It appears that surface frontal wave development will weaken and
remain weak, while quickly migrating eastward across the Mid South
into the Mid Atlantic by early next week, with a southward
advancement of the trailing surface front into the Florida
peninsula, northern Gulf of Mexico and lower Rio Grande Valley. As
it does, continuing severe weather potential becomes more unclear,
but it appears that it may remain relatively limited.

..Kerr.. 05/05/2021
$$
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ACUS48 KWNS 070841
SWOD48
SPC AC 070840

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0340 AM CDT Fri May 07 2021

Valid 101200Z - 151200Z

...DISCUSSION...
Generally zonal flow is forecast across the southern Plains and
Southeast on Day 4/Monday where greater low-level moisture should
reside. Steep mid-level lapse rates should be present across parts
of the southern Plains, and moderate to potentially strong
instability may exist along/near a front. However, the lack of an
obvious large-scale forcing mechanism aloft lends uncertainty
regarding storm coverage and placement on Monday. Somewhat better
severe potential may exist on Day 5/Tuesday across parts of TX as a
shortwave trough ejects eastward across the southern/central Plains.
Still, there does not appear to be a robust low-level mass response
ahead of this feature, and there is some uncertainty with the
evolution and placement of the upper trough.

This upper trough should move eastward from the Plains to the MS
Valley and OH Valley from Day 6/Wednesday into Day 7/Thursday. Rich
low-level moisture may remain confined to coastal portions of TX and
the Southeast in this time frame, with a fairly muted threat for
organized severe storms. Large differences in medium-range guidance
regarding the synoptic-scale pattern across the CONUS by late next
week suggest limited predictability.

..Gleason.. 05/07/2021
$$
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ACUS48 KWNS 090840
SWOD48
SPC AC 090838

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0338 AM CDT Sun May 09 2021

Valid 121200Z - 171200Z

...DISCUSSION...
Organized severe potential appears low across the CONUS on Day
4/Wednesday and Day 5/Thursday, as rich low-level moisture will be
located mostly south of the Gulf Coast. By Day 6/Friday, partially
modified low-level moisture may begin to return northward over parts
of the southern/central Plains. Some instability could develop
across these regions Friday into next weekend, but upper ridging may
exist across the Rockies. Rather weak mid-level flow downstream from
this upper ridge should tend to keep any severe threat fairly muted.
Still, some severe threat may eventually develop across a portion of
the Plains by next weekend, but predictability remains low.

..Gleason.. 05/09/2021
$$
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ACUS48 KWNS 110838
SWOD48
SPC AC 110836

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0336 AM CDT Tue May 11 2021

Valid 141200Z - 191200Z

...DISCUSSION...
Medium-range guidance is starting to come into somewhat better
agreement with the evolution of an upper trough that should move
across the western CONUS from Day 4/Friday through the upcoming
weekend. The return of at least partially modified Gulf moisture is
forecast to occur across parts of the southern/central Plains in the
same time frame. A relatively focused area of isolated severe
potential may develop across portions of the central High Plains on
Friday with a southerly low-level jet. However, mid-level flow is
currently forecast to remain somewhat weak, which casts uncertainty
on the overall magnitude of the severe threat. There also appears to
be some severe potential on Day 5/Saturday and Day 6/Sunday across
portions of the southern/central Plains as low-level moisture
gradually increases. However, mid-level flow may remain fairly weak
as the upper trough moves only slowly eastward over the Great Basin
and Southwest. By Day 7/Monday, considerable differences emerge in deterministic guidance regarding the possible development of a
northern-stream upper trough across the northern Plains and Upper
Midwest. If this occurs, it would impact the ejection of the
southern-steam upper trough over the southern/central Plains.
Predictability of the synoptic-scale pattern clearly remains low at
this extended time frame, but an isolated severe threat may persist
across some part of the southern Plains early next week.

..Gleason.. 05/11/2021
$$
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ACUS48 KWNS 120836
SWOD48
SPC AC 120834

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0334 AM CDT Wed May 12 2021

Valid 151200Z - 201200Z

...DISCUSSION...
A closed upper low should move slowly eastward across CA, the Great
Basin, and Southwest this upcoming weekend. This feature is forecast
to eventually eject east-northeastward across the southern/central
Plains early next week, but there are some timing differences in
medium-range guidance regarding when this may occur. Low-level
moisture should gradually increase across the southern/central
Plains through the weekend and into early next week. Still, the
severe threat across these areas will probably remain rather limited
owing to weak mid-level flow and related deep-layer shear on both
Day 4/Saturday and Day 5/Sunday. Depending on when the upper low
evolves into an open wave and ejects across the southern/central
Plains, some increase in the overall severe threat may occur around
Day 6/Monday and/or Day 7/Tuesday as the mid-level flow strengthens. Regardless, there is too much uncertainty regarding the forecast
evolution of the upper trough/low early next week to include 15%
severe probabilities.

