324 FXUS21 KWNC 231944 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EST December 23 2025 SYNOPSIS: An evolving mid-level pattern is forecast heading into the New Year, with mid-level low pressure expected to bring increased chances for enhanced onshore flow and unsettled conditions over the southwestern contiguous U.S. (CONUS) early in week-2. Thereafter, mid-level low pressure is expected to develop further north across the northeastern Pacific, bringing a renewed risk of Atmospheric River (AR) related hazards across the northwestern CONUS later in week-2. Strong mid-level high pressure over the Bering Sea is likely to promote persistent Arctic high pressure and anomalously cold temperatures for many parts of Alaska. An enhanced risk of high winds remains for much of the northeastern CONUS associated with an area of mid-level and surface low pressure. HAZARDS Slight risk of heavy precipitation for parts of the southern California, and the Desert Southwest, Wed-Fri, Dec 31-Jan 2 Slight risk of heavy snow for the higher elevations of the southern Sierra Nevada mountains and lower Four Corners, Wed-Fri, Dec 31-Jan 2. Slight risk of high winds over the southwestern CONUS, Wed-Fri, Dec 31-Jan 2. Slight risk of heavy precipitation for parts of northern California and the Pacific Northwest, Thu-Tue, Jan 1-6. Slight risk of heavy snow for the higher elevations of northern Sierra Nevada, Klamath, and Cascades, Thu-Tue, Jan 1-6. Slight risk of heavy snow for portions of the higher elevations of the Northern Intermountain, Great Basin, and Rockies, Thu-Tue, Jan 1-6. Slight risk of high winds for the portions of the northeastern CONUS, Wed-Thu, Dec 31-Jan1. Moderate risk of much below-normal temperatures for parts of central and southern Mainland Alaska and Southeast Alaska, Fri-Tue, Jan 2-6. Slight risk of much below-normal temperatures for southern and central Mainland and Southeast Alaska, Wed-Tue, Dec 31-Jan 6. Slight risk of high winds for much of southern Alaska, including the Alaska Range, Wed-Tue, Dec 31-Jan 6. DETAILED SUMMARY FOR FRIDAY DECEMBER 26 - TUESDAY DECEMBER 30: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR WEDNESDAY DECEMBER 31 - TUESDAY JANUARY 06: There continues to be good agreement in the mean 500-hPa week-2 height anomaly forecasts among the dynamical models featuring a highly amplified ridge over the Bering Strait with downstream troughing over the northeastern Pacific. Consistent with previous thinking, this pattern remains favorable for the advection of very cold Arctic air into Alaska, as well as the threat of Atmospheric River (AR) activity over the West Coast of the U.S during week-2. Over the past few days, however, both the GEFS and ECMWF have begun to heavily trend towards building mid-level positive heights over the northwestern CONUS towards the middle of next week, where the bridging between two ridge anomaly centers located over the northern Pacific and central CONUS looks to force a mean cutoff low feature offshore of Baja California. As a result, much of the enhanced onshore flow over the northern half of the West Coast is favored to relax, with unsettled weather becoming more focused further south over the southwestern CONUS heading into the New Year. This pattern shift looks to be temporary as ensembles favor a reloading northeastern Pacific 500-hPa trough, pointing to the return of an enhanced onshore flow regime over the northwestern CONUS later in week-2. Despite being short-lived, such a reprieve from the AR-related hazards over the northern half of the West Coast would be welcomed, as any drier than normal conditions would help mitigate flood risks and allow some recovery of saturated conditions over the Pacific Northwest and northern California following very wet conditions being registered this month. Both the GEFS and ECMWF based Integrated Vapor Transport (IVT) tools are in agreement reflecting this pattern evolution, favoring the highest IVT signals south of 35N tied to the mean cutoff feature early in week-2 before the signals reemerge further north during the second half of the period. Likewise, Probabilistic Extreme Tools (PETs) feature 40-50% chances for 3-day precipitation amounts exceeding the 85th climatological percentile from southern California to the lower Four Corners. While these signals are supportive of a moderate risk designation of heavy precipitation and high elevation heavy snow, both the uncalibrated GEFS and ECMWF are comparably drier, depicting generally 20% chances of 3-day amounts exceeding an inch. Due to better skill of the uncalibrated precipitation guidance over this part of the