Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 756 PM EST Tue Dec 9 2025 Day 1 Valid 01Z Wed Dec 10 2025 - 12Z Wed Dec 10 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PACIFIC NORTHWEST... ...Western Oregon and Washington... The atmospheric river event will continue across the Pacific Northwest tonight as the next surge of heavy rain reaches Washington. As of 00Z, steady light to moderate with occasional heavy rain was ongoing across northern Oregon, but rates have tapered off compared to earlier Tuesday morning. Ahead of an approaching cold front, a plume of IVT with magnitudes in the 500-750 kg/m/s range along the northern Oregon coast will translate north and increase in magnitude into the 800-1000 kg/m/s range along coastal Washington into the southern portions of the Puget Sound through Wednesday morning. The probability of hourly rainfall in excess of 0.5 inches will increase overnight, especially into the 06-12Z time frame across the Olympics into the northern Cascades. 850-700 mb mean layer winds of 50 to 60+ kt are expected across western Washington, coupled with precipitable water values of roughly 1.2 to 1.4 inches. Snow levels will also increase with the surge of warmer air lifting north, with all rain below elevations of 9,000 to 10,000 feet. Peak 12 hour rainfall totals through 12Z Wednesday are forecast to range between 2-4 inches from the Olympics into the northern Cascades. The forecast rain rates and additional rainfall through 12Z carry an increased flood threat on area rivers/streams due to roughly 5 to 10 inches of rain which have fallen over the past 3 days, most over the past 24-36 hours. In addition to potential flooding of low-lying and urban locations, the potential will also exist for landslides and debris flows due to the increase in rainfall intensity overnight. ...Northern Idaho and Adjacent Areas... The Marginal Risk across northern Idaho into neighboring western Montana, northeastern Oregon and southeastern Washington remains unchanged with this update. Similar to coastal locations of the Pacific Northwest, heavier rainfall intensities are expected in the 06-12Z time frame tonight. Hourly rainfall potential will increase up to and beyond 12Z Wednesday with peak values generally under 0.3 inches, but the Blue Mountains of northeastern Oregon will have potential for peak hourly rainfall of 0.3 to 0.4+ inches overnight. Total peak 12 hour rainfall of 1 to 1.5 inches is expected (close to 2 inches for the Blues) through 12Z with snowmelt contributing to rising streams and rivers. Given increased soil moisture due to antecedent rainfall, localized impacts of heavy rain will remain possible. Otto Day 2 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... 2100 UTC update: No significant changes made to the previous outlook areas across the coastal Pac NW and Northern Rockies. Edits to the previous areas were mostly to better fit latest qpf and snowfall forecasts. Oravec ...Previous discussion... ...Western Washington... The wettest day of the next 3 will be on Wednesday across much of western Washington. The continuous westerly flow of 40-50 kt at 850 will continue off the Pacific through the day. A vigorous jet streak to 150 kt will remain just north of Washington before diving southeastward across the upper high Plains. This will keep Washington in the favorable right entrance region. A series of lows will barrel into the British Columbia coast, each one keeping the A.R. plume nearly stationary and highly energetic. Thus, widespread 4-6 inch rainfall totals are expected for the Olympics and the west facing foothills of the Washington Cascades. As with most A.R.s, very high snow levels will keep any snow confined to the highest peaks of the far northern Cascades. This will keep the lion's share of the expected precipitation as heavy rain for most of the Cascades and certainly all of the coastal ranges as well. This being the third day of more-or-less steady rainfall across much of Washington, especially south of the Puget Sound area means the soils are well past saturated, and all of the precipitation should convert to runoff. Further, the high snow levels will promote snowmelt as well, which will only further raise river levels. Finally, the lows moving into the British Columbia coast will turn the surface flow out of the WNW for a time, especially Wednesday night. This will direct the flow straight down the strait of Juan the Fuca and put the low-level flow just slightly off from orthogonal to the mountains, favoring a slight northerly flow of moisture down the foothills and the Puget Sound Region. This should temper the downsloping side east of the mountains