..Gleason.. 05/12/2021
$$
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ACUS48 KWNS 130900
SWOD48
SPC AC 130858

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CDT Thu May 13 2021

Valid 161200Z - 211200Z

...DISCUSSION...
Medium-range guidance remains in fairly good agreement that a closed
upper low will move slowly eastward from CA across the Southwest
from Day 4/Sunday into Day 5/Monday. Any appreciable severe threat
on these days will probably remain confined to parts of the
southern/central Plains where sufficient low-level moisture to
support surface-based storms should reside. But, the fairly weak
mid-level flow and related deep-layer shear forecast across this
region may tend to limit the overall severe threat to some extent on
both Sunday and Monday.

The stronger mid-level flow associated with the upper low will
probably begin to overspread the southern High Plains into TX/OK
from late Monday into Day 6/Tuesday as the upper low continues to
slowly eject eastward. Latest guidance indicates that perhaps a
somewhat greater threat for organized severe storms may occur on
Tuesday across parts of TX, as stronger deep-layer shear overlaps
with greater boundary-layer instability and increasing potential for surface-based storms. If current model trends continue regarding the
timing and evolution of the upper low, then a 15% severe area may
need to be included for some parts of TX in a later outlook update.
By Day 7/Wednesday and continuing through the end of the forecast
period, predictability in the synoptic-scale pattern across the
CONUS quickly decreases.

..Gleason.. 05/13/2021
$$
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ACUS48 KWNS 150847
SWOD48
SPC AC 150846

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0346 AM CDT Sat May 15 2021

Valid 181200Z - 231200Z

...DISCUSSION...
...Tuesday/Day 4 to Thursday/Day 6...
The medium-range models are in good agreement on Tuesday, moving an
upper-level trough into the southern High Plains. A moist airmass is
forecast across much of Texas and Oklahoma, where moderate
instability should develop by afternoon. The models continue to
forecast convective potential from central Texas into southern
Oklahoma but are slightly west of yesterday's model runs. It appears
that deep-layer shear will be strong enough for severe storms.
However, the speed of the upper-level trough will determine exactly
how far west the severe threat ends up being. A slower speed would
put the severe threat further west. Due to this uncertainty, will
not introduce a 15 percent contour at this time.

For Wednesday, most of the models have the upper-level trough in the
southern Plains, but model spread increases concerning the position
of the trough. The models do show a corridor of moderate instability
across the southern Plains. Thunderstorms that develop along this
corridor could develop a severe threat. However, deep-layer shear is
forecast to be on the low-side for supercells. And subsidence in the
wake of the upper-level trough may suppress convective development.
For this reason, any severe threat for Wednesday should be isolated.

On Thursday, the models have south-southwesterly mid-level flow in
place across the Great Plains. Considerable spread exists among the
models concerning instability. Using an ensemble approach, pockets
of moderate instability would be expected to develop Thursday
afternoon in parts of the southern and central Plains. Convective
development would be possible along the western edge of this area
from west Texas northward into southwest Kansas. Deep-layer shear
would support a severe threat with this activity. However, spread
among the models scenarios is too great to consider adding a 15
percent contour at this time.

...Friday/Day 7 to Saturday/Day 8...
On Friday and Saturday, the models continue to develop pockets of
moderate instability across the Great Plains. The stronger mid-level
flow is expected to remain over the Rockies, just to the east of an
upper-level trough in the Intermountain West. Due to the relatively
weak mid-level flow across the Great Plains, convection that
develops on Friday afternoon and Saturday afternoon may struggle to
become organized.

..Broyles.. 05/15/2021
$$
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ACUS48 KWNS 160822
SWOD48
SPC AC 160820

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0320 AM CDT Sun May 16 2021

Valid 191200Z - 241200Z

...DISCUSSION...
As a weakening upper low/trough ejects over the Plains/Midwest on
D4/Wednesday, another upper trough is forecast to move into the
western CONUS. This trough will likely only move slowly eastward
with time as a blocking ridge becomes established to the east.
Extended range guidance is in reasonably good agreement with the
large-scale pattern evolution through D6/Friday, before model spread
increases notably into next weekend.

