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   FOUS30 KWBC 061947   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   247 PM EST Sat Dec 6 2025   
      
   Day 1   
   Valid 16Z Sat Dec 06 2025 - 12Z Sun Dec 07 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Cook/Wegman   
      
      
   Day 2   
   Valid 12Z Sun Dec 07 2025 - 12Z Mon Dec 08 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Orrison   
      
      
   Day 3   
   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...   
      
   The 06Z/12Z model guidance suggests a slightly more suppressed=20   
   axis of greater IVT magnitudes impacting the Pacific Northwest on=20   
   the front end of the upcoming atmospheric event for the region. The   
   NAM/GFS/ECMWF solutions though still support very strong warm air=20   
   advection with enhanced IVT magnitudes reaching 800 to 1000+ kg/m/s   
   across the western WA and northwest OR coastal ranges with the=20   
   first wave that arrives in the 12Z/08 to 00Z/09 time frame. As=20   
   strong surface low pressure and associated shortwave/jet energy=20   
   crashes into British Columbia, the southern flank of the attendant=20   
   cold front will slow as it is crosses WA state and this should=20   
   allow for a relatively longer persistence of greater IVT values=20   
   across southern WA and northern OR going into the 00Z to 12Z/09=20   
   time period.=20   
      
   The deeper layer moisture fetch will have a trans-Pacific origin=20   
   that extends down to just west of Hawaii, and satellite data would   
   suggest a likelihood for tropical and subtropical moisture feeds=20   
   into this atmospheric river event which will drive very efficient   
   and high rainfall rates that may reach well into the 0.50" to   
   1"/hour range near the periods of max IVT and deeper layer ascent. The   
   coastal ranges in particular across southwest WA and northwest OR=20   
   along with the west-facing slopes of the Cascades farther inland=20   
   should see enhanced rainfall totals that reach as high as 4 to 6+=20   
   inches which will heavily aided by deep layer orthogonal flow=20   
   relative to the terrain.   
      
   Given the subtle southward shift in the axis of greater IVT=20   
   magnitudes and overall higher rainfall rates/totals for the=20   
   northern parts of the Olympic Peninsula and the northern WA=20   
   Cascades, the Slight Risk for this area was trimmed just a tad,   
   with very little to no change elsewhere. This is consistent with=20   
   the overall 06Z/12Z guidance QPF footprint. Regardless, the wet=20   
   antecedent conditions and heavy rainfall totals are expected to=20   
   drive concerns for locally enhanced areal and riverine flooding,=20   
   with urban flooding also likely to become a concern. A threat for=20   
   landslides will exist, especially toward the end of the period and   
   thereafter as additional upstream atmospheric river=20   
   activity/moisture transport arrives with more heavy rainfall.   
      
   Orrison   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!9aFOFjXyp5fO2yGDbmAc8SgvmIuD4o_8YXvCRfBrrsRc=   
   Wnn6gPdSZxHnwDSD66SCcJaPG3_6KNsEaRgA3xFh9myQhxI$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!9aFOFjXyp5fO2yGDbmAc8SgvmIuD4o_8YXvCRfBrrsRc=   
   Wnn6gPdSZxHnwDSD66SCcJaPG3_6KNsEaRgA3xFhRlT2zME$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!9aFOFjXyp5fO2yGDbmAc8SgvmIuD4o_8YXvCRfBrrsRc=   
   Wnn6gPdSZxHnwDSD66SCcJaPG3_6KNsEaRgA3xFhunykWzE$=20   
      
   =3D =3D =3D   
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