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   FOUS30 KWBC 162335   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   635 PM EST Tue Dec 16 2025   
      
   Day 1   
   Valid 01Z Wed Dec 17 2025 - 12Z Wed Dec 17 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF   
   THE PACIFIC NORTHWEST...   
      
   Both the 18z HREF and 12z REFS paint a similar QPF picture through   
   12z.  The threat of heavy rainfall across portions of northern=20   
   California continues through the period, while a renewed surge of=20   
   heavy rains impacting portions of WA early on shifts south across=20   
   western OR with time. IVT values are well above normal due to the=20   
   strength of deep layer flow, peaking near 900 kg/m/s. An additional   
   1-2" for the lower coastal ranges are expected with local amounts   
   to 4" across portions of the Olympics, Cascades, and terrain of   
   northern CA by 12z. Only cosmetic changes were made to the broad=20   
   Marginal Risk of Excessive Rainfall.   
      
   Roth   
      
      
   Day 2   
   Valid 12Z Wed Dec 17 2025 - 12Z Thu Dec 18 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Bann/Kleebauer   
      
      
   Day 3   
   Valid 12Z Thu Dec 18 2025 - 12Z Fri Dec 19 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF   
   WESTERN OREGON AND PORTIONS OF ADJOINING STATES...   
      
   20Z Update... Changes were minimal with the forecast remaining   
   steadfast through the 12z NWP suite. The threat for significant   
   rainfall remains elevated across the western half of Oregon with a   
   focal point centered over the southern Cascades and the adjacent   
   valleys on either side. The key in this threat is the projected   
   "spillover" of heavier precip to the leeward side of the Cascades   
   due to a pronounced 700mb jet protrusion inland from the Pacific   
   leading to significant Pacific moisture breaching the terrain and   
   allowing for heavier precip to make it downwind of the mountains.   
   This is the area where ARI forecasts are hovering between the   
   25-100 year exceedance intervals, matching the previous output from   
   overnight's forecast. This allowed for general continuity in the   
   SLGT risk area with modifications only made to match to the latest   
   QPF footprint with emphasis on where 2+" will fall in the D3 time   
   frame, especially away from the coastal plain.   
      
   Kleebauer   
      
   ...Previous Forecast...   
      
   The next in a series of Atmospheric Rivers will arrive early   
   Thursday with its axis of greatest moisture transport being   
   directed primarily along the western coast of Oregon...with maximum   
   IVT values in excess of 800 kg per meter per second largely being   
   directed perpendicular into the coastal ranges early in the period.   
   Global models continue to advertise the possibility of 6 inches or   
   more of rainfall during the period...and comparing the WPC 24hr   
   QPF to ARI yielded a broad swath exceeding the 2-5 year return   
   intervals. It was also worth noting that the highest ARIs were just   
   to the east of the crest of the Cascades where an axis of 25-100   
   year exceedance intervals was depicted. This does seem like a   
   scenario where more spillover into the normally drier downslope   
   areas is plausible, so could be an impactful event even into some   
   of those areas. Consequently, the Day 3 excessive rainfall outlook   
   expanded the Slight Risk eastward in Oregon to cover areas with QPF   
   in excess of 5 inches and the highest ARI values mentioned above   
   given the concern for spillover. Snowfall within this area should   
   be confined to the higher elevations.   
      
   Bann   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!9yJ9QnzkUQBJ6OUonNg1PTGAxZ442AuYmIx86fXKyGQO=   
   XutYRHqIveFmikdfWa17WGJxCsmCmWtYqMhKNt0BJA7vRRM$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!9yJ9QnzkUQBJ6OUonNg1PTGAxZ442AuYmIx86fXKyGQO=   
   XutYRHqIveFmikdfWa17WGJxCsmCmWtYqMhKNt0BUSkFoWs$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!9yJ9QnzkUQBJ6OUonNg1PTGAxZ442AuYmIx86fXKyGQO=   
   XutYRHqIveFmikdfWa17WGJxCsmCmWtYqMhKNt0Bw8fWyvM$=20   
      
   =3D =3D =3D   
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