FOUS30 KWBC 190048 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 748 PM EST Thu Dec 18 2025 Day 1 Valid 01Z Fri Dec 19 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. 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 the south early on Friday. WPC Deterministic QPF in the 1.0 to 1.4 inch range is forecast along the track of the low. Offsetting the potential of excessive rainfall by limiting rainfall rates/amount 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 making headway into northern CA by the end of the period, but 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. Kleebauer ..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://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt