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   FOUS30 KWBC 182018   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   318 PM EST Thu Dec 18 2025   
      
   Day 1   
   Valid 16Z 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 THE   
   NORTHWEST CORNER OF CALIFORNIA...   
      
   16z update:   
   CIRA LPW and buoy observations denote the rapidly surging warm   
   front nearing the OR coast orienting from a developing sub 995mb   
   surface low southeast toward the OR/CA state-line. Mid to upper-   
   level 99th to record LPW values at 850-700 and 700-500mb have over-   
   run the coastal region but the very strong winds will reach the   
   area with the warm front. RAP, HRRR forecasts denote 60-80kts of   
   fairly unidirectional southwest flow from 925 to 700mb through the   
   core of the deep layer moisture. This brings IVT values to be over   
   1000 kg/m/s with some of the hi-res CAMS solutions nearing 1250   
   kg/m/s. As such, the orographic ascent will drive atypical   
   rain-rates for the mid to upper slopes of the coastal range and   
   central to northern Oregon Cascades, likely to be at or above   
   .5"/hr through the forecast period with averages around .75"/hr,   
   occasional up to 1" being possible. Localized totals of 6-8" are   
   probable with HREF probability near 80-100% for 8" for peaks of   
   eastern Linn/Marion counties.   
      
   While FFG values are high enough to accommodate the rates, totals   
   in the 6, 12 and 24hr rates are near exceedance and there are   
   places of 50 to 100 year Average Return Intervals (ARI), denoting   
   the uncommon nature to the rainfall rates/totals. However,   
   rain shadows will limit rainfall totals in the Willamette Valley,   
   the runoff should be high and swell rivers and streams fairly   
   quickly. Please refer to further river/stream forecasts from the   
   River Forecast Center and National Water Center. In collaboration   
   with the NWC, there are atypical response signals that would   
   suggest some rapid onset flooding in the upper reaches/extremities   
   of the watersheds. An upgrade to Moderate Risk was considered for   
   those upper- stretches in the Cascades but the areal extent is   
   considered to small to delineate.   
      
   Gallina   
      
      
      
