826 FXUS21 KWNC 171958 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EST December 17 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 renewed northerly Arctic flow, and anomalously cold temperatures for much of southern Alaska during the period. HAZARDS High risk of heavy precipitation for much of coastal California, Thu, Dec 25. High risk of heavy snow for the Sierra Nevada mountains, Thu, Dec 25. Moderate risk of heavy precipitation for western Nevada, and California, Thu-Fri, Dec 25-26. Moderate risk of heavy snow for the Sierra Nevada mountains, Thu-Fri, Dec 25-26. High risk of heavy precipitation for portions of western Washington, Sat-Sun, Dec 27-28. High risk of heavy snow for parts of the northern Cascades, Sat-Sun, Dec 27-28. Moderate risk of heavy precipitation for the Pacific Northwest and northern California, Fri-Wed, Dec 26-31. Moderate risk of heavy snow for the Cascades and Klamath mountains, Fri-Wed, Dec 26-31. Slight risk of heavy precipitation for the West Coast and portions of the Desert Southwest, Thu-Wed, Dec 25-31. Slight risk of heavy snow for the Cascades, Klamath, and Sierra Nevada, Thu-Wed, Dec 25-31. Moderate risk of heavy snow for the Northern Rockies and Northern Intermountain, Sat-Wed, Dec 27-31. Slight risk of heavy snow for the Central and Northern Rockies, parts of the Great Basin and Northern Intermountain, Thu-Wed, Dec 25-31. Slight risk of heavy snow for the lower Four Corners, Thu-Sat, Dec 25-27. Moderate risk of high winds for much of the northwestern CONUS, Thu-Wed, Dec 25-31. Moderate risk of high winds for many parts of the southwestern CONUS, Thu-Sat, Dec 25-27. Slight risk of high winds for much of the central and western CONUS, Thu-Wed, Dec 25-31. Moderate risk of much below-normal temperatures for much of southern Mainland and Southeast Alaska, Fri-Sun, Dec 26-28. Slight risk of much below-normal temperatures for many parts of the southern and central Mainland and Southeast Alaska, Thu-Wed, Dec 25-31. Slight risk of high winds for much of southern Alaska, including the Alaska Range, Thu-Wed, Dec 25-31. Flooding possible for much of the West Coast. DETAILED SUMMARY FOR SATURDAY DECEMBER 20 - WEDNESDAY DECEMBER 24: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR THURSDAY DECEMBER 25 - WEDNESDAY DECEMBER 31: There continues to be good consistency and agreement in the dynamical models featuring a highly amplified 500-hPa ridge over the north-central Pacific, with strong troughing downstream over the northeastern Pacific to promote Atmospheric River (AR) activity over the West Coast continuing into next week. During week-2, the evolution of this activity closely follows changes in the strength and position of the mean troughing feature, where the period starts with the greatest AR-related risks over the lower half of the West Coast, with these risks shifting northward later in the period as mid-level shortwave energy reintensifies near British Columbia. Both the GEFS and ECMWF have come into better agreement with the timing of this development, resulting in a slightly deleted return of the enhanced onshore flow over the Pacific Northwest than previously advertised. However, stronger mid-level height anomalies are also featured, resulting in a higher risk of AR related hazards in the outlook. Any renewed AR activity over the northwestern CONUS may be particularly impactful in light of the copious precipitation received (over 20 inches in some locations over a 10-day span) triggering recent flooding over many portions of Washington, along with a very wet week-1 forecast. Later in week-2, ensembles show little indication of any progression of the longwave features over North America, which continues to be consistent with a negative Pacific North America (-PNA) pattern. Consequently, this unfortunately implies the persistence of potentially hazardous weather over the western CONUS at least through the end of 2025. Late in week-1 and into early week-2, 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. Based on daily mean precipitation amounts from the ECMWF, GEFS and Canadian, some of the highest precipitation amounts over California (from the Bay Area southward) now appear to be timing off into week-1, though both raw and calibrated Probabilistic Extreme Tools (PETs) tools continue to show elevated chances for 3-day amounts exceeding the 85th percentile and and 1 inch during the first few days of the period. Therefore, a high risk of heavy precipitation remains issued and valid through Dec 25 over much of coastal California, with a broader moderate risk area of heavy precipitation posted through Dec 26 before much of the anomalous troughing aloft deamplifies. Raw snow tools are consistent with this evolving threat of enhanced moisture over California, and an accompanying high and moderate risk of high elevation heavy snow remains posted over the Sierra Nevada mountains though Dec 25 and Dec 26, respectively. 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 by days 9 and 10, there is good agreement into the daily raw tools showing the return of increasing precipitation amounts over the northwestern CONUS through the middle of week-2. The GEFS continues to be the driest when compared to the other models, though it indicates a substantial uptick in the probabilities for 3-day amounts exceeding 2 inches for days 10-12 (Dec 27-29) relative to yesterday. Similarly, the ECMWF and Canadian have also trended wetter, with the latest ensemble solutions favoring greater than 50% chances for daily amounts exceeding an inch over western Washington and northwestern Oregon, with higher chances (>70-80%) exceeding 2 inches over a 3-day period focused over western Washington. Although PETs are less onboard with the heavy precipitation potential, maintaining only 20-30% chances of amounts exceeding the 85th percentile, the more amplified troughing aloft favored and the corresponding precipitation responses in the raw tools over already saturated, and flooding susceptible areas of the Pacific Northwest supports the addition of a high risk of heavy precipitation and high elevation snow for western Washington where wet signals are strongest for Dec 27-28. Broader moderate risks of heavy precipitation and high elevation snowfall remain issued over the Pacific Northwest with added coverage into northern California, where there is increased tool support for the heavy precipitation and snow risks again shifting southward with time, both remaining valid through the end of week-2. To capture the evolution 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 slight 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 Pacific moisture associated with continued AR activity during week-2 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, now valid for Dec 27-31 where signals for amounts exceeding a foot are the highest in the tools. In addition, a separate slight risk area of heavy snow is posted over the lower Four Corners (Dec 25-27) 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 favored 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, anomalously positive 500-hPa heights over the north-central Pacific remain favored to build northward in the Bering Strait, allowing for the development of troughing downstream over the Mainland. As a result of this pattern shift, temperatures are expected to briefly moderate over the Southeast late in week-1 before anomalously cold air begins to redevelop and expand westward into the central and western portions of the Mainland. Since yesterday, both raw and calibrated temperature tools have trended stronger with the cold signals, likely tied to the aforementioned stronger troughing developing over the northeastern Pacific to establish more of a northerly flow component over the state. Therefore, a moderate risk of much below normal temperatures is posted from the northern part of the Alaskan Peninsula eastward through Yakatat in the Southeast, where the ensembles show elevated chances (30-40%) for negative temperature departures exceeding more than 30 degrees F. A broader slight area of much below normal temperatures remains issued for much of the Mainland and Southeast, valid for all of week-2. 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 $$