FOUS30 KWBC 100830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 AM EST Wed Dec 10 2025 Day 1 Valid 12Z Wed Dec 10 2025 - 12Z Thu Dec 11 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF WESTERN WASHINGTON AS WELL AS FOR PORTIONS OF NORTHERN IDAHO AND FAR WESTERN MONTANA... ...Western Washington... An atmospheric river is currently impacting much of Washington State this morning. The narrow but intense plume of deep tropical moisture originating from near Hawaii is being drawn northeastward by a strong subtropical high that is parked off the California coast. This area of high pressure is forming the southeast side the atmospheric river. Meanwhile, polar high pressure over the Aleutians is sending copious Arctic air well to the south through the North Pacific. This cold air has set up a strong baroclinic boundary as it runs into the warm air associated with the subtropical high. Finally, a strong area of low pressure, fed in part by this same baroclinicity, will move into the British Columbia coast today. The low and it's trailing cold front moving into the coast is adding substantial lift to the atmosphere where the A.R. is located in western Washington. The result of these various forcings is a "hose" of deep tropical moisture aligned right into Washington State, then continuing inland through Idaho and Montana. The low and its front are locally enhancing the rain rates, especially on the upwind/west-facing slopes of the coastal ranges and the Cascades. This latest round of heavy rain is coming after 2+ days of nearly steady rainfall into this same region. Saturated soils, full rivers, and abundant additional rainfall are all contributing to flooding concerns through this area. Heavy rain is likely to continue for much of the day, gradually diminishing in intensity tonight, but unlikely to stop completely most anywhere in western Washington. Since the forecast is little changed, particularly with expected rainfall amounts, the Slight Risk area is also little changed. A higher end Slight remains in effect for portions of the western slopes of the Cascades. Very high snow levels between 8,500 and 9,000 ft are keeping all but the northernmost Cascades as rain. A wedge of colder air trapped in the northern Cascades are locally keeping snow levels there down closer to 6,500-7,000 ft. The higher end Slight includes much of the I-90 corridor between Seattle and Snoqualmie Pass. Land and rockslides remain a concern, and numerous Flood Warnings remain in effect for much of the western slopes of the Cascades. ...Northern Idaho and Far Western Montana... The Slight Risk of excessive rainfall also remains in effect for much of the northern Idaho Panhandle. This includes the I-90 corridor roughly between Coeur d'Alene on the west side to Missoula on the east side. The strong atmospheric river moving in from the Pacific will drop the lion's share of its moisture into the coastal ranges and Cascades. However, plentiful leftover moisture will then continue east and be re-lifted into the Bitterroots of Idaho and Montana. While rainfall amounts will be tempered greatly compared to areas further west, they will still be significantly impactful for this region, especially when considering it's been raining off-and-on in this region from this same atmospheric river for the past 48-72 hours. Thus, soils are saturated and rivers are already running high in this area. All additional rainfall will convert to runoff, both worsening and prolonging any resultant flooding impacts. The area south of I-90 and east of Lewiston with west- facing slopes will be hardest hit. It is in this area that a higher end Slight remains in effect. The heaviest rain across this region will be through the day today, with some modest diminishing of rainfall rates through tonight. Regardless, most upslope areas should see steady rainfall throughout this period. Wegman Day 2 Valid 12Z Thu Dec 11 2025 - 12Z Fri Dec 12 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF NORTHERN IDAHO... ...Northern Idaho... Thursday will be the final day of the atmospheric river across all of the Pacific Northwest, including into Idaho. A fast moving shortwave trough diving southeast in the jet streak will reinvigorate the heavy rain across northern Idaho and western Montana during the day Thursday. This shortwave will also usher in an Arctic front to the east of the Rockies across much of the rest of Montana. This much colder and drier air will finally allow conditions to dry out as upper level ridging shoves what's left of the A.R. northward into Canada, and a large low well out over the Pacific cuts off the tropical feed as it moves towards Alaska. The ceasing of tropical moisture will effectively end the heavy rain threat everywhere, though lingering flooding is likely to continue in this area for multiple days to come. Heavy rain will likely be ongoing across northern Idaho at the start of the period on Thursday morning. A surface low will form along the advancing Arctic air, which will draw the Arctic front south across much of Montana Thursday night, turning what moisture is left over to snow east of the range, while putting northern Idaho into the "dry slot", ending the heavy rainfall. While rain rates may stay elevated for much of the day Thursday across northern Idaho, the Slight Risk is at least equally for the ongoing impacts going into the period from the heavy rain that is likely to occur tonight. Rock and mudslides will remain a threat until conditions begin to dry out Thursday night, as well as a continuation of both ongoing flooding and potentially some new flooding as the rain ends across the area. ...Western Washington... A Marginal Risk remains in place across portions of Western Washington, specifically the foothills of the Cascades on Thursday. The Marginal Risk reflects the end of the heavy rainfall from the A.R. from Wednesday and Wednesday night...i.e. diminishing rainfall rates...and the likely much higher impacts that will be ongoing by then along all of the Washington Cascades. Thus, expect Slight level impacts over this corridor to continue, but the heavy rain ending should spell the end of new or worsening impacts going forward. The Marginal attempts to strike a balance between these two competing concerns. Meteorologically, the A.R. will still be in full swing at the start of the period 12Z Thursday, but as the forcings from inland lows, rapidly weakening westerly flow orthogonal to the ranges, and surface high pressure builds in from the south, rain rates will rapidly diminish through the morning, ending entirely for many areas of western Washington by Thursday night. Since the tropical flow responsible for the A.R. will be cut off by Thursday night, this will be the end of the event for the area, and a much needed and "mostly" dry period of weather will follow across most of western Washington, and those areas that do see additional rain into the weekend will find it will be a fraction of the intensity that this A.R. produced, and should be essentially non-impactful. Wegman Day 3 Valid 12Z Fri Dec 12 2025 - 12Z Sat Dec 13 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman 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