659 FXUS21 KWNC 201941 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EST December 20 2025 SYNOPSIS: Following a southward shift in the storm track along the West Coast, mid-level low pressure over the northeastern Pacific increases the risk for additional heavy precipitation, heavy snow, and high winds across the Pacific Northwest to close out 2025 and heading into the New Year. The enhanced Pacific flow favors heavy snow across the northern Rockies and an elevated chance of high winds over much of the western and central U.S. Strong mid-level high pressure over the Bering Sea is likely to promote persistent Arctic high pressure and anomalously cold temperatures for Alaska. HAZARDS High risk of heavy precipitation for western Washington and northwestern Oregon, Sun-Tue, Dec 28-30. High risk of heavy snow for the northern Cascades, Sun-Tue, Dec 28-30. Moderate risk of heavy precipitation for the Pacific Northwest and northern California, Sun-Sat, Dec 28-Jan 3. Moderate risk of heavy snow for the Cascades and Klamath mountains, Sun-Sat, Dec 28-Jan 3. Slight risk of heavy precipitation for the Pacific Northwest and northern California, Sun-Sat, Dec 28-Jan 3. Slight risk of heavy snow for the Cascades, Klamath, and the northern Sierra Nevada Mountains, Sun-Sat, Dec 28-Jan 3. Moderate risk of heavy snow for the Northern Rockies and part of the Northern Intermountain West, Sun-Sat, Dec 28-Jan 3. Slight risk of heavy snow for the Central and Northern Rockies, and parts of the Great Basin and Northern Intermountain West, Sun-Sat, Dec 28-Jan 3. Slight risk of heavy snow for southwestern Colorado, Sun, Dec 28. Moderate risk of high winds for parts of the West, Sun-Sat, Dec 28-Jan 3. Slight risk of high winds for much of the West, Great Plains, and Upper Mississippi Valley, Sun-Sat, Dec 28-Jan 3. Moderate risk of much below-normal temperatures for much of southern Mainland and Southeast Alaska, Sun-Tue and Fri-Sat, Dec 28-30 and Jan 2-3. Slight risk of much below-normal temperatures for southern and central Mainland and Southeast Alaska, Sun-Sat, Dec 28-Jan 3. Slight risk of high winds for much of southern Alaska, including the Alaska Range, Sun-Sat, Dec 28-Jan 3. Flooding possible for parts of northern California and the Pacific Northwest. DETAILED SUMMARY FOR TUESDAY DECEMBER 23 - SATURDAY DECEMBER 27: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR SUNDAY DECEMBER 28 - SATURDAY JANUARY 03: The ECENS, GEFS, and CMCE are in good agreement and consistent with a persistent anomalous 500-hPa ridge (trough) over the Bering Sea (northeastern Pacific) throughout week-2, which is consistent with the ongoing La Nina. The highly anomalous 500-hPa ridge over the Aleutians and Bering Sea remains an anchor to a stable longwave pattern over the forecast domain. The amplified northeastern Pacific trough favors a return of an atmospheric river (AR) to the Pacific Northwest by the end of December. Early in week-2, the 0z ECENS and 6z GEFS depict large probabilities (50% chance or more) for integrated vapor transport (IVT) values > 250 kg/m/s to shift from 50 to 45 degrees N. Based on this IVT guidance and 24-hour precipitation amounts from the ECENS and GEFS, a high risk (> 60% chance) of heavy precipitation is designated for western Washington and northwestern Oregon from December 28 to 30. The heavy precipitation is forecast to initially affect the Olympic Peninsula around Dec 28 and then shift or expand southward to northwestern Oregon on Dec 29 and 30. Model solutions are in good agreement that this heavy precipitation signal continues to expand southward with time to southwestern Oregon and northern California on December 31 and January 1. Since the multi-model ensemble mean solutions depict the amplified 500-hPa trough and enhanced onshore flow persisting through the beginning of January 2026, a moderate risk (40-60% chance) of heavy precipitation is posted from the Pacific Northwest south to northern California for the entirety of week-2. The southward extent of the heaviest precipitation remains uncertain, but 24-hour precipitation amounts from the ensemble means along with the GEFS and ECMWF Probabilistic Extremes Tool (PET) support a slight risk of heavy precipitation as far south as the Bay area of California and northern Sierra Nevada Mountains. Given the ongoing river flooding and saturated soils along with heavy precipitation forecast in the next week, a possible flooding hazard, with an associated risk of landslides and debris flows in steep terrain or burn scars, remains posted for parts of the Pacific Northwest and northern California. The enhanced Pacific moisture is favored to overspread the interior West, leading to an elevated risk of heavy snow for parts of the northern Rockies, Great Basin, and Northern Intermountain West. A moderate risk of heavy snow remains posted for the Northern Rockies for the entirety of week-2 where the ECENS has 7-day amounts of near a foot or more. A broader area is highlighted with a slight risk of heavy snow extending south to northeastern Utah and northwestern Colorado. On December 28, a shortwave trough is forecast to cross the Four Corners Region supporting a slight risk of heavy snow for southwestern Colorado. Accompanying the heavy precipitation and heavy snow risks, the longwave pattern and enhanced Pacific flow support periods of high winds across much of the West throughout week-2. A moderate risk for high winds is designated from the coastal Pacific Northwest and northern California east to the northern and central Rockies, including the Colorado Front Range. Farther east, individual shortwaves are anticipated to periodically be ejected eastward, inducing one or more surface lows over the Great Plains. Therefore, a slight risk of high winds is posted for much of the Great Plains and Upper Mississippi Valley. Arctic high pressure is likely to persist over Alaska into the New Year. Based on the GEFS and ECMWF PETs indicating near a 40 percent chance of temperatures falling below the 15th percentile, a moderate risk of much below normal temperatures remains posted for southern Mainland Alaska, southeastern Alaska, and the eastern Aleutians from December 28 to 30. A brief easing of the anomalous cold is expected during the middle part of week-2 before bitterly cold temperatures return. Therefore, January 2 and 3 were also included in the moderate risk for much below normal temperatures. The slight risk is valid through the entirety of week-2 and includes all of Alaska south of the Brooks Range. Gusty northerly winds may accompany the Arctic air, supporting a slight risk of high winds for the Aleutians and southern Alaska. 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: Brad Pugh $$