For D4/Wednesday - D5/Thursday, some severe threat may evolve across
the eastern Plains into the upper Midwest with the ejecting upper
trough, though details regarding any such threat remain quite
uncertain at this time. Further west, severe thunderstorm potential
may begin to increase across the much of the High Plains from
D5/Thursday into the weekend, as the influence of the longwave
trough over the West spreads slowly eastward. The magnitude of this
threat will depend on the extent to which stronger mid/upper-level
flow associated with the western trough can overlap increasing
low-level moisture that will be returning to much of the Plains.
Predictability regarding such details remains low at this time, but probabilities may eventually be needed across some portion of the
Plains for this scenario.

..Dean.. 05/16/2021
$$
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ACUS48 KWNS 170846
SWOD48
SPC AC 170844

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0344 AM CDT Mon May 17 2021

Valid 201200Z - 251200Z

...DISCUSSION...
...Thursday/Day 4 to Saturday/Day 6....
The pattern from Thursday to Saturday is not expected to change
much. An upper-level trough is forecast to be located in the western
U.S. with an upper-level ridge in the Mississippi Valley. A
south-to-north corridor of low-level moisture should be located in
the Great Plains, where pockets of moderate instability may develop
each day. Thunderstorm development will be possible along
pre-existing boundaries or near thermal axes that develop each
afternoon. Although isolated severe thunderstorm development will be
possible across parts of the Great Plains, mid-level flow is
forecast to be relatively weak due to the large high over the
central and eastern U.S. For this reason, predictability is low from
Thursday to Saturday.

...Sunday/Day 7 and Monday/Day 8...
The upper-level ridge in the Mississippi Valley is forecast to
remain in place on Sunday and Monday as the upper-level trough moves northeastward into the Rockies. Some models suggest that the
combination of instability and deep-layer shear will be strong
enough for organized thunderstorms in the central Plains on Sunday.
This favorable combination of shear and instability is forecast to
shift northward into the northern Plains on Monday where isolated
severe thunderstorms would again be possible. However,
predictability remains low for Sunday and Monday, mainly due to the
extended forecast range.

..Broyles.. 05/17/2021
$$
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ACUS48 KWNS 180856
SWOD48
SPC AC 180855

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0355 AM CDT Tue May 18 2021

Valid 211200Z - 261200Z

...DISCUSSION...
...Friday/Day 4 to Sunday/Day 6...
An upper-level ridge is forecast to be in place across the
Mississippi Valley this upcoming weekend with an upper-level trough
moving northeastward from southern California on Friday to the
northern Rockies on Sunday. South-southwesterly mid-level flow is
forecast across the central and northern Plains. The pattern will
favor the development of pockets of moderate instability in the
central and northern High Plains each day. Convection is expected to
develop each afternoon along the eastern edge of the higher terrain
in the central Rockies, with storms moving northeastward into the
lower elevations. Instability will be greater eastward across the
High Plains where the amount of deep-layer shear should support a
severe threat. Supercells with large hail and damaging wind gusts
will be possible. The greatest risk of severe is forecast from
eastern Colorado northward into western South Dakota. A threat area
will not be added at this time, mainly due to uncertainties
concerning instability and shortwave timing.

...Monday/Day 7 and Tuesday/Day 8...
The medium-range models gradually move the upper-level ridge
eastward from the Mississippi Valley this weekend into the
Appalachians by early in the week. An upper-level trough is forecast
to move northeastward into the northern Plains, with a corridor of
low-level moisture located from the central Plains northeastward
into the western Great Lakes. This pattern would move a cold front southeastward across the central and northern Plains from Monday
into Tuesday, with areas of moderate instability developing along
and ahead of the front each afternoon. Model forecasts suggest that
enough deep-layer shear will exist for a severe threat with storms
that organize along the front. The main uncertainty early in the
week will be the amount of instability that develops each day and
the timing of the upper-level trough coming out of the Rockies.

..Broyles.. 05/18/2021
$$
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ACUS48 KWNS 190743
SWOD48
SPC AC 190741

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0241 AM CDT Wed May 19 2021

Valid 221200Z - 271200Z

...DISCUSSION...
...Saturday/Day 4 and Sunday/Day 5...
An upper-level ridge is forecast to retrograde westward into the
Mississippi Valley on Saturday as an upper-level low remains over
the Intermountain West. The upper-level ridge is forecast to remain
in place on Sunday as the upper-level low moves northeastward across
the western U.S. Isolated thunderstorm development will be possible
across the central and northern High Plains on Saturday, and an
isolated severe threat would be possible. Thunderstorm coverage is
forecast to increase markedly on Sunday as the upper-level system
ejects northeastward toward the High Plains. Moderate instability,
strong deep-layer shear and steep mid-level lapse rates should be
favorable for severe storms Sunday afternoon and evening across
parts of the central and northern High Plains. A threat area may
need to be added if future model runs continue to eject the
upper-level system out of the western states on Sunday.