   ~~~~Prior Discussion~~~~   
   A well define plume of deep moisture will continue to approach the   
   coast and then progress inland bringing widespread coverage of   
   moderate or greater rainfall to portions of western Oregon and a   
   small part of adjacent Washington and the northwest corner of   
   California. After several runs where the guidance has shown a   
   subtle northward shift of the moisture plume and associated   
   rainfall...there was subtle shift southward in the latest suite of   
   guidance necessitating a small shift in the north/south position of   
   the Slight Risk along/near the Oregon Cascades. Overall...the risk   
   within the Slight Risk area remained along the west aspect of the   
   Cascades where the moisture plume encounters the Cascades and is   
   forced to lift orographically. Guidance showed widespread 3 inch to   
   6 inch precipitation amounts within 24hrs with isolated higher   
   totals depicted by the convective allowing models in and near the   
   terrain. Both the HREF and RRFS continued to show high   
   probabilities of 0.50"+/hr rainfall rates especially after 21Z   
   today which persists through 12Z Friday. 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.   
      
   Bann   
      
   Day 2   
   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...   
      
   20Z Update: Little deviation in the 12z model suite leading to   
   general run-to-run continuity across both New England and the   
   Western U.S for the respective ERO risks in place. Small   
   adjustments were made based on the QPF distribution and HREF   
   neighborhood probabilities from the latest numerical suite, but   
   changes were not enough to warrant any considerable detail in   
   either area. Best chance will likely lie within the coastal terrain   
   of northwestern CA where flow orthogonal to the terrain will   
   generate efficient upslope enhancement over the bulk of the period.=20   
      
   Kleebauer   
      
   ..Previous Discussions..   
      
   ...Pacific Northwest...   
      
   The atmospheric river that came on-shore in the Day 1 period will   
   be on-going across Oregon...although the event should be winding   
   down in terms of rainfall rates and additional amounts fairly early   
   in the outlook period. 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 700 units are forecast to cover   
   much of Oregon initially...then weaken as the axis of the moisture   
   plume and associated precipitation settles into northern   
   California. Maintained a large amount of continuity in terms of   
   placement of the Slight Risk area clipping northwest California and   
   a portion of adjacent southwest Oregon.   
      
   Saw little reason to make any significant changes to the expanded   
   coverage of the Slight Risk area made yesterday to cover more into   
   the Trinity Mountains given latest QPF.   
      
   ...Southern New England...   
   A trough crossing the eastern United States will draw increasing   
   amounts of moisture northward into parts of New England...helping   
   set the stage for locally heavy rainfall when flow aloft becomes   
   increasingly diffluent and a surface warm front approaches from=20   
   the south early on Friday. WPC Deterministic QPF in the 1.0 to 1.4=20   
   inch range is forecast along the track of the low. Offsetting the=20   
   potential of excessive rainfall by limiting rainfall rates/amount=20   
   is the meager instability expected to be in place.   
      
   Bann   
      
      
   Day 3   
   Valid 12Z Sat Dec 20 2025 - 12Z Sun Dec 21 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PORTIONS OF   
   NORTHERN CALIFORNIA|...   
      
   20Z Update: The next stronger AR pulse from the Pacific will be=20   
   making headway into northern CA by the end of the period, but=20   
   persistence in the AR overall will maintain alignment from northern   
   Sierra over into the northern CA coast. Overall, the synoptic   
   pattern remains consistent in the evaluation of the run-to-run QPF   
   distribution with ensembles basically overlapping each other in   
   where the heavier precip will occur. The fortunate news is there is   
   still only a small axis of overlap from the previous period, so   
   cumulative impacts are less of a concern this go around. The urban   
   centers between Sacrament to San Francisco will be more at play   
   this period leading to maintenance of the MRGL over the I-80   
   corridor. Overall, changes were minimal with just some minor   
   adjustments on the northern and southern edges of the inherited   
   MRGL risk.=20   
      
   Kleebauer=20   
      
   ..Previous Discussion..   
      
   Renewed rainfall is expected across northern California as yet   
   another push of moisture arrives on the heels of the AR which   
   departed during the day on Friday, The axis of the strongest on-   
   shore winds and the greatest on-shore moisture transport looks to   
   be south of the axis of the previous event, but there is likely to   
   be at least some overlap between the two rainfall footprints.   
   Precipitable water values increase to between 1.2 and 1.4 inches   
   along the northern California coast north of San Francisco south   
   of the Oregon/California border by early Saturday evening. Given   
   such high snow levels...there is not a lot of concern about   
   precipitation type until the moisture laden flow reaches the   
   northern end of the Sierra Nevada range (save for a few mountain   
   tops along the trajectory inland from the coast). With the areal   
   average amount of the rainfall forecast being in the 1 to 2 inch   
   range...will keep the Marginal Risk inherited from yesterdays Day   
   4 ERO and plan to adjust as needed when placement and amounts   
   become more locked in,   
      
   Bann   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!7hjcXr5tNoy7vtk072XHeu2EQaFVEV0ioYYdkl78FhOD=   
   d3NXk9zYIwXBxcApy71515RWE8pjJCdP6fPF2DtKIaUMHV0$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!7hjcXr5tNoy7vtk072XHeu2EQaFVEV0ioYYdkl78FhOD=   
   d3NXk9zYIwXBxcApy71515RWE8pjJCdP6fPF2DtKJ9DasXk$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!7hjcXr5tNoy7vtk072XHeu2EQaFVEV0ioYYdkl78FhOD=   
   d3NXk9zYIwXBxcApy71515RWE8pjJCdP6fPF2DtKq8_DjRY$=20   
      
   =3D =3D =3D   
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