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   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 https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!71Jepf9yev7hmkJDrRvmtLMcUq8j50D7FgYNO7kSskSH7SKK20EaAmLtVqgpFTJ6kFC0=   
   SGJjw80lHp4zyyPnh7ywKDw$  for graphic product...   
      
   ATTN...WFO...OTX...PDT...PQR...SEW...   
      
   ATTN...RFC...PTR...NWC...   
      
   LAT...LON   49202170 49192044 48662002 47232051 46482165=20   
               46642366 47412470 48122487 48372444 48112360=20   
               48162268 48802244=20   
      
   =3D =3D =3D   
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