938 FXUS21 KWNC 271938 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EST December 27 2025 SYNOPSIS: At the outset of week-2, mid-level low pressure remains forecast across western North America favoring continued enhanced onshore flow and increased chances of heavy precipitation (coastal rain and high elevation snow) for the west coast of the contiguous U.S. (CONUS). Uncertainty increases later in the period as dynamical models indicate a trend toward increased mid-level low (high) pressure across the central Pacific (western CONUS). This could result in a drying trend across the West Coast toward the end of week-2 as well as moderating temperatures across Alaska. While the pattern across the eastern U.S. is forecast to be more variable, increasing high latitude blocking combined with incoming shortwave energy digging underneath could favor a more active weather pattern. HAZARDS Moderate risk of much below-normal temperatures for southwestern Mainland Alaska, the Alaska Peninsula, and eastern Aleutians, Sun-Mon, Jan 4-5. Slight risk of much below-normal temperatures for all of Alaska with the exception of the North Slope and Southeast Alaska, Sun-Wed, Jan 4-7. Slight risk of high winds for the Alaska Peninsula, southern Mainland Alaska, and southeastern Alaska, Sun-Wed, Jan 4-7. Slight risk of heavy precipitation across the west coast of the CONUS, Sun-Thu, Jan 4-8. Slight risk of heavy precipitation across portions of the Lower Mississippi Valley, Tennessee Valley, and Southeast, Tue-Sat, Jan 6-10. Slight risk of heavy snow across portions of the Cascades, Klamath, and Sierra Nevada Mountains, Sun-Thu, Jan 4-8. Slight risk of heavy snow across portions of the Upper Mississippi Valley, Great Lakes, and the Northeast, Sun-Tue, Jan 4-6. Slight risk of high winds across much of the CONUS west of the Rockies, Sun-Thu, Jan 4-8. DETAILED SUMMARY FOR TUESDAY DECEMBER 30 - SATURDAY JANUARY 03: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR SUNDAY JANUARY 04 - SATURDAY JANUARY 10: During week-2, the 0z ECMWF and Canadian ensembles depict a quick mid-level pattern transition across the Pacific with a weakening of the ridge to the south of the Aleutians, troughing emerging across the central Pacific, and hints of ridging near the west coast of the CONUS by the end of week-2. This would be consistent with a weakening negative phase of the Pacific North American (-PNA) pattern. The GEFS is slower and weaker with the pattern evolution, adding to some uncertainty with the eventual outcome which could ultimately have implications downstream over the CONUS later in week-2. Following a period of relatively drier weather in week-1 across the West Coast tied to a transient mid-level ridge, another trough is predicted to build over western North America by the outset of week-2 favoring a return to enhanced precipitation chances over the region. Both the GEFS and ECMWF Probabilistic Extremes Tools (PETs) depict at least a 20 percent chance 3-day precipitation totals exceed the 85th climatological percentile and 1-inch across the entire West Coast through the middle of the period. Probabilities for reaching the 85th percentile are higher (above 40 percent) in the ECMWF PET across the Southwest compared to the GEFS PET, with uncalibrated tools favoring higher precipitation totals over the Pacific Northwest. A broad slight risk for heavy precipitation is posted across the entire West Coast, valid Jan 4-8, with a corresponding slight risk of heavy snow across higher elevations of the Cascades, Klamath, and Sierra Nevada Mountains supported by the GEFS snow water equivalent (SWE) PET. Heavy snow is also possible further inland across the Great Basin and Rockies, although probabilities for daily snowfall greater than 4 inches in the ECMWF and GEFS ensembles are lower compared to closer to the coast precluding inclusion in the hazard. There are some indications of a stronger surge of moisture into the Northwest toward the middle of the period, with increased 24-hour precipitation totals in the uncalibrated guidance. Stronger winds may also be possible across the Pacific Northwest, with the ECMWF PET depicting high probabilities (greater than 60 percent) for wind speeds exceeding the 85th climatological percentile associated with a tightening surface pressure gradient. Trends will continue to be monitored for a potential upgrade to a moderate risk tomorrow. Further downstream across the CONUS, shortwave disturbances are likely to propagate through the mean flow. The first is favored early in the period, with another system possible later on in week-2. The ECMWF ensemble along with recent deterministic runs depict a more amplified system and associated surface low pressure across the northeastern U.S. early in week-2 which could bring increased chances for heavy snow initially across the Upper Mississippi Valley and Great Lakes, and then into portions of the Northeast. The GEFS is flatter with this initial wave, although its corresponding deterministic solutions also depict areas of snow or wintry weather across these same areas. Therefore, a slight risk for heavy snow is highlighted across portions of the Upper Mississippi Valley, Great Lakes, and the Northeast, Jan 4-6. A second shortwave later in week-2 may have more of a southern stream component with both the ECMWF and GEFS PETs depicting at least a 20 percent chance 3-day precipitation totals exceed the 85th climatological percentile and 1-inch over parts of the Lower Mississippi Valley, Tennessee Valley, and Southeast during the second half of the period. This supports a slight risk for heavy precipitation over these areas, Jan 6-10. The northward trajectory of this moisture is uncertain, although a substantial high-latitude blocking ridge predicted over northeastern North America could support greater amplification of negative 500-hPa height anomalies underneath this feature at the end of week-2. Surface high pressure is forecast to persist across much of Alaska and the Yukon throughout week-2. Conversely, mean low pressure is favored in the Gulf of Alaska. The pressure gradient between these features results in elevated chances for episodes of high winds across much of southern Alaska from the central Aleutian Islands through Southeast Alaska. High winds would be particularly favored in gaps and inlets. The PETs depict at least a 20 percent chance wind speeds exceed the 85th climatological percentile and 40-mph over some of these areas through the middle of the period. There is still some signal for high winds at the end of week-2, but given increasing model spread and a transitioning pattern there is insufficient confidence to issue a full-week hazard. Therefore a slight risk of high winds is posted for most of southern and southeastern Alaska for Jan 4-7. PETs continue to highlight enhanced chances for temperatures to fall to near hazardous thresholds across much of southern, Interior, and Southeast Alaska, driven in part by the pressure gradient discussed above. A moderate risk of much below-normal temperatures is posted for southern Alaska west of Prince Williams Sound, the lower Yukon, and the Alaskan Peninsula, valid Jan 4-5, while a broader slight risk is posted for all of the Alaskan Mainland with the exception of the North Slope and Southeast Alaska for Jan 4-7. The combination of 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. As the ridge over the central Pacific begins to break down, moderation of temperatures is likely toward the second half of week-2, particularly across interior and eastern parts of Mainland Alaska. FORECASTER: Thomas Collow $$