Mesoscale Precipitation Discussion 1261
NWS Weather Prediction Center College Park MD
1107 PM EST Wed Dec 10 2025

Areas affected...Western WA (including the Olympic Peninsula and
Cascades)

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

Valid 110407Z - 111530Z

SUMMARY...Strong atmospheric river activity to continue overnight
across much of western WA with a focus for additional heavy rain
and locally significant areal flooding, including potential for
debris flows, landslides and localized flash flooding.

DISCUSSION...The latest GOES-W WV suite along with OSPO/CIRA ALPW
data sets show a well-defined and strong atmospheric river
continuing to advance inland across western WA, with an upstream
orthogonal orientation of the deep Pacific moisture axis/plume
relative to the Olympic Peninsula and the Cascades. Satellite and
radar data continue to show areas of heavy rain impacting these
areas with recent rainfall rates in the 0.25" to 0.40"/hour range.

Much of the deeper layer trans-Pacific moisture transport into the
region continues to be aided by the positioning of a strong
subtropical ridge near and offshore of CA, and an elongated axis
of mid-level troughing from the Gulf of AK southwestward to
30N/40N and 160W. Offshore experimental CIRA LVT data is showing
some gradual slackening of the low and mid-level moisture
transport around the top side of the ridge axis, and this may
allow for some of the more widespread heavier rainfall rates to
gradually subside a bit going through the overnight hours.

However, there will be the arrival of a new Pacific cold front
across the region as shortwave energy moves into British Columbia,
and this boundary should slowly push inland and become more
oriented west/east across southern WA Thursday morning. IVT
magnitudes along the front should remain elevated for the next 6
to 12 hours, but by early Thursday morning, these values are
forecast to drop down into the 600 to 800 kg/m/s range with the
overall slackening of the low to mid-level kinematic wind field
environment.

The 00Z HREF guidance does continue to support rainfall rates in
the Olympic Peninsula of around a 0.25"/hour, but with heavier
rates occasionally reaching a 0.50"+/hour in the central and
northern WA Cascades. Going through 15Z (7AM PST) on Thursday, an
additional 1 to 2 inches of rain is expected for the Olympic
Peninsula, but with an additional 2 to 4 inches in the central and
northern WA Cascades.

These additional rains will exacerbate ongoing areal flooding and
especially to the river basins which are locally seeing
significant impacts. Given the amount of rainfall that has
occurred over the last few days in the Cascades in particular
(with 12 to 15+ inches locally), the terrain is particularly
sensitive, with the additional rains favoring concerns for debris
flows, landslides and potential flash flooding around areas of
steep terrain.

Orrison

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

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

LAT...LON   49202170 49192044 48662002 47232051 46482165
            46642366 47412470 48122487 48372444 48112360
            48162268 48802244