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   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   315 AM EST Mon Dec 8 2025   
      
   Day 1   
   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...   
      
   ...Western Washington and Oregon...   
      
   The inherited Slight Risk across Western Washington and Oregon   
   remains little changed this morning. A hauling 150+ kt upper level   
   jet will bring a vigorous shortwave trough into British Columbia   
   north of the Canadian border Monday afternoon and evening. The   
   trailing front associated with the surface low that is being   
   strengthened by that shortwave will plow into the coast Monday   
   afternoon. Behind that disturbance, mid-level 850 mb flow will   
   rapidly increase out of the due west at at least 50 kt, while PWATs   
   also rapidly increase from 0.75 inches to 1.25 inches as the head   
   of the incoming atmospheric river (A.R.) makes its way ashore. On   
   the synoptic scale, the overall rainfall footprint will be greatest   
   today as compared with the expected rainfall over the next few   
   days. This in turn will set the stage for multiple days of flooding   
   concerns as the rain continues through much of the upcoming   
   workweek. For 24 hour rainfall totals, expect a widespread 4-6   
   inches of rain for this period for much of the upslope areas of the   
   Olympics/Coastal Ranges/Cascades. The typical downslope areas in   
   due westerly flow, which includes metro Seattle and Portland,   
   should be spared the heaviest rainfall, with totals in the 1-2 inch   
   range in those metros likely to be more common. Soils in these   
   areas are already near saturation, which represents an average   
   amount of soil moisture going into today's big rainfall event, so   
   at least the second half of the rainfall should largely convert to   
   runoff, leading to significant rises in streams, creeks, and   
   eventually the rivers that drain the Willamette and Puget Sound   
   areas.=20   
      
   The abundance of Pacific moisture that will be moving into the   
   Pacific Northwest with the atmospheric river is originating from   
   the tropics well west of Hawaii. This plume of moisture is moving=20   
   over multiple thousands of miles of Pacific Ocean before it reaches   
   the coast, which is further enhancing the amount of moisture=20   
   available to fall as rain once that moisture plume runs into the   
   wall of mountains that is the coastal ranges and the Cascades.   
   Strong subtropical high pressure centered off the California coast   
   is directing the moisture plume's south side around its   
   northwestern periphery, while strong Pacific lows are drawing the   
   moisture northeastward on the plumes north side. This is acting to   
   focus locally enhance the rainfall within the moisture   
   plume/atmospheric river, which is why the heavy rain is   
   concentrated into western Washington and Oregon.   
      
   Expect that there will be a bit more "slop-over" of rainfall onto   
   the immediate downwind side of the Cascades and Olympics, so a few   
   minor expansions of the Marginal were made to include some of the   
   east-facing slopes of the Cascades, and a Slight Risk area   
   expansion was made on the east side of the Olympics.=20   
      
   ...Northern Idaho and Adjacent Areas...   
      
   The inherited Marginal Risk was adjusted slightly to account for   
   small changes in the expected rainfall across interior areas of the   
   Pacific Northwest. The Marginal was trimmed from the Glacier N.P.   
   area due to most of the precipitation expected to fall as snow, but   
   expanded into far southeastern Washington and far northeastern   
   Oregon, where upslope potential will be increased in the terrain of   
   those areas. The A.R. will feature warm conditions, which will   
   raise snow levels to several thousand feet above ground level,   
   assuring that except in the highest elevations, the precipitation   
   type will be all rain, especially west of the Continental Divide.   
   While today should also bring the most rain to the area compared to   
   the upcoming several days, all of the soils in this area are nearly   
   saturated. In fact, recent colder weather and lower snow levels may   
   locally enhance the flash flooding threat once the heavier rain and   
   warmer temperatures move in as snowmelt becomes a contributing   
   factor to rising stream and river levels.=20   
      
   Wegman   
      
   Day 2   
   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...   
      
   ...Western Washington and Oregon...   
      
   The atmospheric river (A.R.) plume on Tuesday morning will be=20   
   aimed at the northwestern quadrant of Oregon, after the plume=20   
   shifts south down the coast Monday night. The A.R. will remain   
   across northwestern Oregon for much of the day Tuesday. Expect an   
   additional 3-5 inches of rain across the west facing slopes of both   
   the coastal ranges and the Oregon and far southern Washington   
   Cascades during the day Tuesday. This will have followed the   
   roughly 3-5 inches of rain expected Monday night. Thus, expect high   
   creek, stream, and river levels by Tuesday night after all that   
   rainfall, as well as low-lying and urban flooding impacts. The   
   Slight Risk area remains largely unchanged.=20   
      