....Monday/Day 6 to Wednesday/Day 8...
From Monday to Wednesday, the medium-range models move the
upper-level ridge eastward across the eastern U.S. as a
low-amplitude upper-level trough moves across the northern U.S. A
cold front is forecast to advance southeastward from the northern
Plains into the mid Mississippi Valley. Thunderstorm development
will be possible along parts of the front each afternoon and evening
from Monday to Wednesday. The day with the greatest severe threat
would be Tuesday, when a corridor of moderate instability is
forecast from northeast Kansas into Iowa. This combined with
moderate deep-layer shear would be favorable for severe storms
Tuesday afternoon and evening. A threat area could be considered if
model runs increase convective coverage for Tuesday and show
consistency concerning the frontal position over the next couple of
days.

..Broyles.. 05/19/2021
$$
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ACUS48 KWNS 200829
SWOD48
SPC AC 200827

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0327 AM CDT Thu May 20 2021

Valid 231200Z - 281200Z

...DISCUSSION...
...Sunday/Day 4 to Tuesday/Day 6...
The medium-range models are in fairly good agreement for Sunday. A negatively-tilted upper-level trough is forecast to eject
northeastward into the central Rockies. Ahead of the trough, a
corridor of instability is forecast to develop by afternoon in the
central and northern Plains. In addition, deep-layer shear will
become strong across this area as a 60 to 80 kt mid-level jet moves
in from the south-southwest. Thunderstorms that develop in the
higher terrain will likely move northeastward into the lower
elevations during the late afternoon. The instability combined with
the strong deep-layer shear should support an isolated severe
threat. Hail and marginally severe wind gusts will be the primary
threats. At this time, instability is forecast to remain weak across
the central and northern High Plains on Sunday, which is expected to
keep any threat marginal.

On Monday and Tuesday, an upper-level trough is forecast to move
eastward into the central and northern Plains. An associated cold
front is forecast to advance southeastward from the northern Plains
into the Upper Midwest and mid Missouri Valley by Tuesday afternoon.
This boundary should be a focus for thunderstorm development each
afternoon. Moderate deep-layer shear will make an isolated severe
threat possible, with the magnitude of the threat dependent upon how
much instability materializes.

...Wednesday/Day 7 and Thursday/Day 8...
On Wednesday and Thursday, an upper-level trough is forecast to move
eastward from the Mississippi Valley to the Eastern Seaboard.
Thunderstorm development may take place ahead of the trough each
afternoon as surface temperatures warm and instability become
maximized. The greatest chance for severe storms on Wednesday would
be in the Ohio Valley with that area shifting eastward into the
Mid-Atlantic on Thursday. Although an isolated severe threat could
develop ahead of the trough each day, instability is forecast to
remain relatively weak. This along with differences in the model
solutions suggest that predictability is low at this extended range
in the forecast period. At this time, the severe threat is expected
to remain marginal both Wednesday and Thursday.

..Broyles.. 05/20/2021
$$
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ACUS48 KWNS 220901
SWOD48
SPC AC 220859

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0359 AM CDT Sat May 22 2021

Valid 251200Z - 301200Z

...DISCUSSION...
Medium-range model guidance indicates a series of mid-level
disturbances will move from the central U.S. into the Great
Lakes/Northeast during the upcoming work week. It seems plausible
that strong to severe storms will focus in mesoscale corridors given
the overall pattern (i.e., void of a large, eastward-advecting
elevated mixed layer, lack of a rich moisture reservoir in the low
levels) being unfavorable for large-scale severe outbreaks.
Nonetheless, strong to severe storms will likely associate with the aforementioned disturbances but some timing/placement issues at this
time preclude potential severe highlights. Towards the end of the
extended period, uncertainty increases due to model variability.

..Smith.. 05/22/2021
$$
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ACUS48 KWNS 230847
SWOD48
SPC AC 230845

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0345 AM CDT Sun May 23 2021

Valid 261200Z - 311200Z

...DISCUSSION...
Medium-range model guidance is in better agreement in showing a
mid-level disturbance moving eastward across the central U.S. into
the OH Valley during the Wednesday-Friday (Day 4-6) period. A risk
for strong to severe thunderstorms will probably accompany this
disturbance but uncertainty remains too high regarding placement of
potential severe probabilities. Model spread increases considerably
by next weekend with predictability concerns apparent, despite some
signal for a progressive mid-level trough moving across the West and
towards the High Plains.

