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   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   1040 AM EST Thu Dec 11 2025   
      
   Day 1   
   Valid 16Z Thu Dec 11 2025 - 12Z Fri Dec 12 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   WESTERN WASHINGTON AS WELL AS FOR NORTHERN IDAHO INTO FAR WESTERN   
   MONTANA...   
      
   ...16Z Outlook Update...   
   The ongoing forecast is on track. Refer to Mesoscale Precipitation   
   Discussion #1262 for shorter-term updates on precipitation/flash   
   flood threats across western Washington State. Refer to the   
   discussion below for more details on flood/flash flood potential in   
   Idaho and Montana today.   
      
   Cook   
      
   ...Previous Discussion...   
      
   ...Western Washington...   
      
   The multi-day atmospheric river (A.R.) event of much of the past   
   week will finally conclude across western Washington today.   
   Northward-building ridging both at the surface and at the jet   
   stream level will push the remaining moisture plume from the   
   already weakening A.R. north into Canada, leaving much of western   
   Washington with residual moisture from the weakening and drying   
   westerly flow. Heavy rain is ongoing across much of western   
   Washington, roughly from Tacoma's latitude north. CAMS guidance   
   shows the rain should progress a bit further south over the next   
   few hours as a cold front offshore pushes inland. After that it   
   will stall out, and the rain rates will diminish in place. Thus,   
   the heaviest rain over the area for this period will be over the   
   next few hours this morning, followed by tapering through the   
   afternoon and into tonight.   
      
   Record and in some areas historic flooding is ongoing from the   
   multiple consecutive days of heavy rain with the A.R. Comparatively   
   speaking, today's rainfall totals won't be anything near previous   
   days' totals. Nevertheless, the ongoing moderate to heavy rain   
   across northern Washington will continue to keep river levels up,   
   or even rising through the morning until rain rates can diminish a   
   bit. The Marginal Risk therefore is a compromise between the   
   ongoing highly impactful flooding and steady rain expected to   
   continue into the morning, and the fact that the second half to   
   two-thirds of the period should feature only light rain with the   
   conclusion of the A.R. event. That is to say that for all of the   
   Marginal Risk area, ongoing major flooding will continue through   
   today despite the diminishing rain, and precautions and avoidance   
   of flooded areas should be taken. The Marginal Risk may be able to   
   be dropped with the evening update.   
      
   ...Northern Idaho and Far Western Montana...   
      
   Continuous steady rain is expected to continue through the entire   
   period across the Marginal Risk area in northern Idaho and far   
   western Montana today. While the A.R. is ending out west, it will   
   take longer to do so for Idaho and Montana as the forcings   
   supporting the steady rain will persist throughout the period.   
   Despite the heavy rain on the order of 1-2 inches over the past 24   
   hours, in addition to the multiple inches that fell in this same   
   area in days prior, impacts from flooding have been minimal across   
   this region. Now, with today being the tail end of the A.R., it's   
   probable that if flooding impacts were to occur, they would happen   
   today. Thus, balancing these two offsetting factors, and in   
   coordination with OTX/Spokane, WA forecast office, the inherited   
   Slight Risk for this region was downgraded. Isolated instances of   
   flash flooding are still possible given the expectation of another   
   24 hours or so of steady rain into the west facing slopes of the   
   Bitterroots. However, given the lack of impacts thus far, it seems   
   probable that the same trends will continue through tonight. Steady   
   precipitation should taper off soon after the 12Z Friday end time   
   for the period, as an Arctic air mass further east also works to   
   transition any remaining rain over to snow.   
      
   Wegman   
      
   Day 2   
   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 3   
   Valid 12Z Sat Dec 13 2025 - 12Z Sun Dec 14 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!7rBhq94JwtEik5j7ztY0pOyPCUS1WZfJuOizydDdYTAJ=   
   uk4DXBPBEljRmcSPGB--84qG3QBnHXasWtcMrpOfRQPRwxo$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!7rBhq94JwtEik5j7ztY0pOyPCUS1WZfJuOizydDdYTAJ=   
   uk4DXBPBEljRmcSPGB--84qG3QBnHXasWtcMrpOf_VdDb5M$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!7rBhq94JwtEik5j7ztY0pOyPCUS1WZfJuOizydDdYTAJ=   
   uk4DXBPBEljRmcSPGB--84qG3QBnHXasWtcMrpOfpk_LSAA$=20   
      
   =3D =3D =3D   
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