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   FOUS30 KWBC 072009   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   309 PM EST Sun Dec 7 2025   
      
   Day 1   
   Valid 16Z Sun Dec 07 2025 - 12Z Mon Dec 08 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Cook/Wegman   
      
   Day 2   
   Valid 12Z Mon Dec 08 2025 - 12Z Tue Dec 09 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE   
   PACIFIC NORTHWEST...   
      
   ...21Z Outlook Update...   
   The ongoing forecast across the Pacific Northwest is on track, with   
   3-6 inches of rainfall expected - greatest along windward/terrain-   
   favored areas of the Cascades and Olympics. The previous forecast   
   discussion covers the impending event well.   
      
   In addition to headlines/probabilities across western WA/OR, a   
   second Marginal Risk area has been added to portions of   
   northeastern Oregon, Idaho, western Montana, and far eastern   
   Washington State. Mid-level flow in these areas will increase to   
   around 70-90 knots during the forecast period, resulting in strong   
   orographic ascent against windward terrain and peaks, resulting in   
   many areas of 1-2 inches of rainfall. In addition, snow levels will   
   rise, potentially resulting in rain-enhanced snow melt in some   
   areas. Excessive runoff is expected in a few areas.   
      
   Cook   
      
   ...Previous Discussion...   
      
   The first in a series of lows is expected to slam into the coast of   
   the Pacific Northwest on Monday. While the low centers will track   
   well north of the area, the trailing cold fronts will be steered   
   right into the coast and will be the primary forcing for heavy   
   rain. The deep Pacific and tropical moisture will originate well   
   west of Hawaii, then track ENE to NE along the northwestern   
   periphery of a strong, stationary subtropical high parked off the   
   central California coast. This will act as the "train track"   
   directing moisture and any front associated with lows well to the   
   north into the same stretch of coast (Washington and Oregon) for   
   the next several days. The air mass will feature PWATs of up to   
   1.25 inches, which is above the 95th percentile of atmospheric   
   moisture for this part of the world in December. This is also 3   
   sigma above normal. In other words, there will be plentiful   
   moisture for the coastal mountains and the fronts to wring out into   
   the foothills and adjacent valleys in the form of rainfall. For   
   just this Day 2 period, expect a widespread 3-5 inches of rain in   
   the Slight Risk area, with the highest amounts into the highest   
   terrain. For portions of the northern Washington Cascades, where   
   most of the precipitation falls as snow, it will be measured in   
   feet. Note all of these statistics are for just this Day 2 period.   
   Expect several more rounds, albeit slightly weaker than this one,   
   to impact these same areas over the coming days.   
      
   Contrasting the heightened flooding threat will be that the area is   
   highly resistant to flooding, at least flash flooding, as the area   
   topographically is quite efficient at moving rainfall to the local   
   rivers and the ocean. Thus, the highest flooding threat is in the   
   urban areas such as Portland, Olympia and the eastern suburbs of   
   Seattle. Elsewhere flood-sensitive low lying areas will also be at   
   greater risk for localized flash flooding. Meanwhile, the rivers   
   may take a couple days of heavy rainfall to really respond and   
   potentially threaten bankside communities.   
      
   The Slight Risk area was largely unchanged with this update. It   
   should be noted there was a small northward shift in the latest 00Z   
   guidance, but so as to not flip-flop the forecast along the   
   Canadian border and the Juan de Fuca Strait, the Marginal Risk   
   there was left unchanged so as to allow for another cycle of model   
   guidance to determine whether the flood risk should be returned to   
   the Slight category in these areas. For the vast majority of the   
   area however, the flooding risk remains steady, with no major   
   changes to the overall rainfall forecast, just where some of the   
   heavier rainfall amounts may occur.   
      
   Wegman   
      
   Day 3   
   Valid 12Z Tue Dec 09 2025 - 12Z Wed Dec 10 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE   
   PACIFIC NORTHWEST...   
      
