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   FOUS30 KWBC 150832   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   332 AM EST Mon Dec 15 2025   
      
   Day 1   
   Valid 12Z Mon Dec 15 2025 - 12Z Tue Dec 16 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF   
   WASHINGTON AND OREGON TODAY AND TONIGHT...   
      
   Overall...the changes were fairly minor as yesterday's Day 2=20   
   Excessive Rainfall Outlook propagated into today's Day 1 outlook.=20   
   There were a few run to run differences in the deterministic and=20   
   probabilistic runs...especially over northeast Washington- northern   
   Idaho and northwest Montana. The broad forecast reasoning did not=20   
   fundamentally shift.=20   
      
   The areal coverage of rainfall expands and rain rates increase   
   early today as a well defined atmospheric river continues to=20   
   direct abundant moisture into the Washington coastal ranges and=20   
   Cascades. IVT values in excess of 700 kg per meter per second early   
   today will be spreading inland but weaken as the axis makes its way   
   inland and as the axis of best moisture transport gets directed   
   southward into Oregon. Given antecedent conditions...the forecast   
   of another 3 or 4 inches in the terrain raises concerns of worsening   
   any on-going flooding and result in new areas. In   
   addition...considering how water-logged the soils became in the   
   recent heavy to excessive rainfall event...the potential is there=20   
   for additional landslides or mudslides. Latest guidance still   
   showed the nose of the moisture plume getting as far east as parts   
   of eastern Washington....northern Idaho and northwest Montana where   
   a second and more focused Marginal Risk area remained. The focus=20   
   of the heavy rainfall should gradually shift southward into an area   
   that did not receive as much rainfall from the previous atmospheric   
   river and has had a chance to drain off some of that water.=20=20   
      
   Bann   
      
   Day 2   
   Valid 12Z Tue Dec 16 2025 - 12Z Wed Dec 17 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF   
   THE PACIFIC NORTHWEST...   
      
   Maintained the previously issued Marginal Risk area from=20   
   Washington southward along the Cascades and the coastal ranges into   
   northwest California on Tuesday and early Wednesday. Latest=20   
   deterministic and ensemble guidance showed the highest QPF in the=20   
   complex terrain...rivaling the QPF on Day 1 in some places in=20   
   Washington. The main reason for holding with a Marginal Risk area=20   
   was due to the lowering snow-levels given post-frontal lowering of=20   
   heights/thickness values that result in snow. Even so...did expand=20   
   the eastern boundary of the Marginal Risk area a bit to the east in   
   Washington and a portion of nearby Oregon. While this resulted in=20   
   some additional overlap with snow areas...the idea was to capture=20   
   some of the higher QPF amounts from regions where (presumably) the=20   
   precipitation started as rain and then transitioned to snow. Tried=20   
   to avoid overlap with the Winter Weather Desk snow amounts of 6=20   
   inches or greater.=20   
      
   The overall synoptic pattern continued to show the deepest moisture   
   and broadest coverage of rainfall arriving in association with the   
   surface cold front around 17/00Z and the diminishing QPF as the   
   area gets whisked eastward.   
      
   Bann   
      
   Day 3   
   Valid 12Z Wed Dec 17 2025 - 12Z Thu Dec 18 2025   
      
   ...THE MARGINAL RISK OF EXCESSIVE RAINFALL AREA LINGERS OVER PARTS   
   OF THE PACIFIC NORTHWEST. ON WEDNESDAY...   
      
   There should be a relative lull in activity across parts of   
   Washington...Oregon and northern California in the wake of a system   
   on Tuesday into early Wednesday. Even though there may still be   
   some rainfall that lingers into Day 3...the Marginal Risk area was   
   maintained in deference to the approach of the next and seemingly   
   stronger AR on Day 4 and to account for the possibility that the=20   
   guidance could be too slow with the onset of rainfall with this   
   system. Refer to guidance from the WPC Medium Range Desk for   
   additional info. Even if the rainfall associated with the next   
   system does not arrive prior to 12Z Thursday as shown by the   
   models...any lingering rainfall still has the potential for be   
   impactful in the most hydrologically sensitive areas.   
      
   Bann   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!5EjH_1ZQ6D4ZDlLOoXLMyMgTdOjxIkqnF7_YFDK5M9TZ=   
   825cFnHr32W5bL_XuxNbiCHyt3aHmrRJbPd6I8b_dJcEJ6Q$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!5EjH_1ZQ6D4ZDlLOoXLMyMgTdOjxIkqnF7_YFDK5M9TZ=   
   825cFnHr32W5bL_XuxNbiCHyt3aHmrRJbPd6I8b_FUXIHa0$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!5EjH_1ZQ6D4ZDlLOoXLMyMgTdOjxIkqnF7_YFDK5M9TZ=   
   825cFnHr32W5bL_XuxNbiCHyt3aHmrRJbPd6I8b_IPAtlc0$=20   
      
   =3D =3D =3D   
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