AWUS01 KWNH 281758
FFGMPD
CAZ000-290300-

Mesoscale Precipitation Discussion 0018
NWS Weather Prediction Center College Park MD
1257 PM EST Thu Jan 28 2021

Areas affected...Central California Coast to Southern California

Concerning...Heavy rainfall...Flash flooding possible

Valid 281800Z - 290300Z

SUMMARY...Slow eastward advancement of Atmospheric River will keep
flash flooding possible across Transverse Ranges and lower slopes
into urban corridor this evening. Lingering moisture but isolated
shallow convection post frontal may cross areas already in
flooding conditions to maintain it through the southern Santa
Lucia Range.

DISCUSSION...GOES-W WV suite depicts a narrow and sharpening
longer wave trough between 125-130W with strong meridional jet
features on both sides of it. The base of the trof is generally
moving east along 33N, with weak mesoscale wave features sliding
northward along the interface with the mid to upper level cirrus
shield associated with broad diffluence in the 3H jet. A
stronger 130kt nose digging along the backside of the trof will
continue to sharpen the trof, which in turn will increase
height-gradient across the California Bight leading to
acceleration of the nearly unidirectional flow within the main
moisture plume. This moisture plume continues to narrow as it was
severed upstream, but a ribbon of near 1" Total PWats along/ahead
of the pre-frontal trof zone along with this deep unidirectional
flow will continue to support deep Integrated Vapor Transport
(IVT) values to near 600 kg/m/s which remains in the 99th
percentile and near 4 standard deviations from normal, through
00z.

Low level flow in the Bight is backed through channelized flow
ahead of the cold front, enhancing surface moisture convergence in
a narrow band that shows up well across KVBX and VTX RADAR
projections. Stronger divergence aloft is expected by 00z as the
western meridional jet coupled with subtropical jet influence will
round the base into northern Baja California. This will leave the
region in a very favorable left exit dynamical ascent coupled with
the accelerating/enhanced right entrance of the polar jet
(100-110kts) to support continued ~.75"/hr totals with the cold
front passage along the south coast of Santa Barbara, Ventura into
W Los Angeles county. Increased orographic ascent with 25-30kts
increasing toward 40kts at 85H into the Transverse range will
likely help to reach scattered 1"/hr totals, particularly before
height-falls slowly drop freezing levels across the highest peaks
of the San Rafael at or just after 00z (expanding into the San
Gabriel by 03z). While the peak moisture flux convergence and
deep layered flow is along/ahead of the cold front, post-frontal
shield precip will remain solid ahead of the upper-level trof
axis(total PWats .75"). for .15-.25"/hr rates and generally 3-4"
totals across SB and Ventura counties by 03z. Combine this with
general 3-5" totals over the last few days, flash flooding may be
possible with debris/rock/mudslides.

Upstream...Cold air advection aloft with the sharpening
height-falls will provide steepening lapse rates while modest
moisture (Total PWats .75") still lingers through the lower
tropospheric profile. Hi-Res CAMs suggest modest instability for
shallow 'polar' convection. As such rates of .33-.5"/hr with the
line of scattered convective cells (as currently seen approaching
S Monterey county into SLO county). At worst this may yield and
additional 1-2" totals across areas already experiencing flooding
conditions, further continuing the threat generally sliding south
along the south Central Coast (18z) to Point Conception by 03z.

Gallina

ATTN...WFO...LOX...MTR...SGX...

ATTN...RFC...CNRFC...NWC...

LAT...LON 36292172 35912105 35912046 35472017 35102001
34811958 34681913 34651833 34561785 34251752
33771785 33551819 33831899 33801967 33922033
34572077 35242111 35792151 36162173
#
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AWUS01 KWNH 281838
FFGMPD
TNZ000-KYZ000-MSZ000-MOZ000-ARZ000-TXZ000-OKZ000-010035-

Mesoscale Precipitation Discussion 0032
NWS Weather Prediction Center College Park MD
137 PM EST Sun Feb 28 2021

Areas affected...southeast OK, central AR into southeast MO,
southwest KY, west TN

Concerning...Heavy rainfall...Flash flooding possible

Valid 281835Z - 010035Z

SUMMARY...A southwest to northeast axis of heavy rain is expected
through 00Z from the Red River Valley into central AR and portions
of southeastern MO, southwestern KY and western TN. Rainfall rates
of 1-2 in/hr will likely lead to a few areas of 2-4 inch totals
through 00Z and possible flash flooding. Some overlap with heavier
rainfall over the past 72 hours may enhance runoff.

DISCUSSION...18Z observations placed a cold front stretching from
central IL, southwestward across the Ozarks into southeastern OK
and north-central TX. Radar imagery across the region showed a few
strong to severe storms along the front near a weak wave in the
Red River Valley and expanding convection ahead of the cold front
in western and central AR. The SPC mesoanalysis page indicated
500-1500 J/kg MLCAPE at 18Z, extending from the TX/OK border near
the front into AR and western TN, with the highest values for
southern and western locations. Ahead of the front was a low level
axis of convergence, located between 925 and 850 mb, oriented in a
SW to NE line from near DEQ to SRC to UCY. Co-located with the
pre-frontal convergence axis were precipitable water values over
1.5 inches from central AR into central KY and 30-40 kt of low
level flow leading up to the south side of the axis.

