FOUS30 KWBC 162335 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 635 PM EST Tue Dec 16 2025 Day 1 Valid 01Z Wed Dec 17 2025 - 12Z Wed Dec 17 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PACIFIC NORTHWEST... Both the 18z HREF and 12z REFS paint a similar QPF picture through 12z. The threat of heavy rainfall across portions of northern California continues through the period, while a renewed surge of heavy rains impacting portions of WA early on shifts south across western OR with time. IVT values are well above normal due to the strength of deep layer flow, peaking near 900 kg/m/s. An additional 1-2" for the lower coastal ranges are expected with local amounts to 4" across portions of the Olympics, Cascades, and terrain of northern CA by 12z. Only cosmetic changes were made to the broad Marginal Risk of Excessive Rainfall. Roth Day 2 Valid 12Z Wed Dec 17 2025 - 12Z Thu Dec 18 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann/Kleebauer Day 3 Valid 12Z Thu Dec 18 2025 - 12Z Fri Dec 19 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF WESTERN OREGON AND PORTIONS OF ADJOINING STATES... 20Z Update... Changes were minimal with the forecast remaining steadfast through the 12z NWP suite. The threat for significant rainfall remains elevated across the western half of Oregon with a focal point centered over the southern Cascades and the adjacent valleys on either side. 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 protrusion inland from the Pacific leading to significant Pacific moisture breaching the terrain and allowing for heavier precip to make it downwind of the mountains. This is the area where ARI forecasts are hovering between the 25-100 year exceedance intervals, matching the previous output from overnight's forecast. This allowed for general continuity in the SLGT risk area with modifications only made to match to the latest QPF footprint with emphasis on where 2+" will fall in the D3 time frame, especially away from the coastal plain. Kleebauer ...Previous Forecast... The next in a series of Atmospheric Rivers will arrive early Thursday with its axis of greatest moisture transport being directed primarily along the western coast of Oregon...with maximum IVT values in excess of 800 kg per meter per second largely being directed perpendicular into the coastal ranges early in the period. Global models continue to advertise the possibility of 6 inches or more of rainfall during the period...and comparing the WPC 24hr QPF to ARI yielded a broad swath exceeding the 2-5 year return intervals. It was also worth noting that the highest ARIs were just to the east of the crest of the Cascades where an axis of 25-100 year exceedance intervals was depicted. This does seem like a scenario where more spillover into the normally drier downslope areas is plausible, so could be an impactful event even into some of those areas. Consequently, the Day 3 excessive rainfall outlook expanded the Slight Risk eastward in Oregon to cover areas with QPF in excess of 5 inches and the highest ARI values mentioned above given the concern for spillover. Snowfall within this area should be confined to the higher elevations. 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