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   FOUS30 KWBC 180025   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   725 PM EST Wed Dec 17 2025   
      
   Day 1   
   Valid 01Z Thu Dec 18 2025 - 12Z Thu Dec 18 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Hurley   
      
      
   Day 2   
   Valid 12Z Thu Dec 18 2025 - 12Z Fri Dec 19 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY OVER WESTERN   
   OREGON EXTENDING INTO A SMALL PORTION OF WASHINGTON AND   
   CALIFORNIA...   
      
   2000 UTC Update: Have added a 25% area -- which is an enhanced or   
   'higher- end' Slight Risk -- across eastern portions of the   
   Willamette Valley into the western slopes of the Cascades, while   
   also clipping the far eastern reaches of Portland metro along with   
   the western Columbia Gorge into far southern WA. The 12Z guidance   
   came in even wetter, owing to the strong AR which given better   
   sampling, the intensity of which is coming into better view now   
   within 24-36 hours of the main event. CW3E global ensemble guidance   
   (GEFS and ECENS) show peak IVT values ~1000 kg/m-s, while the   
   Western Region WRF mean peaks closer to 1200. Guidance QPFs show   
   widespread 4-8" amounts within 24hrs, with higher totals depicted   
   from the CAM (particularly in the 'enhanced' Slight Risk area). 12Z   
   HREF probabilities of QPF exceeding 5" within the Day 2 period are   
   90% or higher withing the enhanced Slight Risk area, while within   
   the same area, probabilities of exceeding 8" peak between 60-80+   
   percent. Both the HREF and RRFS show high probabilities of   
   0.50"+/hr rainfall rates between 16Z Wed through 12Z Thu (probs   
   over 50%; in many areas over 70-80%), with quite a bit of overlap   
   within the same areas through ~03Z Thu.   
      
   Therefore, the potential for small stream and/or river flooding   
   continues to increase with this event (per collaboration with the   
   National Water Center). However the potential for flash flooding   
   (more short-fused inundation), especially over burn scars, is also   
   increasing within the enhanced or 'higher-end' Slight Risk area.   
   Next shift will evaluate the potential need for a Moderate Risk.   
      
   Hurley   
      
   Previous Discussion below...   
      
   The next in a series of atmospheric rivers will arrive along the   
   west coast of North America early on Thursday. The threat for   
   significant rainfall remains elevated across the western half of   
   Oregon with a focal point for maximum precipitation centered over   
   the southern Cascades and the adjacent valleys on either   
   side...although a subtle northward shift was noted in the 17/00Z   
   guidance. The key in this threat is the projected "spillover" of   
   heavier precip to the leeward side of the Cascades due to a   
   pronounced 700mb jet from the Pacific leading to significant   
   Pacific moisture being drawn inland. This is the area where ARI   
   forecasts are hovering between the 25-100 year exceedance   
   intervals, matching the output from the past several model runs and   
   deterministic QPF from WPC. As a result, Day 2 excessive rainfall   
   outlook was expanded the Slight Risk northward across portions of   
   the Washington Cascades in response to the subtle northward shift   
   noted earlier...with little reason to expand any farther eastward.   
      
   Bann   
      
      
   Day 3   
   Valid 12Z Fri Dec 19 2025 - 12Z Sat Dec 20 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY OVER   
   PORTIONS OF NORTHWEST CALIFORNIA...   
      
   The atmospheric river that came on-shore on Thursday will be on-   
   going across much of Oregon as the Day 3 period begins at 12Z on   
   Friday. A well-defined plume of deep layer moisture will continue   
   to be drawn from the eastern North Pacific Ocean eastward into   
   parts of the northern Great Basin by fast westerly flow aloft. IVT   
   values at or above 600 units is forecast to cover much of Oregon   
   initially...then weaken as the axis of the moisture plume and   
   associated precipitation settles into northern California. The   
   system looks to have trended a bit more progressive on Friday than   
   suggested by earlier runs...so some of the QPF values over Oregon   
   and northwest California have come down a bit. Even so...opted to   
   maintain a large amount of continuity from the initial overnight   
   issuance with the Slight Risk area over northwest California and a   
   portion of adjacent southwest Oregon given the possibility of   
   further wobbles in the placement of the axis or variations in model   
   QPF. The only notable change for the 20z update was to expand the   
   slight risk eastward and more into the Trinity Mountains where QPF   
   values and snow levels are high. Area average precipitation   
   amounts are currently in the range of 1-2 inches with localized   
   higher amounts maxima in the terrain, some of which may fall as   
   snow.   
      
   Santorelli/Bann   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!-HOgv9M7e3nZ4pcsap5fDpS4JwT71sAS9RAqdViThRDt=   
   XsRbQj5pk7LaLisjaPSIJhkz8ng6pybkEbqpXMzmmBl-DE4$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!-HOgv9M7e3nZ4pcsap5fDpS4JwT71sAS9RAqdViThRDt=   
   XsRbQj5pk7LaLisjaPSIJhkz8ng6pybkEbqpXMzmyK5ZSzE$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!-HOgv9M7e3nZ4pcsap5fDpS4JwT71sAS9RAqdViThRDt=   
   XsRbQj5pk7LaLisjaPSIJhkz8ng6pybkEbqpXMzmn2jAbs8$=20   
      
   =3D =3D =3D   
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