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   FOUS30 KWBC 152016   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   316 PM EST Mon Dec 15 2025   
      
   Day 1   
   Valid 16Z 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...   
      
   ...16Z update...   
      
   IVT values approaching 1000 kg/m/s were found offshore and to the   
   coastline of southern Washington/northern Oregon between 12-15Z   
   this morning with 60-70 kt of flow in the 850-700 mb layer. Peak   
   3-hr rainfall of 1.0 to 1.3 inches was observed for the Olympics   
   and isolated spots within the northern Washington Cascades ending   
   15Z. IVT values will drop off across western Washington/Oregon   
   through the day today as the moisture axis pushes east and south,   
   out ahead of a cold front. Snow levels will also fall behind the   
   cold front to about 6000 ft by 00Z and lower to near 3000 ft for   
   the northern Cascades by 12Z Tuesday.   
      
   The Slight Risk was maintained given the wet antecedent conditions   
   and ongoing rainfall. The potential will exist for an additional 2   
   to 3 inches of rain through 12Z (maximum values), although rain   
   rates will be much lower than this morning with occasional 0.25+   
   in/hr rates and perhaps an isolated spot with 0.5 in/hr where weak   
   instability combines with strong left-exit jet stream forcing and   
   continued onshore flow.   
      
   Elsewhere, the forecast remains on track with a surge of heavier   
   rainfall likely to impact portions of the northern Rockies over the   
   next 3-6 hours followed by locally high rain rates in excess of   
   0.5 in/hr for the coast of Oregon into northern California where   
   the Marginal Risk was maintained.   
      
   Otto   
      
   ...previous discussion follows...   
      
      
   Overall...the changes were fairly minor as yesterday's Day 2   
   Excessive Rainfall Outlook propagated into today's Day 1 outlook.   
   There were a few run to run differences in the deterministic and   
   probabilistic runs...especially over northeast Washington- northern   
   Idaho and northwest Montana. The broad forecast reasoning did not   
   fundamentally shift.   
      
   The areal coverage of rainfall expands and rain rates increase   
   early today as a well defined atmospheric river continues to   
   direct abundant moisture into the Washington coastal ranges and   
   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   
   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   
   of the heavy rainfall should gradually shift southward into an area   
   that did not receive as much rainfall from the previous=20   
   atmospheric river and has had a chance to drain off some of that=20   
   water.   
      
   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   
   Washington southward along the Cascades and the coastal ranges into   
   northwest California on Tuesday and early Wednesday. Latest   
   deterministic and ensemble guidance showed the highest QPF in the   
   complex terrain...rivaling the QPF on Day 1 in some places in   
   Washington. The main reason for holding with a Marginal Risk area   
   was due to the lowering snow-levels given post-frontal lowering of   
   heights/thickness values that result in snow. Even so...did expand   
   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   
   some additional overlap with snow areas...the idea was to capture   
   some of the higher QPF amounts from regions where (presumably) the   
   precipitation started as rain and then transitioned to snow. Tried   
   to avoid overlap with the Winter Weather Desk snow amounts of 6   
   inches or greater.   
      
   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.   
      
   Cook/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   
   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.   
      
   Cook/Bann   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!41TtQ_L5IEVVj3LCzu9o5i1FvXVLhZiCRv-i15hU1NRB=   
   fnSkYcArsrIeQm1axtdO5Jkku8zkOeB6IYpu9uUY916pxF0$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!41TtQ_L5IEVVj3LCzu9o5i1FvXVLhZiCRv-i15hU1NRB=   
   fnSkYcArsrIeQm1axtdO5Jkku8zkOeB6IYpu9uUY5AT1nGw$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!41TtQ_L5IEVVj3LCzu9o5i1FvXVLhZiCRv-i15hU1NRB=   
   fnSkYcArsrIeQm1axtdO5Jkku8zkOeB6IYpu9uUYYhOufH0$=20   
      
   =3D =3D =3D   
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   --- SBBSecho 3.28-Linux   
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