413 FXUS21 KWNC 162004 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EST December 16 2025 SYNOPSIS: A persistent mid-level pattern featuring strong low pressure over the northeastern Pacific favors an elevated risk of Atmospheric River (AR) activity, supporting multiple weather related hazards, including high winds, heavy precipitation, and high-elevation snow across the western Contiguous U.S. (CONUS) throughout week-2. The greatest AR potential is expected over the lower half of the West Coast before shifting northward to the Pacific Northwest later in the week-2 period. Surface low pressure over the Great Plains along with surface high pressure over the Southeast U.S. are expected to induce potentially hazardous winds over much of the Great Plains, Mississippi Valley, and western Great Lakes. Farther north, building mid-level high pressure over the Bering Strait is likely to lead to more northerly flow and anomalously cold temperatures for much of southern Alaska during the period. HAZARDS High risk of heavy precipitation for southwestern Oregon and coastal California (excluding areas south of Los Angeles, CA), Wed-Thu, Dec 24-25. High risk of heavy snow for the Sierra Nevada and Klamath mountain ranges, Wed-Fri, Dec 24-26. Moderate risk of heavy precipitation for southwestern Oregon, western Nevada, and California, Wed-Fri, Dec 24-26. Moderate risk of heavy snow for the Klamath Range, and Sierra Nevada, Wed-Fri, Dec 24-26. Moderate risk of heavy precipitation for the Pacific Northwest, Fri-Tue, Dec 26-30. Moderate risk of heavy snow for the Cascades, Fri-Tue, Dec 26-30. Slight risk of heavy precipitation for the West Coast and portions of the Desert Southwest, Wed-Tue, Dec 24-30. Slight risk of heavy snow for the Cascades, Klamath, and Sierra Nevada, Wed-Tue, Dec 24-30. Moderate risk of heavy snow for the Northern Rockies and Northern Intermountain, Wed-Tue, Dec 24-30. Slight risk of heavy snow for the Central and Northern Rockies, parts of the Great Basin and Northern Intermountain, Wed-Tue, Dec 24-30. Slight risk of heavy snow for the lower Four Corners, Wed-Fri, Dec 24-26. Moderate risk of high winds for much of the northwestern CONUS, Wed-Tue, Dec 24-30. Moderate risk of high winds for many parts of the southwestern CONUS, Wed-Sat, Dec 24-27. Slight risk of high winds for much of the central and western CONUS, Wed-Tue, Dec 24-30. Slight risk of much below-normal temperatures for much of southern Mainland and Southeast Alaska, Wed-Tue, Dec 24-30. Slight risk of high winds for much of southern Alaska, including the Alaska Range, Wed-Tue, Dec 24-30. Flooding possible for much of the West Coast. DETAILED SUMMARY FOR FRIDAY DECEMBER 19 - TUESDAY DECEMBER 23: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR WEDNESDAY DECEMBER 24 - TUESDAY DECEMBER 30: The week-2 hazards perspective remains on track since yesterday. Dynamical models continue to feature a highly amplified 500-hPa ridge over north-central Pacific, with strong troughing downstream over the eastern Pacific heading into the period. Analysis of the mean 6-10 day 500-hPa height pattern from the GEFS and ECMWF ensembles reveals two minima in the negative height anomaly fields, with both of these features indicating the evolution of the potential Atmospheric River (AR) activity favored over the West Coast during the period. The first feature is a deep trough that peaks in strength by the outset of week-2, promoting a southward surge of enhanced tropospheric moisture into the lower half of the West Coast. As this initial mid-level feature gradually deamplifies, the second feature is a redeveloping 500-hPa trough further north offshore of British Columbia. While this is expected to bring drier conditions across California (mainly south of the Bay Area), enhanced onshore flow looks to migrate northward to many parts of the Pacific Northwest towards the middle of the period. Any renewed AR activity over the northwestern CONUS may be particularly impactful in light of the recent flooding over many portions of Washington, along with a very wet week-1 forecast. Later in week-2, the 500-hPa height pattern looks to remain pretty stable, which unfortunately implies the persistence of potentially hazardous weather over the western CONUS through the end of December. There remains good agreement in the GEFS and ECMWF Integrated Vapor Transport (IVT) tools depicting the southward solution of the AR activity, with enhanced probabilities for IVT values exceeding 250 kg/m/s reaching as far south as 30N though before retreating northward. Based on continued support in the raw and calibrated Probabilistic Extreme Tool (PET) guidance, a high risk of heavy precipitation remains posted for coastal California and into southwestern Oregon where the GEFS and ECMWF depict 50-70% chances for 3-day amounts exceeding the 85th climatological percentile, valid through Dec 25. Within the highlighted area, there is good agreement among uncalibrated tools showing greater than 50% chances of 3-day amounts exceeding 1 inch, with pockets of greater than 30% of amounts exceeding 2 inches early in the period. Based on raw tools indicating 50-60% chances for 3-day snowfall amounts exceeding a foot through the middle of week-2, an accompanying high risk of heavy snow remains posted over the higher elevations of the Sierra Nevada and Klamath ranges though Dec 26. Surrounding these high risk areas, moderate risks of heavy precipitation and high elevation snow are posted over California and southern Oregon through Dec 26 before the enhanced onshore flow begins