just a bit, allowing for more rainfall into Seattle, Olympia, and the Puget Sound region broadly. Thus, Wednesday and Wednesday night will also be the wettest day in the urban cities as well. Given all of the above, the inherited Slight Risk was expanded with this update to include the Seattle metro and the surrounding areas around southern Puget Sound with this update. A higher end Slight was expanded south down the Cascades to near the Columbia River/Oregon state line. Once the impacts from all the rainfall seen across Western Washington become more clear, it will be easier to ascertain what the impacts from this round of heavy rain on Wednesday are likely to be. A Moderate Risk upgrade may be needed with future updates or all or a portion of the Cascades and western foothills with future updates. ...Northern Idaho and Far Western Montana... The atmospheric river (A.R.) impacting Western Washington on Wednesday will continue inland and impact the Rockies of northern Idaho and western Montana as well on Wednesday. Just like further west, the A.R. will cause snow levels to rise to as high as 8,000 ft. This will introduce a significant snowmelt component to the resulting runoff from the upslope rainfall. Over the last 24 hours, the area has picked up between 1-2 inches of rain, causing soils to become fully saturated. Today, expect even more rain, with 2-3 inch totals expected to be common. The steady rain, snowmelt, and abnormally warm conditions will increase the risk for avalanches in the prone areas, especially where a heavy wet snow falls on top of much weaker snow layers. Also expect an increase in rock and landslides, as well as rising streams, creeks, rivers, and localized urban flooding. Given all these expected impacts, a large portion of northern Idaho and far western Montana were upgraded to a Slight Risk with this update, and a smaller portion of northern Idaho along the Montana state line from I-90 south is in a higher end Slight Risk, as this area is expected to see the highest rainfall amounts to around 3 inches through the period, almost all of which falling as rain. Wegman Day 3 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... 2100 UTC update: The previous slight risk area over northern ID was decreased in size to better fit the latest day 3 qpf. The previous marginal risk area was removed along the WA coast, into far northern OR coast based on the latest light day 3 qpf across these areas. The marginal risk was confined to the upslope area of the WA Cascades. Oravec ...Previous discussion... ...Northern Idaho and Western Montana... A double upgrade from no risk area to a Slight Risk was introduced for a portion of northern Idaho, generally south of I-90 and along the Montana state line with this update. This upgrade was introduced largely as a continuation of heavy rain from Wednesday night across this region. Rainfall rates should be gradually diminishing through the day Thursday, and as far as the heaviest rain and immediate impacts, the rain on Wednesday should be more impactful in this region. However, given the steady rain continues into the day Thursday, the impacts resulting therefrom should also continue. Meanwhile further east into Montana, a strong Arctic front pushing south down the Plains should turn all of the precipitation over central Montana over to snow. Thus, the flooding threat is largely confined to northern Idaho and far western Washington where the warm air associated with the A.R. will remain in place on Thursday. Rock slides, avalanches, low-lying flooding, sporadic urban impacts, and very high river levels will all be ongoing Thursday morning over this area, supporting the Slight Risk upgrade. ...Western Washington... A Marginal Risk was expanded mainly for the Thursday morning time period across much of western Washington with this update. This expanded Marginal Risk includes the Seattle metro and the Puget Sound area more broadly. As with further east, it will be a continuation of the steady rain from Wednesday night into Thursday across the area. It's likely a Slight Risk upgrade will be needed, as the rain looks to take its time until Thursday afternoon to significantly diminish in intensity. The A.R. event broadly will be diminishing through this period. A large low well out over the Pacific will take a more northward track, as compared to its predecessors, as the pattern amplifies. This will effectively cut off the A.R. as the moisture plume gets directed north towards the Gulf of Alaska, and therefore, the flow with the A.R. shifts north into British Columbia and substantially diminishes. Wegman Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt