AWUS01 KWNH 150936
FFGMPD
ORZ000-WAZ000-152135-

Mesoscale Precipitation Discussion 1263
NWS Weather Prediction Center College Park MD
435 AM EST Mon Dec 15 2025

Areas affected...Western WA and Northwest OR

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

Valid 150935Z - 152135Z

SUMMARY...A new atmospheric river will be arriving this morning
across the Pacific Northwest. New rounds of heavy rain will be
associated with this across especially western WA and northwest
OR, and this will gradually pose a renewed threat of flooding.

DISCUSSION...The latest GOES-W WV suite along with OSPO/CIRA ALPW
data shows a new trans-Pacific atmospheric river approaching the
Pacific Northwest. The source region of the long-fetch moisture
transport is situated well southwest of Hawaii along 160W to 170W,
with a northeast advection of it up along and ahead of a
well-defined frontal zone that extends from Hawaii to offshore of
the West Coast. Satellite imagery and surface data also shows
additional northern stream energy and a reinforcing cold front
approaching from areas near and south of the Gulf of AK.

Over the next 6 to 12 hours, strong deep layer southwest flow with
embedded shortwave energy will approach and begin crossing the
Pacific Northwest which will drive the aforementioned offshore
fronts gradually inland across the coastal ranges. Strong warm air
advection and enhanced low to mid-level flow will drive increasing
rainfall rates across the orographically favored coastal ranges
from northwest OR up into the Olympic Peninsula and eastward into
the foothills and higher terrain of the Cascades.

Microwave-based CMORPH2 data shows heavy rainfall rates associated
with the offshore shortwave energy focused along 130W, with some
rates on the order of 0.75" to 1"/hour. This is associated with
broken areas of convection as validated by GOES-W GLM data
indicating intermittent lightning activity. Some of these heavier
rates should arrive gradually throughout the morning, and the 00Z
HREF guidance shows high probabilities (>60%) of rainfall rates
exceeding a 0.50"/hour across the Olympic Peninsula and the
foothills of the Cascades, with some low-end probabilities
(approaching 30%) of seeing these rates max out near 1"/hour in
the central and southern WA Cascades.

These high rates will coincide with the arrival of high IVT
magnitudes that will be on the order of 800 to 1000 kg/m/s into
the coastal ranges, with an inland penetration of the core IVT
plume into the Cascades. Some modest instability with MUCAPE
values up around 250+ J/kg along and just ahead of the frontal
boundaries will also be a contributor to these elevated rates.

Expect as much as 3 to 5 inches of new rainfall over the next 12
hours, with the heaviest totals over the Olympic Peninsula and the
windward slopes of the WA Cascades. Lesser amounts will be noted
over northwest OR with as much as 1 to 3 inches here involving the orographically favored terrain.

The region is extremely sensitive given the residual impacts from
the recent high-impact multi-day atmospheric river. Elevated/high
streamflows and saturated ground will support the additional rains
going into runoff that will be capable of driving renewed areas of
areal flooding. Some localized burn scar flash flooding, debris
flows and landslide activity also cannot be ruled out today.

Orrison

ATTN...WFO...OTX...PDT...PQR...SEW...

ATTN...RFC...PTR...NWC...

LAT...LON 49262217 49252090 48782034 47882038 46802093
45402127 44392175 44302231 44662313 44682399
45742427 47102424 48002466 48222357 47692293
47822252 48592230

$$
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