FOUS30 KWBC 171531 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1031 AM EST Wed Dec 17 2025 Day 1 Valid 12Z 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... 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 with 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. As it stands...the area average precipitation amounts are currently in the range of 1-2 inches with localized higher amounts maxima in the terrain. 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