to ease over the lower half of the West Coast. It is worth noting that should the strongest AR potential be realized over the more heavily populated areas of coastal California, this is likely to cause major disruptions to travel over the holiday season. Tied to the reloading 500-hPa trough off British Columbia, ensembles remain relatively consistent with the magnitude and coverage of the precipitation responses over the Pacific Northwest since yesterday. The GEFS remains comparably the driest among the model mean solutions, but its daily tools do nonetheless point to the return of above-normal and potentially heavy precipitation further north from day 10 (Dec 26) and beyond. Based on the good agreement between raw ECMWF and Canadian ensembles indicating 40-50% chances of daily amounts exceeding an inch over portions of western Washington and Oregon (with similar chances for 3-day amounts exceeding 2 inches in the ECMWF), the moderate risk areas of heavy precipitation and high elevation heavy snow remain posted over the Pacific Northwest, both valid beginning Dec 26. While the GEFS is quicker to dry out over the highlighted areas, all ensembles favor the persistence of anomalous mean troughing through the end of December, supportive of the corresponding hazards remaining valid through the remainder of week-2. Given the meridional nature of the AR activity favored, broader slight risk areas of heavy precipitation and high elevation heavy snow remain posted over the West Coast for the entirety of week-2, with the heavy precipitation risk also extending into the Desert Southwest based on continued support of 3-day amounts exceeding the 85th percentile and half an inch in the PETs. Based on the aforementioned antecedent conditions and the week-1 precipitation forecast from the Weather Prediction Center (WPC QPF), the possible flooding hazard remains issued for most of the entire West Coast excluding areas south of Los Angeles, CA. Continued precipitation accumulations on saturated soils also brings the additional risk of landslides and debris flows in steep terrain or burn scars. The incoming moisture associated with this enhanced jet is favored to overspread much of the Interior West, leading to an elevated risk of heavy snow for many parts of the Rockies, Great Basin and Northern Intermountain. This continues to be supported by both the precipitation and Snow Water Equivalent PETs, as well as the raw snow tools from the GEFS and ECMWF. A slight risk of heavy snow remains posted for a broad area of the Interior West, along with a moderate risk embedded within this area over portions of the Central and Northern Rockies where signals for amounts exceeding a foot are the highest, both valid for all of week-2. In addition, a separate slight risk area of heavy snow is posted over the lower Four Corners (Dec 24-26) associated with the deep troughing favored initially in the period, as locally heavy snowfall is possible over the higher elevations of Arizona and New Mexico. Accompanying the risks of heavy precipitation and heavy snow, the elevated potential for AR activity also supports periods of high winds reaching hazard criteria throughout the period. At the base and ahead of the mean troughing favored early in the period, a moderate risk is posted for much of the southwestern CONUS where PETs show elevated chances for winds exceeding the 85th percentile early in week-2. A separate moderate risk area for high winds is issued for all of week-2 over the northwestern CONUS where anomalous troughing remains predominately favored. Farther east, individual shortwaves are anticipated to periodically be ejected eastward, inducing one or more surface lows developing over the Great Plains. Surface high pressure should be fairly persistent downstream from the axis of the mid-level ridge covering the CONUS. Between these systems, a tight pressure gradient may periodically be established across the central CONUS, increasing the odds for episodes of high winds across the central CONUS. With the pattern changing little over the course of week-2, a slight risk of high winds is supported for the entire period. For Alaska, much of the anomalously positive 500-hPa heights over the north-central Pacific are favored to build northward along the Bering Strait, allowing for the development of troughing downstream over the Mainland. As a result of this shift, temperatures are expected to briefly moderate over the Southeast before anomalously cold air begins to redevelop and expand westward into the central and western portions of the Mainland. Based on the PETs, the strongest cold signals are favored to be offshore in the Gulf of Alaska and are not as potent as the cold air favored over the Southeast late in week-1. As a result, a slight risk of much below normal temperatures remains posted, and is expanded northward in the Mainland where there is better support for minimum temperatures falling below the 15th percentile. High winds also remain possible in gaps and inlets, and close to the lowest surface pressures. Despite day to day variation regarding the precise locations of the highest winds, a slight risk of high winds is posted for most of southern and southeastern Alaska for all of week-2. The Probabilistic Extremes Tools (PETs) show enhanced chances of wind speeds exceeding the 85th climatological percentile in these areas.. The very cold weather and high winds along the southern tier of the state increases the threat for heavy freezing spray at times near larger bodies of water. FORECASTER: Nick Novella $$