..Smith.. 05/23/2021
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ACUS48 KWNS 240841
SWOD48
SPC AC 240839

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0339 AM CDT Mon May 24 2021

Valid 271200Z - 011200Z

...DISCUSSION...
Medium-range model guidance is coming into better agreement in the
depiction of a mid-level shortwave trough over the north-central
U.S. on Thursday (day 4). Additionally, it appears that a
lower-latitude belt of stronger flow will overspread the TX
Panhandle and OK where a moist/unstable sector will reside. There
is uncertainty about the eastern extent of a moist/unstable airmass
across the mid MS Valley and the probable weakening of an MCS
Thursday morning. Nonetheless, it seems prudent to introduce a
15-percent severe risk for Thursday for much of OK northeast into
southeast KS and into MO. Uncertainty increases by Friday (day 5)
owing to convective influences in the prior days across the central
U.S. The upper-air pattern begins to appear less favorable for
organized severe storms by this weekend but model variability is
relatively high at this point.

..Smith.. 05/24/2021
$$
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ACUS48 KWNS 250853
SWOD48
SPC AC 250852

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0352 AM CDT Tue May 25 2021

Valid 281200Z - 021200Z

...DISCUSSION...
Recent medium-range model guidance is beginning to converge towards
a scenario with modest west-northwest flow aloft, coupled with
reservoir of strong instability south of a front/outflow across the
southern High Plains on Friday (day 4). Convective evolution prior
to the extended period casts uncertainty and precludes the
introduction of severe probabilities at this time. Farther east, a
mid-level disturbance is forecast to move from the Midwest to the
Mid-Atlantic states on Friday-Saturday. Severe potential may be
lowest on Saturday before a series of disturbances potentially move
from the West into the High Plains during the latter part of the
extended period.

..Smith.. 05/25/2021
$$
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ACUS48 KWNS 260739
SWOD48
SPC AC 260737

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0237 AM CDT Wed May 26 2021

Valid 291200Z - 031200Z

...DISCUSSION...
Medium-range models show a series of disturbances moving across the
central and eastern CONUS during the extended period. Severe
thunderstorm activity will probably be relegated to parts of the
central and southern High Plains during the early part of the
extended period. Increasing low-level moisture across the southern
Great Plains early next week is progged by model guidance. This may
result in potential severe as minor disturbances move east from the
Desert Southwest towards the southern High Plains.

..Smith.. 05/26/2021
$$
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ACUS48 KWNS 270802
SWOD48
SPC AC 270800

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0300 AM CDT Thu May 27 2021

Valid 301200Z - 041200Z

...DISCUSSION...
Medium-range models continue to show an upper-air pattern largely
void of widespread severe weather potential through the extended
period. However, it appears severe thunderstorm activity is most
likely on an isolated to widely scattered basis for parts of the
central and southern High Plains on Sunday (day 4). A mean
mid-level trough over the Desert Southwest will gradually shift
eastward into the southern High Plains during the early part of the
work week. An associated uptick in strong to severe storm potential
may focus over the southern High Plains during this timeframe. By
mid week next week, overall high-end severe potential appears very
limited but a risk for strong to severe storms could materialize
across parts of the central U.S. while a mid-level ridge amplifies
over the West Coast.

..Smith.. 05/27/2021
$$
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ACUS48 KWNS 280858
SWOD48
SPC AC 280856

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0356 AM CDT Fri May 28 2021

Valid 311200Z - 051200Z

...DISCUSSION...
The extended period is characterized by low potential for widespread
severe but organized severe thunderstorms will probably occur for
much of the period. Medium-range models show a weak mid-level
trough over the Desert Southwest moving into the south-central U.S.
during the early part of the extended period. By Tuesday into
Wednesday, models are indicating mid-level troughing from a northern
stream disturbance over the Great Plains. Severe thunderstorm
potential will probably begin to shift from the south-central states
into the mid to lower MS Valley towards the end of the period as a
large ridge builds over the West.

..Smith.. 05/28/2021
$$
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ACUS48 KWNS 310817
SWOD48
SPC AC 310815

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0315 AM CDT Mon May 31 2021

Valid 031200Z - 081200Z

...DISCUSSION...
Severe thunderstorm potential will probably concentrate at least on
an isolated basis across parts of the eastern U.S. on Thursday and
Friday (days 4-5) as a mid-level trough lifts from the TN Valley
northeastward during the timeframe. By next weekend, models show a
strong speed max moving into the northwestern CONUS. Severe
activity may increase across the north-central U.S. with any
ejecting mid-level disturbance during the latter part of the
extended period. Details concerning this evolution are nebulous and
preclude a spatial highlight until greater confidence can be
attained.

