AWUS01 KWNH 270246
FFGMPD
CAZ000-ORZ000-271200-

Mesoscale Precipitation Discussion 1195
NWS Weather Prediction Center College Park MD
945 PM EST Thu Dec 26 2024

Areas affected...Southwest OR...Northern and Central CA

Concerning...Heavy rainfall

Valid 270245Z - 271200Z

SUMMARY...New atmospheric river surge arriving overnight will
bring additional rounds of heavy rainfall and potentially some
localized flooding and runoff concerns.

DISCUSSION...The latest GOES-W Airmass RGB satellite imagery shows
a new shortwave trough amplifying offshore of the West Coast which
will drive a new area of cyclogenesis overnight. The developing
low center will arrive across the coastal ranges of northwest OR
and southwest WA by early Friday morning. To the south of this low
center will be a renewed deep layer fetch of Pacific moisture that
will overrun the coastal ranges down the coast, and especially
across southwest OR and into northern and central CA.

A warm front offshore of the West Coast will be approaching as
rather strong IVT magnitudes encroach on the region. Enhanced low
to mid-level flow around the southern flank of the shortwave
trough will drive IVT values upwards of 750 to 1000 kg/m/s at
least for several hours (generally between 03Z and 09Z) as the
warm front arrives and advances inland. Later tonight and toward
12Z, the low center entering the Pacific Northwest will drive a
cold front inland across the coastal ranges.

PWs are forecast to reach as high as 1 to 1.25+ inches, and
reaching locally as high as 2 to 3+ standard deviations above
normal with the greater anomalies forecast to set up in the 06Z to
12Z time frame down across the Bay Area. Even areas a bit farther
south down the coast around Monterey are expected to see a notable
increase in PWs overnight as this latest atmospheric river surge
arrives.

The latest HREF guidance shows rather high probabilities of seeing
at least a few hours of potential 0.50"+/hour rainfall rates
impacting the orographically favored terrain from southwest OR
down through northwest CA and also portions of the Bay Area in the
06Z to 12Z time frame. Some of these heavier rains may also at
least locally spillover into the far northern Sacramento Valley
and the foothills of the northern Sierra Nevada, but generally the
heaviest rates should be across the coastal ranges. This will
coincide with the strongest window of forcing from warm air
advection and higher IVT parameters. After 12Z, the rates should
diminish rather substantially as the energy associated this latest
system advances rapidly inland.

Additional rainfall totals going through 12Z (4AM PST) Friday
morning are expected to reach 2 to 3 inches with isolated heavier
amounts possible. Given the wet antecedent conditions, these
additional rains may result in some localized flooding concerns
and this will include some urban flooding potential around the Bay
Area tonight.

Orrison

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!8784PPpwQjwvhq15F2dET7JKppLElsnfqNyj9XokRKUxZ74gHfsylSnY2rK2uCnw_wQK=
8WHCS1WHNlAuclfAjx9eiDQ$  for graphic product...

ATTN...WFO...EKA...MFR...MTR...REV...STO...

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

LAT...LON   43142392 42382323 41932229 41422172 41052171=20
            40552133 40062088 39642075 39182077 39102137=20
            40002199 39922247 39572247 38952222 38262196=20
            37342135 36822148 36582193 36972258 38182341=20
            39262412 40792456 41792449 42912460=20

=3D =3D =3D
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