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   FOUS30 KWBC 172023   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   323 PM EST Wed Dec 17 2025   
      
   Day 1   
   Valid 16Z Wed Dec 17 2025 - 12Z Thu Dec 18 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   ...Middle Texas Coast...   
   A fairly defined southern stream shortwave is noted across the   
   Northwest Gulf with impressive shield expansion within the larger   
   scale diffluent flow. Strong southerly western Gulf LLJ initiated   
   thunderstorms along/parallel to the southern Texas coast and cold   
   pool interaction and deeper layer steering flow is allowing for   
   cells/convective line to propagate eastward and remain well   
   offshore. However, as the upper- level cirrus shield clears to the   
   east, but low level moisture flux continues toward the Middle Texas   
   coast and coastal plain; insolation will allow for some increased   
   modest instability for an isolated thunderstorm or two to develop.   
   Cell motions will be very slow and may allow for longer duration   
   increased rainfall totals. Sandy soils will largely absorb these   
   modest rates/totals, but there remains a non-zero potential for   
   some flooding concerns particularly if the rates intersect with   
   poor drainage urban centers, though the coverage does not warrant   
   an increase in categorical level.   
      
   Gallina   
      
      
   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=20   
   'higher- end' Slight Risk -- across eastern portions of the=20   
   Willamette Valley into the western slopes of the Cascades, while=20   
   also clipping the far eastern reaches of Portland metro along with=20   
   the western Columbia Gorge into far southern WA. The 12Z guidance=20   
   came in even wetter, owing to the strong AR which given better=20   
   sampling, the intensity of which is coming into better view now=20   
   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=20   
   Western Region WRF mean peaks closer to 1200. Guidance QPFs show=20   
   widespread 4-8" amounts within 24hrs, with higher totals depicted=20   
   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=20   
   the same area, probabilities of exceeding 8" peak between 60-80+=20   
   percent. Both the HREF and RRFS show high probabilities of=20   
   0.50"+/hr rainfall rates between 16Z Wed through 12Z Thu (probs=20   
   over 50%; in many areas over 70-80%), with quite a bit of overlap=20   
   within the same areas through ~03Z Thu.=20   
      
   Therefore, the potential for small stream and/or river flooding=20   
   continues to increase with this event (per collaboration with the=20   
   National Water Center). However the potential for flash flooding=20   
   (more short-fused inundation), especially over burn scars, is also=20   
   increasing within the enhanced or 'higher-end' Slight Risk area.=20   
   Next shift will evaluate the potential need for a Moderate Risk.=20   
      
   Hurley   
   =20   
   Previous Discussion below...   
      
   The next in a series of atmospheric rivers will arrive along the=20   
   west coast of North America early on Thursday. The threat for=20   
   significant rainfall remains elevated across the western half of=20   
   Oregon with a focal point for maximum precipitation centered over=20   
   the southern Cascades and the adjacent valleys on either=20   
   side...although a subtle northward shift was noted in the 17/00Z=20   
   guidance. The key in this threat is the projected "spillover" of=20   
   heavier precip to the leeward side of the Cascades due to a=20   
   pronounced 700mb jet from the Pacific leading to significant=20   
   Pacific moisture being drawn inland. This is the area where ARI=20   
   forecasts are hovering between the 25-100 year exceedance=20   
   intervals, matching the output from the past several model runs and   
   deterministic QPF from WPC. As a result, Day 2 excessive rainfall=20   
   outlook was expanded the Slight Risk northward across portions of=20   
   the Washington Cascades in response to the subtle northward shift=20   
   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=20   
   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=20   
   slight risk eastward and more into the Trinity Mountains where QPF=20   
   values and snow levels are high. Area average precipitation=20   
   amounts are currently in the range of 1-2 inches with localized=20   
   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!5Aq_kH0Hoa5mggkNl__4_GgGtMdxGizZYCq8uXJw7zPI=   
   wKshy5T910RvNmHmPJteGbKqHp1DNOvp2paSoXLSYWEOxGE$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!5Aq_kH0Hoa5mggkNl__4_GgGtMdxGizZYCq8uXJw7zPI=   
   wKshy5T910RvNmHmPJteGbKqHp1DNOvp2paSoXLSIxMep1I$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!5Aq_kH0Hoa5mggkNl__4_GgGtMdxGizZYCq8uXJw7zPI=   
   wKshy5T910RvNmHmPJteGbKqHp1DNOvp2paSoXLSUEUTZrk$=20   
      
   =3D =3D =3D   
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   --- SBBSecho 3.28-Linux   
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