..Smith.. 05/31/2021
$$
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ACUS48 KWNS 010901
SWOD48
SPC AC 010859

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0359 AM CDT Tue Jun 01 2021

Valid 041200Z - 091200Z

...DISCUSSION...
In the wake of an upper trough that will exit the eastern U.S.
seaboard Thursday, model consensus is that weak winds aloft will
prevail over most of the nation through day 8 with a blocking
pattern becoming established by day 7. An exception will be from the
Pacific Northwest into the northern Plains when a belt of stronger
winds aloft associated with a low-amplitude trough will spread
across this region. While some severe threat might evolve across a
portion of the Northern Plains day 5 into day 6, uncertainty exists
regarding timing of shortwave troughs and degree of moisture return
beneath the belt of stronger westerlies. Beyond day 6 consensus is
that a synoptic upper trough will amplify across the western states
with an upper ridge in the east. Generally weak flow will prevail
east of the Rockies where richer low-level moisture will reside,
resulting in limited prospects for organized severe storms.

..Dial.. 06/01/2021
$$
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ACUS48 KWNS 020839
SWOD48
SPC AC 020838

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0338 AM CDT Wed Jun 02 2021

Valid 051200Z - 101200Z

...DISCUSSION...
Day 4 (Saturday) - A belt of stronger westerlies will prevail from
MT to along the international border across northern ND. A leading
shortwave trough will move through southern Manitoba during the day,
but the associated cold front will advance southeast through ND and
eastern MT. Given this impulse will reside well north of the surface
front, along with limited low-level moisture in the warm sector,
thunderstorm initiation appears unlikely during the day. A better
chance for storms may occur overnight in post-frontal region as
forcing for ascent increases downstream from an approaching
shortwave trough. While some severe threat might accompany this
activity the post-frontal nature of the storms along with limited
low-level moisture lower confidence in a more robust threat.

Days 5-7 (Sunday through Tuesday) - Models still indicate an upper
trough will amplify across the Pacific NW into the northern Rockies
with corresponding upper ridging building over the northern Plains.
This pattern will generally keep the stronger flow aloft displaced
west of the corridor of greater low-level moisture situated over the
Plains, lowering confidence in a more robust severe threat.
Moreover, models vary considerably regarding the evolution of this
amplifying upper trough, with resulting low predictability.

..Dial.. 06/02/2021
$$
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ACUS48 KWNS 040919
SWOD48
SPC AC 040917

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0417 AM CDT Fri Jun 04 2021

Valid 071200Z - 121200Z

...DISCUSSION...
For the day 4-8 period a blocking pattern is expected to evolve with
troughing confined to the western and Northeast States and synoptic
ridging over the central U.S.. Embedded within the ridge, a cutoff
low will drift very slowly into the mid MS Valley region and
gradually weaken. The best chance for severe storms within this
pattern will exist across the northern Plains as a series of
shortwave troughs eject northeast and intercept greater low-level
moisture. However, uncertainty remains high regarding the amplitude
and timing of these features.

Day 4 (Monday) - Heights begin to rise over the northern Plains as
an upper trough amplifies across the western states. While some
severe weather is possible in association with an advancing warm
front across the northern Plains, uncertainty exists regarding storm
coverage. Farther south, a few strong to severe storms may spread
southeast within the northwest flow regime across the southern High
Plains.

Day 5-6 (Tuesday - Wednesday) - Upper ridge will continue to build
across the northern Plains, and it appears ejecting impulses will
remain mostly west of the moist axis. However, a few high-based
severe storms will be possible, mainly across a portion of central
and eastern MT. A few severe storms will also be possible over the
Northeast States day 6 in association with an evolving northwest
flow regime.

Day 7 (Thursday) - Models indicate a more substantial impulse will
eject farther east, intercepting western periphery of richer
low-level moisture across the northern Plains. This suggests
potential for a more substantial severe threat might evolve.
However, models have not demonstrated sufficient run-to-run
consistency for a categorical forecast at this time frame.

..Dial.. 06/04/2021
$$
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ACUS48 KWNS 070858
SWOD48
SPC AC 070856

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0356 AM CDT Mon Jun 07 2021

Valid 101200Z - 151200Z

...DISCUSSION...
...D4/Thursday: Northern and central High Plains...
A negatively tilted mid/upper-level trough is forecast to eject into
the northern Rockies and northern High Plains on Thursday. In
response, a surface cyclone will deepen across eastern MT during the
day, before migrating northeastward Thursday night as a trailing
cold front begins to sweep through the northern and central High
Plains. Steep midlevel lapse rates and sufficient low-level moisture
will support at least moderate buoyancy by late afternoon near and
just north of the surface low, and southward down the High Plains
along/east of a surface trough/dryline.

