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   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   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!8CSOBZPag-i-C1aXm6jQzW12n-4RBwiZpBMzlfmSn5drA27YKyMJLZE9060HaBqpt42K=   
   23iAiSQIlsvBBUL7myAJ6NQ$  for graphic product...   
      
   ATTN...WFO...OTX...PDT...PQR...SEW...   
      
   ATTN...RFC...PTR...NWC...   
      
   LAT...LON   49262217 49252090 48782034 47882038 46802093=20   
               45402127 44392175 44302231 44662313 44682399=20   
               45742427 47102424 48002466 48222357 47692293=20   
               47822252 48592230=20   
      
   =3D =3D =3D   
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