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   FOUS30 KWBC 091550   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   1050 AM EST Tue Dec 9 2025   
      
   Day 1   
   Valid 16Z Tue Dec 09 2025 - 12Z Wed Dec 10 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE   
   PACIFIC NORTHWEST...   
      
   16z Update: Forecast still looks on track so only minimal changes   
   needed to the inherited risk areas. Impacts should ramp up after=20   
   00z as the atmospheric river lifts back north and into western WA=20   
   and IVT magnitudes increase.   
      
   Chenard   
      
   ...Previous Discussion...=20   
      
   ...Western Oregon and Washington...   
      
   The atmospheric river (A.R.) event is well underway across the   
   Pacific Northwest this morning. The plume of anomalous moisture   
   associated with the A.R. is currently aligned across northwestern   
   Oregon. Radar indicates rainfall rates are between 1/4 inch and 1/2   
   inch per hour. As is fairly typical of most A.R.s, the impact from   
   the rain isn't necessarily from the instantaneous rates from heavy   
   rain, but rather the long duration of steady rain, which can be   
   heavy at times on the windward (west-facing) sides of the coastal   
   ranges and the Cascades. A strong and vertically stacked   
   subtropical high is parked well off the California coast.   
   Meanwhile, frequent lows are tracking northeast across the   
   northeastern Pacific and slamming into the Canadian coast. The   
   trailing cold fronts are tapping into the plume of moisture that is   
   riding the gradient between the strong high to the south and the   
   lows to the north.   
      
   A Pacific low moved into the British Columbia coast earlier this   
   evening. This shoved the A.R. to its current position into   
   northwestern Oregon. As the next low tracks a little further north,   
   the subtropical high will begin ridging towards the north in   
   response. This will push the A.R. north with time. A moving A.R. is   
   usually a very good thing, as it reduces the time it's heavily   
   raining in any one area. Unfortunately this time, it will simply   
   move the A.R. back to the already hard hit areas of western   
   Washington that were hard hit yesterday. This northward shift is   
   expected this evening as the low moves ashore near Haida Gwaii, or   
   just south of the southern Alaska Panhandle. As the trailing front   
   follows into the Washington coast tonight, rainfall rates will   
   increase significantly as the A.R. aligns into Washington. This   
   will allow the rain rates to diminish substantially in most of   
   Oregon, though light rain near the Columbia River is likely to   
   continue to some much lesser degree. The A.R. is characterized by   
   PWATs to 1.25 inches, which is between 2 and 3 sigma above normal   
   for this time of year. Orthogonal westerly flow into the mountains   
   at 40-50 kt, which is plenty strong enough for substantial upslope   
   enhancement.   
      
   In Oregon, expect the heavy rain, ongoing at present, to continue   
   for much of the day, while gradually shifting north and weakening.   
   In Washington the heaviest rain will be overnight tonight. The   
   Slight Risk area was expanded north with this update to include all   
   of the Olympic Peninsula as well as for a portion of the foothills   
   of the Cascades east of Seattle. The surrounding Marginal was   
   trimmed on the southern end in central Oregon as the trend   
   throughout the period will be a northward readjustment of the A.R.   
      
   ...Northern Idaho and Adjacent Areas...   
      
   The Marginal Risk into northern Idaho, a small sliver of far   
   western Montana, and the northeast corner of Oregon is largely   
   unchanged with this update. Over the last 24 hours, the area has   
   seen anywhere from 1-1.5 inches of rain. At least that much is   
   expected over many of the same areas of the Sawtooths, Bitterroots,   
   and west into northeastern Oregon. Since the A.R. is originating   
   from the tropics, in addition to the anomalous moisture, much   
   warmer air will also accompany the rain, causing snow levels to   
   rise. Expect a significant snowmelt contribution to the rising   
   streams and rivers as a result. A Slight Risk upgrade may become   
   necessary if rainfall forecasts increase further through the day.   
   NASA Sport soil moisture analyses show that soils all across   
   northern Idaho are at saturation, so all new rainfall today will   
   convert to runoff. The impact of this runoff is unclear, hence the   
   possibility of a Slight Risk upgrade.   
      
   Wegman   
      
   Day 2   
   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...   
      
   The wettest day of the next 3 will be on Wednesday across much of   
   western Washington. The continuous westerly flow of 40-50 kt at 850   
   will continue off the Pacific through the day. A vigorous jet   
   streak to 150 kt will remain just north of Washington before diving   
   southeastward across the upper high Plains. This will keep   
   Washington in the favorable right entrance region. A series of lows   
   will barrel into the British Columbia coast, each one keeping the   
   A.R. plume nearly stationary and highly energetic. Thus, widespread   
   4-6 inch rainfall totals are expected for the Olympics and the west   
   facing foothills of the Washington Cascades. As with most A.R.s,   
   very high snow levels will keep any snow confined to the highest   
   peaks of the far northern Cascades. This will keep the lion's share   
   of the expected precipitation as heavy rain for most of the   
   Cascades and certainly all of the coastal ranges as well. This   
   being the third day of more-or-less steady rainfall across much of   
   Washington, especially south of the Puget Sound area means the   
   soils are well past saturated, and all of the precipitation should   
   convert to runoff. Further, the high snow levels will promote   
   snowmelt as well, which will only further raise river levels.   
   Finally, the lows moving into the British Columbia coast will turn   
   the surface flow out of the WNW for a time, especially Wednesday   
   night. This will direct the flow straight down the strait of Juan   
   the Fuca and put the low-level flow just slightly off from   
   orthogonal to the mountains, favoring a slight northerly flow of   
   moisture down the foothills and the Puget Sound Region. This should   
   temper the downsloping side east of the mountains just a bit,   
   allowing for more rainfall into Seattle, Olympia, and the Puget   
   Sound region broadly. Thus, Wednesday and Wednesday night will also   
   be the wettest day in the urban cities as well.   
      