While the usual uncertainty remains regarding the exact placement of
relevant surface features at this forecast range, confidence remains
high that an organized severe-thunderstorm threat will evolve during
the afternoon and evening near/north of the surface low and
southward down the High Plains. Initial supercell development will
be possible across far eastern MT into western ND in closer
proximity to the surface low, potentially posing a threat of all
severe hazards. Upscale growth is possible with time, especially as
the cold front accelerates eastward into western portions of the
Dakotas and Nebraska, which may sustain a severe wind threat for
much of the evening.

...D5/Friday...
Model spread begins to increase on Friday regarding the evolution of
the ejecting shortwave trough, the surface cyclone, and attendant
cold front. Some severe threat may spread into the upper MS Valley
along the weakening cold front, and also potentially into portions
of the Plains and mid-MS Valley within a northwesterly flow regime.
The magnitude of any threat appears relatively limited at this time,
with low predictability.

...D6/Saturday - D7/Sunday...
While some severe threat may evolve as midlevel
northerly/northwesterly flow impinges on a moderately unstable
airmass from the southern Plains into portions of the MS/OH Valleys, predictability is much too low to delineate any probabilities for
this weekend.

..Dean.. 06/07/2021
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ACUS48 KWNS 080836
SWOD48
SPC AC 080834

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0334 AM CDT Tue Jun 08 2021

Valid 111200Z - 161200Z

...DISCUSSION...
On Friday/D4, models are in reasonable agreement depicting a
negative-tilt shortwave trough and surface low moving across the
Dakotas during the day, and into Manitoba by Saturday/D5 morning.
Storms may be ongoing along a cold front Friday/D5 morning over the
eastern Dakotas, or perhaps into western MN should storm outflow
move farther than currently forecast. This activity is generally
forecast to wane as the upper wave lifts northeast. An unstable air
mass will remain ahead of the front/outflow boundary over the upper
MS Valley and additional storms may occur, but predictability is
low.
For the Saturday/D5 through Tuesday/D8 period, models generally
build an upper ridge over the Plains and Four Corners states, but
some ECMWF members show height falls over the upper Great Lakes
region beginning on Sunday/D6 as a wave develops out of the
northwest. Even if this solution occurs, the air mass in that region
is forecast to be somewhat stabilized by the passage of a cold front
on Saturday/D5.

Elsewhere, large quantities of instability are forecast to remain
over the southern Plains where dewpoints will be in the 70s F. A few
diurnally driven storms cannot be ruled out over the southern High
Plains near a dryline where heating will be most effective at
removing CIN. However, the upper ridge will likely only support an isolated/disorganized severe threat, with low predictability.

..Jewell.. 06/08/2021
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ACUS48 KWNS 090832
SWOD48
SPC AC 090831

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0331 AM CDT Wed Jun 09 2021

Valid 121200Z - 171200Z

...DISCUSSION...
An upper high is forecast to retrograde from the southern High
Plains on Sat/D4 across the Four Corners states through the
remainder of the period, resulting in hot conditions and weak shear.
Meanwhile, a low-amplitude shortwave trough is forecast to round the
ridge and turn southeastward across the upper MS Valley and Great
Lakes beginning on Sunday/D5. A relatively stable air mass will
already be in place across this region, minimizing thunderstorm
chances with this trough.

The aforementioned shortwave trough will eventually phase with a
deep upper low that is forecast to drop south out of James Bay,
resulting in a large area of cyclonic flow aloft over Great Lakes
and Northeast for the D6/Mon through D8/Wed period. As a result of
this pattern, high pressure will spread south across the northern
Plains, Great Lakes and Northeast, shunting any appreciable moisture
and instability south. While daily thunderstorms are expected along
the southward-moving cold front, this setup would not favor
organized severe storms.

Strong instability and robust low-level moisture will remain across
the southern Plains, lower MS Valley and across the Gulf Coast
states through Tuesday/D7. However, shear will remain weak, with
only sporadic areas of daytime thunderstorms with attendant low
predictability.

..Jewell.. 06/09/2021
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ACUS48 KWNS 100857
SWOD48
SPC AC 100855

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0355 AM CDT Thu Jun 10 2021

Valid 131200Z - 181200Z

...DISCUSSION...
For the D4/Sunday to D8/Thursday period, the large-scale pattern
will feature an upper high building over the Four Corners states,
with ridge extending north across the Rockies and High Plains. To
the east, an upper low is forecast to drop south out of Hudson Bay,
with increasing cyclonic flow aloft from the Great Lakes into the
Northeast. The result will generally be hot temperatures across the
West, with stabilizing high pressure from the Midwest into the
Northeast. Over the Plains, southerly winds will maintain a warm
moist air mass, with only isolated diurnal activity anticipated.