   ...21Z Outlook Update...   
      
   Only minimal changes are in place from the ongoing forecast   
   philosophy (see below). The Slight Risk area for western WA/OR is   
   still in place with no changes. 2-4 inch rainfall totals (with=20   
   locally higher amounts) are still expected into the Oregon coastal=20   
   ranges and far southern Washington Cascades. Slightly lower=20   
   rainfall totals are now depicted in most guidance across=20   
   southwestern Oregon and far northwestern California. The Marginal=20   
   Risk in place across parts of that area has been adjusted northward   
   as a result. Some consideration was given to a northward expansion   
   of Slight into the Olympics, but model spread/confidence was too   
   low for an adjustment for this forecast cycle.   
      
   An additional 1-2 inches of rainfall is expected across portions of   
   northeastern Oregon and much of Idaho through the early Wednesday.   
   Strong mid-level flow will create orographic ascent across those=20   
   areas - especially across windward portions of higher terrain. This   
   rainfall - along with local snowmelt at higher peaks - should=20   
   result in a few areas of excessive rainfall potential. A Marginal=20   
   Risk area has been added to account for this potential.   
      
   Cook   
      
   ...Previous Discussion...   
      
   The atmospheric river (A.R.) directing a plume of Pacific moisture   
   into the coast will shift a bit southward through much of the day   
   and the first part of Tuesday night. This will focus the heaviest   
   rainfall into Oregon, with a brief break from the steadiest heavy   
   rain across northern portions of Washington. While this will allow   
   for a break in the heaviest rainfall for northern Washington, it   
   will keep the flooding threat elevated for much of western Oregon,   
   including portions of southwestern Oregon, which will be largely   
   missed by Monday's rainfall. Rainfall amounts come down a touch   
   compared to Monday, with a broad 2-4 inches of new rain expected   
   into the Oregon coastal ranges as well as the Oregon and far   
   southern Washington Cascades. Given the rain that is likely to have   
   impacted this area on Monday, by Tuesday the slightly lesser amount   
   of new rain will be well offset by the higher stream, creek, and   
   river levels from the coast to the Western Cascades. Thus, the   
   inherited Slight Risk for this area remains on track. Due to the   
   aforementioned southward shift in location, the Slight was expanded   
   south further into west-central Oregon. However, the risk areas   
   were unchanged into Washington as the 1-3 inches of rain expected   
   there should still cause isolated to widely scattered instances of   
   flash flooding since it closely follows Monday's heavy rains over   
   much of the western half of the state. 50 kt orthogonal flow of   
   PWATs at or even a bit above 1.25 inches will still be nearly ideal   
   for producing upslope heavy rain on the west facing slopes of the   
   Oregon coastal ranges and Cascades, which will continue to support   
   the heavy rain threat.   
      
   The large subtropical high centered well off the California coast   
   will continue to be the steering mechanism for the atmospheric   
   river. This high will be nudged south to allow for the heavy rain   
   to primary impact Oregon on Tuesday, but this shift will be short-   
   lived as the high begins to advance back to the north as soon as   
   Tuesday night, which will gradually shift the atmospheric river   
   back into Washington.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!439YMNSeHWHurhLn7bYAdXu_6dIS4UaBavvs4-qFg1b5=   
   2eIEOAXeg6vTojmJZxQm0dvlYv7xFFJ2NtPHAg0RMaQKpPw$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!439YMNSeHWHurhLn7bYAdXu_6dIS4UaBavvs4-qFg1b5=   
   2eIEOAXeg6vTojmJZxQm0dvlYv7xFFJ2NtPHAg0RR_W4ZBY$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!439YMNSeHWHurhLn7bYAdXu_6dIS4UaBavvs4-qFg1b5=   
   2eIEOAXeg6vTojmJZxQm0dvlYv7xFFJ2NtPHAg0RgKtU0LI$=20   
      
   =3D =3D =3D   
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