   Given all of the above, the inherited Slight Risk was expanded with   
   this update to include the Seattle metro and the surrounding areas   
   around southern Puget Sound with this update. A higher end Slight   
   was expanded south down the Cascades to near the Columbia   
   River/Oregon state line. Once the impacts from all the rainfall   
   seen across Western Washington become more clear, it will be easier   
   to ascertain what the impacts from this round of heavy rain on   
   Wednesday are likely to be. A Moderate Risk upgrade may be needed   
   with future updates or all or a portion of the Cascades and western   
   foothills with future updates.   
      
   ...Northern Idaho and Far Western Montana...   
      
   The atmospheric river (A.R.) impacting Western Washington on   
   Wednesday will continue inland and impact the Rockies of northern   
   Idaho and western Montana as well on Wednesday. Just like further   
   west, the A.R. will cause snow levels to rise to as high as 8,000   
   ft. This will introduce a significant snowmelt component to the   
   resulting runoff from the upslope rainfall. Over the last 24 hours,   
   the area has picked up between 1-2 inches of rain, causing soils   
   to become fully saturated. Today, expect even more rain, with 2-3   
   inch totals expected to be common. The steady rain, snowmelt, and   
   abnormally warm conditions will increase the risk for avalanches in   
   the prone areas, especially where a heavy wet snow falls on top of   
   much weaker snow layers. Also expect an increase in rock and   
   landslides, as well as rising streams, creeks, rivers, and   
   localized urban flooding. Given all these expected impacts, a large   
   portion of northern Idaho and far western Montana were upgraded to   
   a Slight Risk with this update, and a smaller portion of northern   
   Idaho along the Montana state line from I-90 south is in a higher   
   end Slight Risk, as this area is expected to see the highest   
   rainfall amounts to around 3 inches through the period, almost all   
   of which falling as rain.   
      
   Wegman   
      
   Day 3   
   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 and Western Montana...   
      
   A double upgrade from no risk area to a Slight Risk was introduced   
   for a portion of northern Idaho, generally south of I-90 and along   
   the Montana state line with this update. This upgrade was   
   introduced largely as a continuation of heavy rain from Wednesday   
   night across this region. Rainfall rates should be gradually   
   diminishing through the day Thursday, and as far as the heaviest   
   rain and immediate impacts, the rain on Wednesday should be more   
   impactful in this region. However, given the steady rain continues   
   into the day Thursday, the impacts resulting therefrom should also   
   continue. Meanwhile further east into Montana, a strong Arctic   
   front pushing south down the Plains should turn all of the   
   precipitation over central Montana over to snow. Thus, the flooding   
   threat is largely confined to northern Idaho and far western   
   Washington where the warm air associated with the A.R. will remain   
   in place on Thursday. Rock slides, avalanches, low-lying flooding,   
   sporadic urban impacts, and very high river levels will all be   
   ongoing Thursday morning over this area, supporting the Slight Risk   
   upgrade.   
      
   ...Western Washington...   
      
   A Marginal Risk was expanded mainly for the Thursday morning time   
   period across much of western Washington with this update. This   
   expanded Marginal Risk includes the Seattle metro and the Puget   
   Sound area more broadly. As with further east, it will be a   
   continuation of the steady rain from Wednesday night into Thursday   
   across the area. It's likely a Slight Risk upgrade will be needed,   
   as the rain looks to take its time until Thursday afternoon to   
   significantly diminish in intensity. The A.R. event broadly will be   
   diminishing through this period. A large low well out over the   
   Pacific will take a more northward track, as compared to its   
   predecessors, as the pattern amplifies. This will effectively cut   
   off the A.R. as the moisture plume gets directed north towards the   
   Gulf of Alaska, and therefore, the flow with the A.R. shifts north   
   into British Columbia and substantially diminishes.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!_Vu226vQNWjBQ27w1LIgtkJbBI16kqPVgA8hzKAR8axw=   
   uoLgN9A7Uoa0Y78bZYnB1cOb2anC_N4l_a2yiWoB6A076YY$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!_Vu226vQNWjBQ27w1LIgtkJbBI16kqPVgA8hzKAR8axw=   
   uoLgN9A7Uoa0Y78bZYnB1cOb2anC_N4l_a2yiWoBS2VV8MU$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!_Vu226vQNWjBQ27w1LIgtkJbBI16kqPVgA8hzKAR8axw=   
   uoLgN9A7Uoa0Y78bZYnB1cOb2anC_N4l_a2yiWoBv26NIms$=20   
      
   =3D =3D =3D   
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