Around D6/Tuesday, some overlap of strong flow aloft and instability
may develop over parts of the Carolinas, on the southern periphery
of the deepening upper trough over the Northeast. Models are not in particularly good agreement with the airmass quality in this region
on D6/Tuesday, thus predictability is low.

For the D6/Tuesday to D7/Wednesday time frame, several models
indicate a shortwave trough moving across the Pacific Northwest, and
perhaps skirting parts of MT and the northern Plains as the upper
ridge flattens. If this occurs, at least a minimal severe risk could
develop, mainly over MT, but predictability is low.

..Jewell.. 06/10/2021
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ACUS48 KWNS 200856
SWOD48
SPC AC 200855

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0355 AM CDT Sun Jun 20 2021

Valid 231200Z - 281200Z

...DISCUSSION...
Medium-range guidance is starting to come into better agreement
regarding the synoptic-scale pattern across the CONUS from the
middle to latter portion of the upcoming week. Upper ridging
centered over the Rockies on Day 4/Wednesday should become
suppressed through Day 6/Friday as multiple shortwave troughs evolve
across south-central Canada, the northern Plains, and Midwest.
Low-level moisture is still forecast to return northward ahead of
these mid-level perturbations, and an EML emanating from the High
Plains may advect as far east as the mid/upper MS Valley and Midwest
by Day 5/Thursday.

A cap may inhibit robust storm development across the Midwest on Day 4/Wednesday. Still, the severe risk should increase as both
instability and shear are forecast to strengthen by Day 5/Thursday.
Portions of southern MN into IA and vicinity may have the best
potential for severe storms on Day 5/Thursday, but predictability
regarding the timing and placement of these shortwave troughs
remains too uncertain to introduce a 15% severe area. Some severe
threat may continue on Day 6/Friday across parts of the Midwest and
mid MS Valley ahead of a cold front, as an upper trough/low possibly
amplifies and develops from the north-central CONUS towards the
Great Lakes.

..Gleason.. 06/20/2021
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ACUS48 KWNS 220854
SWOD48
SPC AC 220852

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0352 AM CDT Tue Jun 22 2021

Valid 251200Z - 301200Z

...DISCUSSION...
Some severe threat may continue on Day 4/Friday across parts of the
central Plains into the mid MS Valley, and perhaps OH Valley and
southern Great Lakes. A convectively enhanced shortwave trough
should develop northeastward across these regions through Friday
evening. Forecast instability ahead of a surface cold front and
deep-layer shear across the warm sector appear sufficient for
organized severe storms. However, there is still substantial
uncertainty regarding storms that may be ongoing Friday morning, and
their impact on destabilization through the day. Although severe
probabilities will likely be needed in a later outlook, it appears
premature to include a 15% severe area for Friday at this time.

Medium-range guidance is in somewhat good agreement that a
positively tilted upper trough will persist over parts of the
central CONUS and into eastern Canada this upcoming weekend and into
early next week. Any lingering severe threat should remain confined
to the south of a ill-defined front extending from parts of the
southern Plains to the OH Valley/Great Lakes vicinity. But,
mid-level flow is forecast to remain generally weak across the warm
sector, and the potential for organized severe storms may remain
fairly low, especially from Day 6/Sunday onward.

..Gleason.. 06/22/2021
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ACUS48 KWNS 260900
SWOD48
SPC AC 260859

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0359 AM CDT Sat Jun 26 2021

Valid 291200Z - 041200Z

...DISCUSSION...
Recent medium-range guidance suggests the upper ridge anchoring the
omega block over the Pacific Northwest and southwest Canada will not
begin shifting eastward until at least D5/Wednesday. Upper pattern
east of the Rockies will also be slow to change, but some modest
dampening of the subtropical ridge over the East Coast is
anticipated on D4/Tuesday and D5/Wednesday as an upper trough moves
gradually eastward across Ontario and the Upper Midwest/Upper Great
Lakes. Some deepening of this upper trough is forecast to begin on D5/Wednesday, and could eventually result in the development of an
upper low. However, guidance shows significant variability after
D5/Wednesday.

In general, the air mass across the majority of the central and
eastern CONUS will be moist and unstable throughout the period. As a
result, an isolated severe threat may develop each day where
sufficient destabilization can overlap modest winds aloft.

..Mosier.. 06/26/2021
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