Showers and thunderstorms are expected to continue expanding in
coverage throughout the afternoon ahead of the cold front and
along the pre-frontal convergence axis within the already moist
and unstable environment. Little movement of the pre-frontal
convergence axis is expected until the cold front catches up to it
near 00Z. Average cell motions are expected to parallel the
orientation of low level convergence axis, supporting repeating
and periods of training. Rainfall rates of 1-2 in/hr are likely
within the stronger cores, leading to 2-4 inches of rain through
00Z in a few locations along with possible flash flooding. Given
some of the expected rainfall will overlap locations which picked
up 2-4 inches of rain within the past 72 hours, runoff may be
enhanced.

Otto

ATTN...WFO...LZK...MEG...OHX...PAH...SHV...TSA...

ATTN...RFC...ABRFC...LMRFC...OHRFC...NWC...

LAT...LON 36668779 36538725 36278719 35928740 35358852
34948994 34349135 33559320 33229426 33389482
33769503 34169496 34889410 36019164 36419007
36478959 36638866
$
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AWUS01 KWNH 281949
FFGMPD
VAZ000-WVZ000-KYZ000-010105-

Mesoscale Precipitation Discussion 0033
NWS Weather Prediction Center College Park MD
248 PM EST Sun Feb 28 2021

Areas affected...eastern KY into southern WV

Concerning...Heavy rainfall...Flash flooding possible

Valid 281946Z - 010105Z

SUMMARY...Locally heavy rain moving through eastern KY and
southern WV may generate localized flash flooding into the early
evening. While rainfall rates are expected to remain below 1
in/hr, ongoing flooding and very low flash flood guidance values
will continue to leave these locations susceptible to enhanced
runoff concerns.

DISCUSSION...Loops of regional radar imagery through 1930Z showed
clusters of heavy rain moving eastward through the Ohio Valley,
with the heaviest rainfall rates in the region over eastern KY
around 0.5 in/hr. This region of the U.S. has precipitable water
values ranging from 1 inch to 1.5 inches, higher to the west, well
above average for this time of year (+2 to +3 standard
deviations). One to two inches of rain over the past 24 hours and
several inches of rain over the past 48 hours has contributed to
widespread flooding across the region. While moist adiabatic
vertical profiles have resulted in only weak instability across
the area, generally less than 500 J/kg MUCAPE, rainfall has been
persistent within a long fetch of low level moisture transport
atop relatively cooler air in place over the Cumberland Plateau.

Short term forecasts from the RAP continue the axis of low level
moisture transport into the eastern half of KY and southern WV
through 00Z, with little (if any) increase in instability.
Therefore, while the ongoing cluster of heavier rain in eastern KY
at 1930Z will continue to track east over the next 1-3 hours
followed by a brief lull, additional rounds of heavy rain are
likely to move in from the west. Peak rainfall rates of 0.5 to
less than 1 in/hr are expected along with an additional inch or
two of rain from eastern KY into southern WV through 01Z on top of
saturated soils. Given flash flood guidance values of an inch or
less in 3 hours for the bulk of the region, flash flooding will be
possible.

Otto

ATTN...WFO...JKL...LMK...LWX...MRX...RLX...RNK...

ATTN...RFC...MARFC...OHRFC...NWC...

LAT...LON 38498052 38457999 38297977 37868005 37438070
37238203 37008326 37098411 37398428 37848420
38098377 38288282 38448147
$$
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AWUS01 KWNH 282233
FFGMPD
TNZ000-KYZ000-ILZ000-010330-

Mesoscale Precipitation Discussion 0034...Corrected
NWS Weather Prediction Center College Park MD
533 PM EST Sun Feb 28 2021

Corrected for typos

Areas affected...Kentucky into northern Tennessee

Concerning...Heavy rainfall...Flash flooding likely

Valid 282140Z - 010330Z

SUMMARY...Additional rainfall across KY into northern TN through
03Z is expected to continue areas of flash flooding across the
region. Rainfall rates of 1-2 in/hr and an additional 2-4 inches
of rain are expected through 03Z. Overlap with areas that received
3-5 inches of rain since midnight in southwestern KY may result in
major impacts.

DISCUSSION...At 21Z, regional radar imagery continued to show
numerous showers with embedded thunderstorms moving from WSW to
ENE across the Mississippi Valley into KY. While the heaviest
rainfall rates have moved east across I-65 in south-central KY,
additional activity was blossoming upstream across the MO/IL
border, southward into AR as noted by cooling cloud tops on GOES
East infrared imagery. The upstream activity was occurring ahead
of a cold front moving through southern IL/MO and an area of
pre-frontal convergence across AR. Lift was also being augmented
by placement within the right entrance region of a strengthening
250 mb jet streak over northern IL.

Numerous Wunderground.com observations have verified a narrow
stripe of 3-5 inches of rainfall since midnight in southwestern KY
(Calloway County, east-northeastward into Warren County), visible
through KHPX storm total precipitation. Over the next 3 to 6
hours, the axis of greatest low level moisture convergence
(oriented SW to NE) will continue to parallel mean storm movement,
supporting continued narrow axes of heavy rainfall with rates of
1-2 in/hr. However, a gradual south and east translation is
expected to occur after 00Z with the low level axis of lift as the
cold front nears from the west. This should continue heavy rain
across much of KY through the evening but the heaviest rain over
the next 6 hours is expected to occur over far northern TN into
south-central KY, just south of the area that picked up 3 to 5
inches of rain in southwestern KY. Flash flooding continues to be
likely as much of the area has saturated soils from earlier
rainfall.

Otto

ATTN...WFO...JKL...LMK...MEG...MRX...OHX...PAH...

ATTN...RFC...LMRFC...OHRFC...NWC...

LAT...LON 37758456 37618427 37348417 37038410 36768433
36418505 36168610 36058691 36058733 36248754
36398790 36348891 36378913 36548922 36838902
37148835 37468669 37718530
$$
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