country during the past 30 days, only slight risks of heavy precipitation and high elevation snow are posted and are valid for through Jan 2 before the mid-level flow becomes more zonal. Elevated wind speeds are also likely at the base and ahead of the mean cutoff feature aloft, thus a slight risk of high winds is also posted over a broad portion of the southwestern CONUS for the same period. By days 9 and 10 (Jan 1-2), daily raw tools show the return of increasing precipitation amounts over the Pacific Northwest, as 500-hPa troughing reestablishes itself over the northeastern Pacific. By comparison, the anomalous troughing is strongest in the GEFS and Canadian, with all models showing this mid-level feature peaking in amplitude by day 12 (Jan 4). A slight risk of heavy precipitation is posted over northern California and the Pacific Northwest where PETs show 20-30% chances for 3-day amounts exceeding the 85th percentile for Jan 1-6. Based on the Snow Water Equivalent (SWE) PET from the GEFS, and raw snow tools, a slight risk of heavy snow is also issued for the higher elevations of the West Coast, as well as, slight risks of heavy snow and high winds (Jan 1-6) over the parts of the Interior West due to the anomalous troughing and accompanying enhanced moisture favored to shift further inland with time. Corresponding moderate risk areas were considered for inclusion, though the raw tools, calibrated PETs, and IVT tools are not quite robust enough with the precipitation and enhanced moisture signals, which precludes a higher risk designation at this time. The addition of moderate risk areas will be revisited in upcoming hazard outlooks should there be stronger support in the probabilistic guidance as well as better run-to-run continuity with the transition from anomalous ridging to troughing over the western CONUS. The possible flooding hazard is removed in the updated outlook due to a reduction in flooding responses in the water models through early January, however additional flooding over parts of the West Coast cannot be ruled out during week-2. East of the Rockies, potentially strong troughing downstream of the ridge anomaly center favored over the central CONUS supports an increased risk of high winds over the northeastern CONUS early in week-2. Mean surface low pressure remains favored over southeastern Canada, where raw ensembles show more of a stronger gradient further west into the Upper Midwest. Therefore, a slight risk of high winds remains posted over the northeastern and is expanded westward into the Upper Mississippi Valley, and valid through Jan 1. Due to more ridging favored over the northwestern CONUS early in the period, there are higher chances for near-normal temperatures east of the Mississippi. While the ECMWF PET shows increased chances for minimum temperatures falling below the 15th percentile, the GEFS PET is less supportive of this risk resulting in no corresponding temperature hazards. Arctic high pressure remains favored to persist over Alaska during week-2. A brief easing of the anomalous cold is expected through Jan 1, before building back again by January 2 associated with the reloading troughing downstream of the potent ridge . Therefore a moderate risk is designated for these dates for much of southern Mainland and Southeast Alaska. The slight risk goes through the entirety of week-2 and includes all of Alaska south of the Brooks Range. With high pressure over the Yukon and surface low pressure over the Gulf of Alaska, a tight pressure gradient may lead to high winds (gap winds for the southeastern Mainland and Southeast Alaska), supporting a slight risk of high winds for the Aleutians and southern Alaska. The PETs (especially the ECENS) indicate at least a 20% chance of wind speeds exceeding the 85th percentile and 40 mph. The very cold weather and high winds along the southern tier of the state increases the threat for heavy freezing spray at times in marine areas in addition to hazardous low wind chills possible, impacting vulnerable populations. Over the central Pacific, mid-level troughing is favored underneath the strong ridge center over the north-central Pacific, which may lead to Kona low development to the west of the Hawaiian Islands early in week-2. PETs indicate elevated (>60%) chances for 3-day amounts exceeding the 85th percentile with elevated chances of totals exceeding two inches. As these signals are favored to shift eastward with time, locally heavy precipitation and other adverse impacts looks to overspread more of the Hawaii Islands (mainly from Maui westward) throughout week-2. FORECASTER: Nick Novella $$