AWUS01 KWNH 261911
FFGMPD
CAZ000-270610-

Mesoscale Precipitation Discussion 0014
NWS Weather Prediction Center College Park MD
210 PM EST Tue Jan 26 2021

Areas affected...Central California Coast

Concerning...Heavy rainfall

Valid 261910Z - 270610Z

Summary...A strong atmospheric river event will bring widespread
rainfall to the central California coast later today into tonight.
Hourly totals between 0.5-0.75 inches and amounts through 06Z of
2-3" are likely. This heavy rainfall could cause flash flooding,
especially over burn scars and other sensitive areas.

Discussion...Water vapor imagery and the latest blended PW product
shows an anomalously strong plume of moisture off the western U.S.
coast. As the system approaches this evening, showers associated
with the warm front are expected to move onshore the coast between
00-03Z then quickly followed by the more intense rain associated
with the cold front after 03Z as it drops south along the coast.
Precipitable water values associated with the atmospheric river
are forecast to be between 1-2 standard deviations above normal
while IVT values by this evening are forecast to be near 4
standard deviations above normal (approaching 5 std later on
Wednesday). 850 mb flow increases to 50-60 kts by 04Z and is
aligned nearly orthogonal to the terrain across the North Bay
areas, which should enhance rain rates.

The 12Z HREF supports hourly totals between 0.50 and 0.75" between
03-06Z across the North Bay then advancing southward later in the
night. The various hi-res models depict some higher-end potential
for 1" hourly totals, though that risk is greater beyond the
current MPD time frame. Through 06Z, 2-3" will be common across
the North Bay areas, some locally higher amounts 3-4" will be
possible.

These rainfall rates and totals over the burn scars and other
sensitive areas could lead to flash flooding, possible
mudslides/debris flows. Even outside burn scar areas, rapid onset
runoff and flooding will be possible.

Taylor

ATTN...WFO...EKA...MTR...STO...

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

LAT...LON 39192309 39032255 38772217 38492199 38272203
38192242 37972260 37942286 38252308 38552326
38802353 39172384
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AWUS01 KWNH 011727
FFGMPD
CAZ000-ORZ000-020000-

Mesoscale Precipitation Discussion 0021
NWS Weather Prediction Center College Park MD
1227 PM EST Mon Feb 01 2021

Areas affected...Northwest CA...Southwest OR

Concerning...Heavy rainfall

Valid 011720Z - 020000Z

SUMMARY...Periods of moderate-to-heavy rain are expected late this
morning and into the afternoon as Pacific moisture moves ashore
ahead of an approaching frontal boundary.

DISCUSSION...GOES-17 satellite imagery depicted a stream of
Pacific moisture out ahead of a deep -2 to -2.5 STD longwave
trough. 15Z surface analysis indicated an elongated frontal
boundary was located just off the Pacific Northwest coastline. PVA
and WAA surging out of the base of the upper trough is advancing
the core of rich Pacific moisture east towards the Pacific
Northwest coastline. PWs topping out as high as 1.00", just shy of
~2 STDs above normal, are being transported within a strong
southwesterly mean 850-300mb flow. These atmospheric factors
support the likelihood for ongoing periods of rain, potentially
heavy at times, as the axis of Pacific moisture reaches the coast.

Latest CAM guidance show the heaviest period of rainfall occurring
between late morning and early afternoon. Hourly rainfall rates
will be maximized along the more orthogonally favored upslope
regions of northwest California and southwest Oregon. Rates in
these areas could range between 0.25-0.50" with several hours
worth of these rates possible. Total rainfall estimates during
this MPD suggests up to 1.5" of precipitation is likely along the
coastal range of northern California and southwest Oregon with
locally higher amounts possible. While totals and rates are not
overly impressive, antecedent conditions show these same areas
have received between 150-300% of normal rainfall over the last 7
days. FLASH guidance also highlights some areas with over 85% soil
saturation. These factors along with some additional snow melt
could result in rising streams and rivers with very localized
flooding possible.

Mullinax

ATTN...WFO...EKA...MFR...

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

LAT...LON 43162426 42762391 42002354 41142352 39962336
39452331 39262397 39852460 40922481 41582470
42042470 42482470 42882468 43122462
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AWUS01 KWNH 112117
FFGMPD
ORZ000-WAZ000-120845-

Mesoscale Precipitation Discussion 0006
NWS Weather Prediction Center College Park MD
416 PM EST Mon Jan 11 2021

Areas affected...northwestern OR into western WA

Concerning...Heavy rainfall

Valid 112113Z - 120845Z

SUMMARY...The likelihood of rainfall rates in excess of 0.5 in/hr
will increase after 03Z across western WA into northwestern OR.
Through 09Z, maximum rainfall totals in the Olympics are expected
to be between 3 and 4 inches, while 2-3 inch max totals are
anticipated from southwestern WA into northwestern OR. This will
be the beginning of a prolonged atmospheric river event for the
Pacific Northwest.

DISCUSSION...A 72 hour loop of Blended TPW imagery showed a
continuous axis of 1+ inch precipitable water values extending
offshore of the West Coast, back into the tropical western
Pacific. Embedded higher values of precipitable water existed
within the plume with over 1.5 inches centered near 35N 150W at
14Z, or about 1500 miles offshore of the OR coastline. The eastern
end of this moisture axis is located south of a slow moving closed
mid to upper-level low centered near Kodiak Island. At the
surface, two fronts were analyzed at 20Z, the lead boundary
extending southwestward from a surface low located near 44N 134W,
and a second (stronger) surface low and associated cold front near
47N 139W.

As the shortwave tied to the western surface low near 139W
advances toward the British Columbia coast tonight, a corridor of
strong southwesterly to south-southwesterly 850 mb flow between
60-70 kt (occassionally higher) is forecast to arrive along the WA
and OR coasts near 03Z according to a consensus of the latest
model guidance. The strengthening low level flow along the coast,
combined with precipitable water values between 1.0 and 1.1
inches, is expected to support IVT values over 750 kg/m/s by
roughly 06Z over northwestern OR into far southwestern WA.

The orientation of the low level flow will favor southwest facing
slopes of the Olympic Peninsula and Coastal Ranges into
northwestern OR with rainfall rates occasionally climbing above
0.5 in/hr. While peak rainfall rates are expected to be below 0.75
in/hr through 09Z, the transition from fairly light rainfall to
heavy rainfall rates is expected to occur somewhat quickly, and
high rainfall rates should persist for hours, continuing into the
day on Tuesday.

Given above average rainfall across the region over the past two
weeks (150 to 300 percent of average from northwest OR into
western WA), locations may be more susceptible to flooding
concerns. Peak 12 hour rainfall totals of over 3 inches are
expected into the favored terrain.

Otto

ATTN...WFO...PQR...SEW...

ATTN...RFC...NWRFC...NWC...

LAT...LON 48072474 48032425 47772392 47622322 47392318
46972338 46752343 46422324 46182328 45602343
44892365 44842416 45302434 46032441 46592449
47632484 47982494
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