AWUS01 KWNH 110408

FFGMPD

WAZ000-111530-



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



...Please see www.wpc.ncep.noaa.gov for graphic product...



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



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



LAT...LON   49202170 49192044 48662002 47232051 46482165 

            46642366 47412470 48122487 48372444 48112360 

            48162268 48802244