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   TID: SBBSecho 3.28-Linux master/123f2d28a Jul 12 2025 GCC 12.2.0   
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   FOUS30 KWBC 100056   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   756 PM EST Tue Dec 9 2025   
      
   Day 1   
   Valid 01Z Wed Dec 10 2025 - 12Z Wed Dec 10 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE   
   PACIFIC NORTHWEST...   
      
   ...Western Oregon and Washington...   
      
   The atmospheric river event will continue across the Pacific=20   
   Northwest tonight as the next surge of heavy rain reaches=20   
   Washington. As of 00Z, steady light to moderate with occasional=20   
   heavy rain was ongoing across northern Oregon, but rates have=20   
   tapered off compared to earlier Tuesday morning. Ahead of an=20   
   approaching cold front, a plume of IVT with magnitudes in the=20   
   500-750 kg/m/s range along the northern Oregon coast will translate   
   north and increase in magnitude into the 800-1000 kg/m/s range=20   
   along coastal Washington into the southern portions of the Puget=20   
   Sound through Wednesday morning.   
      
   The probability of hourly rainfall in excess of 0.5 inches will=20   
   increase overnight, especially into the 06-12Z time frame across=20   
   the Olympics into the northern Cascades. 850-700 mb mean layer=20   
   winds of 50 to 60+ kt are expected across western Washington,=20   
   coupled with precipitable water values of roughly 1.2 to 1.4=20   
   inches. Snow levels will also increase with the surge of warmer air   
   lifting north, with all rain below elevations of 9,000 to 10,000=20   
   feet.=20   
      
   Peak 12 hour rainfall totals through 12Z Wednesday are forecast to   
   range between 2-4 inches from the Olympics into the northern=20   
   Cascades. The forecast rain rates and additional rainfall through=20   
   12Z carry an increased flood threat on area rivers/streams due to=20   
   roughly 5 to 10 inches of rain which have fallen over the past 3=20   
   days, most over the past 24-36 hours. In addition to potential=20   
   flooding of low-lying and urban locations, the potential will also=20   
   exist for landslides and debris flows due to the increase in=20   
   rainfall intensity overnight.   
      
   ...Northern Idaho and Adjacent Areas...   
      
   The Marginal Risk across northern Idaho into neighboring western=20   
   Montana, northeastern Oregon and southeastern Washington remains=20   
   unchanged with this update. Similar to coastal locations of the=20   
   Pacific Northwest, heavier rainfall intensities are expected in the   
   06-12Z time frame tonight. Hourly rainfall potential will increase   
   up to and beyond 12Z Wednesday with peak values generally under=20   
   0.3 inches, but the Blue Mountains of northeastern Oregon will have   
   potential for peak hourly rainfall of 0.3 to 0.4+ inches=20   
   overnight. Total peak 12 hour rainfall of 1 to 1.5 inches is=20   
   expected (close to 2 inches for the Blues) through 12Z with=20   
   snowmelt contributing to rising streams and rivers. Given increased   
   soil moisture due to antecedent rainfall, localized impacts of=20   
   heavy rain will remain possible.   
      
   Otto   
      
      
   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...   
      
   2100 UTC update: No significant changes made to the previous   
   outlook areas across the coastal Pac NW and Northern Rockies. Edits   
   to the previous areas were mostly to better fit latest qpf and   
   snowfall forecasts.   
      
   Oravec   
      
   ...Previous discussion...   
      
   ...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...   
      
   2100 UTC update: The previous slight risk area over northern ID was   
   decreased in size to better fit the latest day 3 qpf. The previous   
   marginal risk area was removed along the WA coast, into far   
   northern OR coast based on the latest light day 3 qpf across these   
   areas. The marginal risk was confined to the upslope area of the WA   
   Cascades.   
      
   Oravec   
      
   ...Previous discussion...   
      
      
   ...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!-Wbn7Te9c7pbBzrByKOqJyHAYVemc2ymNmLY572lvAKF=   
   MTDZJ6pXQLP8DN4lfA7vfdOMDNtTDBNHvmYsp7tJQzZABko$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!-Wbn7Te9c7pbBzrByKOqJyHAYVemc2ymNmLY572lvAKF=   
   MTDZJ6pXQLP8DN4lfA7vfdOMDNtTDBNHvmYsp7tJQfSA_w4$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!-Wbn7Te9c7pbBzrByKOqJyHAYVemc2ymNmLY572lvAKF=   
   MTDZJ6pXQLP8DN4lfA7vfdOMDNtTDBNHvmYsp7tJalJUwbk$=20   
      
   =3D =3D =3D   
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