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   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   330 AM EST Wed Dec 10 2025   
      
   Day 1   
   Valid 12Z Wed Dec 10 2025 - 12Z Thu Dec 11 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF WESTERN   
   WASHINGTON AS WELL AS FOR PORTIONS OF NORTHERN IDAHO AND FAR   
   WESTERN MONTANA...   
      
   ...Western Washington...   
      
   An atmospheric river is currently impacting much of Washington   
   State this morning. The narrow but intense plume of deep tropical   
   moisture originating from near Hawaii is being drawn northeastward   
   by a strong subtropical high that is parked off the California   
   coast. This area of high pressure is forming the southeast side the   
   atmospheric river. Meanwhile, polar high pressure over the   
   Aleutians is sending copious Arctic air well to the south through   
   the North Pacific. This cold air has set up a strong baroclinic   
   boundary as it runs into the warm air associated with the   
   subtropical high. Finally, a strong area of low pressure, fed in   
   part by this same baroclinicity, will move into the British   
   Columbia coast today. The low and it's trailing cold front moving   
   into the coast is adding substantial lift to the atmosphere where   
   the A.R. is located in western Washington. The result of these   
   various forcings is a "hose" of deep tropical moisture aligned   
   right into Washington State, then continuing inland through Idaho   
   and Montana. The low and its front are locally enhancing the rain   
   rates, especially on the upwind/west-facing slopes of the coastal   
   ranges and the Cascades. This latest round of heavy rain is coming   
   after 2+ days of nearly steady rainfall into this same region.   
   Saturated soils, full rivers, and abundant additional rainfall are   
   all contributing to flooding concerns through this area.   
      
   Heavy rain is likely to continue for much of the day, gradually   
   diminishing in intensity tonight, but unlikely to stop completely   
   most anywhere in western Washington. Since the forecast is little   
   changed, particularly with expected rainfall amounts, the Slight   
   Risk area is also little changed. A higher end Slight remains in   
   effect for portions of the western slopes of the Cascades. Very   
   high snow levels between 8,500 and 9,000 ft are keeping all but the   
   northernmost Cascades as rain. A wedge of colder air trapped in the   
   northern Cascades are locally keeping snow levels there down closer   
   to 6,500-7,000 ft. The higher end Slight includes much of the I-90   
   corridor between Seattle and Snoqualmie Pass. Land and rockslides   
   remain a concern, and numerous Flood Warnings remain in effect for   
   much of the western slopes of the Cascades.   
      
   ...Northern Idaho and Far Western Montana...   
      
   The Slight Risk of excessive rainfall also remains in effect for   
   much of the northern Idaho Panhandle. This includes the I-90   
   corridor roughly between Coeur d'Alene on the west side to Missoula   
   on the east side. The strong atmospheric river moving in from the   
   Pacific will drop the lion's share of its moisture into the coastal   
   ranges and Cascades. However, plentiful leftover moisture will then   
   continue east and be re-lifted into the Bitterroots of Idaho and   
   Montana. While rainfall amounts will be tempered greatly compared   
   to areas further west, they will still be significantly impactful   
   for this region, especially when considering it's been raining   
   off-and-on in this region from this same atmospheric river for the   
   past 48-72 hours. Thus, soils are saturated and rivers are already   
   running high in this area. All additional rainfall will convert to   
   runoff, both worsening and prolonging any resultant flooding   
   impacts. The area south of I-90 and east of Lewiston with west-   
   facing slopes will be hardest hit. It is in this area that a higher   
   end Slight remains in effect.=20   
      
   The heaviest rain across this region will be through the day today,   
   with some modest diminishing of rainfall rates through tonight.   
   Regardless, most upslope areas should see steady rainfall   
   throughout this period.=20   
      
   Wegman   
      
   Day 2   
   Valid 12Z Thu Dec 11 2025 - 12Z Fri Dec 12 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF   
   NORTHERN IDAHO...   
      
   ...Northern Idaho...   
      
   Thursday will be the final day of the atmospheric river across all   
   of the Pacific Northwest, including into Idaho. A fast moving   
   shortwave trough diving southeast in the jet streak will   
   reinvigorate the heavy rain across northern Idaho and western   
   Montana during the day Thursday. This shortwave will also usher in   
   an Arctic front to the east of the Rockies across much of the rest   
   of Montana. This much colder and drier air will finally allow   
   conditions to dry out as upper level ridging shoves what's left of   
   the A.R. northward into Canada, and a large low well out over the   
   Pacific cuts off the tropical feed as it moves towards Alaska. The   
   ceasing of tropical moisture will effectively end the heavy rain   
   threat everywhere, though lingering flooding is likely to continue   
   in this area for multiple days to come.=20   
      
   Heavy rain will likely be ongoing across northern Idaho at the   
   start of the period on Thursday morning. A surface low will form   
   along the advancing Arctic air, which will draw the Arctic front   
   south across much of Montana Thursday night, turning what moisture   
   is left over to snow east of the range, while putting northern   
   Idaho into the "dry slot", ending the heavy rainfall. While rain   
   rates may stay elevated for much of the day Thursday across   
   northern Idaho, the Slight Risk is at least equally for the ongoing   
   impacts going into the period from the heavy rain that is likely to   
   occur tonight. Rock and mudslides will remain a threat until   
   conditions begin to dry out Thursday night, as well as a   
   continuation of both ongoing flooding and potentially some new   
   flooding as the rain ends across the area.   
      
   ...Western Washington...   
      
   A Marginal Risk remains in place across portions of Western   
   Washington, specifically the foothills of the Cascades on Thursday.   
   The Marginal Risk reflects the end of the heavy rainfall from the   
   A.R. from Wednesday and Wednesday night...i.e. diminishing rainfall   
   rates...and the likely much higher impacts that will be ongoing by   
   then along all of the Washington Cascades. Thus, expect Slight   
   level impacts over this corridor to continue, but the heavy rain   
   ending should spell the end of new or worsening impacts going   
   forward. The Marginal attempts to strike a balance between these   
   two competing concerns.   
      
   Meteorologically, the A.R. will still be in full swing at the start   
   of the period 12Z Thursday, but as the forcings from inland lows,   
   rapidly weakening westerly flow orthogonal to the ranges, and   
   surface high pressure builds in from the south, rain rates will   
   rapidly diminish through the morning, ending entirely for many   
   areas of western Washington by Thursday night. Since the tropical   
   flow responsible for the A.R. will be cut off by Thursday night,   
   this will be the end of the event for the area, and a much needed   
   and "mostly" dry period of weather will follow across most of   
   western Washington, and those areas that do see additional rain   
   into the weekend will find it will be a fraction of the intensity   
   that this A.R. produced, and should be essentially non-impactful.=20   
      
   Wegman   
      
   Day 3   
   Valid 12Z Fri Dec 12 2025 - 12Z Sat Dec 13 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!_YfyAo0OJ2BkiXnw9JHmGbYkyINKLxAxi8nIYf6WgAER=   
   X1JQoz1XdJN5uJNhDcUNETOQX1eTe6ypuWr94iIwy2dfiz4$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!_YfyAo0OJ2BkiXnw9JHmGbYkyINKLxAxi8nIYf6WgAER=   
   X1JQoz1XdJN5uJNhDcUNETOQX1eTe6ypuWr94iIwV1wvoBU$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!_YfyAo0OJ2BkiXnw9JHmGbYkyINKLxAxi8nIYf6WgAER=   
   X1JQoz1XdJN5uJNhDcUNETOQX1eTe6ypuWr94iIwcbVZHqI$=20   
      
   =3D =3D =3D   
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