   By Tuesday night, a much weaker low than Monday's will track   
   northeast into British Columbia. This low will draw the A.R. back   
   towards the north before its trailing cold front plows into the   
   coast. This will reignite the heavy rainfall across much of western   
   Washington State, while rain rates trail off across west-central   
   Oregon. Unfortunately the heavy rain will continue into the coastal   
   ranges and far northern Oregon Cascades through the period. Expect   
   another 1-3 inches of rain along the Columbia River Tuesday night.   
   Rain rates will be notably lower than on Monday due to the weaker   
   low and a slightly weaker onshore flow at 850 (40-50 kts). Due to   
   the return of steady rain across the Olympics and central Cascades   
   Tuesday night, the inherited Slight Risk area was expanded north to   
   include portions of those ranges and their foothills. Meanwhile,   
   the highest peaks of the Cascades should still remain cold enough   
   for mostly snow.=20   
      
   ...Northern Idaho and Adjacent Areas...   
      
   Weakening, but continuous 20-40 kt westerly flow at 850 across the   
   plains of eastern Washington will allow for locally heavy upslope   
   rain to continue into much of northern Idaho and northeastern   
   Oregon. The inherited Marginal is little changed, and mostly   
   reflects carving out the highest peaks expected to remain all or   
   mostly snow, while expanding into low lying areas where forecast   
   rainfall has increased. Regardless, snow levels will remain plenty   
   high enough to ensure all populated lower lying areas remain all   
   rain, and thus continue to pose a flooding threat.=20   
      
   Wegman   
      
   Day 3   
   Valid 12Z Wed Dec 10 2025 - 12Z Thu Dec 11 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE   
   PACIFIC NORTHWEST...   
      
   ...Western Washington and Oregon...   
      
   The Slight Risk across portions of the Pacific Northwest remains   
   largely unchanged with this update. A northwest to southeast   
   oriented jet stream will remain in place from southern British   
   Columbia through to the northern Plains. This will put the Pacific   
   Northwest in the favorable right entrance region of the jet. A   
   series of upper level shortwaves will zip down the jet, which in   
   turn will keep the atmospheric river (A.R.) plume in place and   
   barely moving through the entire period. Due to the flow   
   orientation changing to out of the west-northwest. The moisture   
   will flow largely unimpeded through the Strait of Juan de Fuca and   
   past the coastal/Olympic ranges and into the Cascades. Thus, the   
   rainfall footprint is much more heavily skewed towards the Cascades   
   getting much heavier rainfall than the windward side of the coastal   
   ranges. That said, it will hardly be dry along the coast. Expect an   
   additional 3-5 inches of rain in the foothills of all of the   
   Washington Cascades through the period, while the coastal ranges   
   can expect just 1-3 inches of new rainfall. Given this setup, it   
   appears the most likely day where a future upgrade may be needed to   
   the ERO risk areas will be on Wednesday. This of course will be in   
   no small part due to the antecedent rainfall in nearly all areas   
   from both Monday and Tuesday, compounding high water concerns day   
   by day.=20   
      
   ...Northern Idaho and Adjacent Areas...   
      
   A secondary peak of heavy precipitation into northern Idaho is   
   expected on Wednesday. The same A.R. impacting areas further west   
   will continue into the interior of the Pacific Northwest on   
   Wednesday as well. Renewed upslope flow into the west facing slopes   
   of the Rockies will re-enhance precipitation rates, as well as   
   raise snow levels after having crashed due to a brief cold air   
   intrusion on Tuesday. Thus, expect another round of snowmelt to go   
   along with the heavier rainfall rates. Just as further west, here   
   too the forecast will need to be monitored for a potential Slight   
   Risk upgrade with future updates. A large contributing factor to   
   the need for upgrades in both areas will be seeing what impacts   
   there are from the heavy rainfall expected especially today, but   
   possibly also again on Tuesday.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!5Hh95HCgjHgME5fYZqAQTC9cjVetXAub5kZzp6IkeDpq=   
   MMjWQr6F0GjkNWb5Ju-Jww0OoMGc79ExgdCCKsYKxVpgomY$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!5Hh95HCgjHgME5fYZqAQTC9cjVetXAub5kZzp6IkeDpq=   
   MMjWQr6F0GjkNWb5Ju-Jww0OoMGc79ExgdCCKsYKvXxlJfk$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!5Hh95HCgjHgME5fYZqAQTC9cjVetXAub5kZzp6IkeDpq=   
   MMjWQr6F0GjkNWb5Ju-Jww0OoMGc79ExgdCCKsYKT9_4zVw$=20   
      
   =3D =3D =3D   
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