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   FOUS30 KWBC 161944   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   244 PM EST Tue Dec 16 2025   
      
   Day 1   
   Valid 16Z Tue Dec 16 2025 - 12Z Wed Dec 17 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF   
   THE PACIFIC NORTHWEST...   
      
   16z update:   
   The timing/placement of the quick hitting A.R. surge remains on   
   track with re-invigoration of the NW California portion will   
   generally start around 20-21z before expanding northward with the   
   core of the northern stream southwesterly deep jet streak reaching   
   NW Oregon just before 06z. IVT values are well above normal due to   
   the strength of deep layer flow, peaking near 900 kg/m/s. The   
   duration of the core of the AR surge will likely be ending about   
   12z-15z. Additional 1-2" for the lower coastal ranges are expected   
   with spots of 2-3" in the mid-slopes of the Cascade, Olympic ranges   
   with only a few highest peaks near the Klamath Range near 4" by   
   12z. As such, no changes were made to the broad Marginal Risk of   
   Excessive Rainfall.   
      
   Gallina   
      
   ~~~~Prior Discussion~~~~   
   Maintained the previously issued Marginal Risk area from   
   Washington southward along the Cascades and the coastal ranges into   
   northwest California today and tonight. Yet another round of   
   precipitation will be spreading from west to east across portions   
   of Washington and Oregon. Cooler air coming in behind the system   
   should result in mountain snows and rain for the lowlands and   
   mountain passes today into tonight. There should be a separate area   
   of mainly rain and some higher level snows that gets directed into   
   the southwest portion and northwest corner of California with   
   amounts locally enhanced by orographic influences. The previous   
   outlook largely covered this scenario so few changes were   
   needed.Once again...some overlap between the excessive rainfall   
   areas and mountain snowfall amounts was unavoidable but we tried to   
   keep the ERO away from the placement of the WPC Winter Weather   
   Desk snow amounts of 6 inches or greater.   
      
   The overall synoptic pattern continued to show the deepest moisture   
   and broadest coverage of rainfall arriving in association with the   
   surface cold front around 17/00Z and the diminishing QPF as the   
   area gets whisked eastward.   
      
   Bann   
      
   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.=20   
      
   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!-ypAPJJuuaAwceezEnqnBKRX50q3ocr24Py2XIZ2prBU=   
   Lh__nb7IrznLLio1RNCoUjxxupOwOEXC9ejVdRCDef3GmH0$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!-ypAPJJuuaAwceezEnqnBKRX50q3ocr24Py2XIZ2prBU=   
   Lh__nb7IrznLLio1RNCoUjxxupOwOEXC9ejVdRCDsnST7bY$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!-ypAPJJuuaAwceezEnqnBKRX50q3ocr24Py2XIZ2prBU=   
   Lh__nb7IrznLLio1RNCoUjxxupOwOEXC9ejVdRCDTu6tGL8$=20   
      
   =3D =3D =3D   
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