773 FXUS21 KWNC 171801 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT October 17 2026 SYNOPSIS: By next weekend, developing mid-level low pressure along the West Coast supports the onset of enhanced onshore flow, leading to an increased risk of heavy precipitation and high elevation snowfall over parts of California and Oregon during the week-2 period. Meanwhile, renewed surface low development is favored over the western Atlantic following a nor'easter system potentially impacting the Eastern Seaboard during week-1. While the heaviest precipitation amounts are favored offshore, the synoptic pattern supports periods of elevated wind speeds across the Mid-Atlantic and Southeast. HAZARDS Slight risk of heavy precipitation for portions of central and northern California, and adjacent southern Oregon, Sun-Sat, Oct 25-31. Slight risk of high elevation heavy snow for the Sierra Nevada Mountains, Sun-Sat, Oct 25-31. Slight risk of high winds for portions of the Mid-Atlantic and the Southeast, Sun-Thu, Oct 25-29. DETAILED SUMMARY FOR TUESDAY OCTOBER 20 - SATURDAY OCTOBER 24: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR SUNDAY OCTOBER 25 - SATURDAY OCTOBER 31: Consistent with previous ensemble mean guidance, 500-hPa height forecasts from the ECENS and GEFS are in good agreement featuring an amplified ridge axis overspreading much of the Plains and Mississippi Valley, bookended by a pair of mean troughs, with one over the Four Corners, and the other centered deep into Florida Peninsula by the start of next weekend. As the upstream mean trough quickly deamplifies and lessens the risk of unsettled weather in the lee of the Rockies by start of week-2 (Oct 25), the latest 0z ECENS has become very eager to develop a potent 500-hPa trough encroaching the West Coast early in the period. While the 0z GEFS is slower to develop this mid-level feature, this ensemble run is an outlier compared to the 0z GEFS-AI, 0z Canadian, and the 6z GEFS ensemble based guidance whose development timing of the trough better aligns with the 0z ECENS. As a result, there is improved outlook confidence in regards to the onset of enhanced onshore flow and the potential for heavy precipitation and high elevation snow hazards out West early in week-2. Meanwhile over the southeastern CONUS, much of the anomalous mid-level troughing is favored to gradually ease early in the period, though models generally maintain broadly cyclonic flow over the part of the country and adjacent portions of the Gulf and western Atlantic through the remainder of week-2. Based on precipitation tools, a slight risk of heavy precipitation is posted over portions of central and northern California, and adjacent southern Oregon. Within the highlighted slight risk area, the ECENS Probabilistic Extremes Tool (PET) depicts increased chances for 3-day amounts exceeding the 85th percentile and one inch, as well as the uncalibrated ECMWF also indicating similar chances for amounts exceeding an inch. Due to the duration of anomalous troughing favored in the models, with some ensemble indications of the mean trough reloading across the West Coast later in October (in conjunction with a restrengthening ridge center upstream over the Aleutians), the slight risk remains valid for entirely of week-2 which is also supported by the ECENS PET. The enhanced onshore flow also supports periods of accumulating, and potentially heavy snowfall across the higher elevations of the Sierra Nevada Mountains during the period. Therefore, a slight risk of heavy snowfall is likewise posted for all of week-2, where uncalibrated tools show increased chances for 3-day snowfall amounts exceeding 6 inches throughout week-2. Periods of high winds are also possible at the base and ahead of the mean trough axis favored over the West Coast. However, in the absence of strong surface pressure gradients favored, and weak signals in the Integrated Vapor Transports (IVT) tools reflective of atmospheric river activity, a corresponding high wind risk is omitted in the updated outlook for the time being. This potential will be reassessed in upcoming outlooks. Downstream, surface high pressure is favored to overspread the Great Plains and Mississippi Valley in the wake of a departing surface low next weekend. Although some lingering frontal activity from week-1 may bring enhanced and possibly precipitation into portions of the Southern Plains as favored in the 0z GEFS, other dynamical models are more supportive of drier conditions developing throughout the central CONUS by the start of week-2, and the slight risk of heavy precipitation over Texas and Oklahoma is discontinued in the updated outlook. Across the eastern U.S. and western Atlantic, the latest ECMWF based guidance has become better distinguished in regards to the evolution of offshore low activity during the next two weeks. A nor'easter-like event remains favored during week-1; as this low weakens, the ECENS points to a new clustering of deepening low members further south to the north of the Bahamas late in week-1. Despite the lack of a closed low in the ECENS ensemble mean, a renewal of cyclonic surface flow is favored, where combined with deep tropical moisture is supportive of a heavy precipitation risk in proximity to the lower Eastern Seaboard. Similar to yesterday, the highest precipitation amounts in the raw tools are favored just offshore ahead of the trough axis, and PETs remain unsupportive of heavy precipitation protruding onshore. This precludes a heavy precipitation hazard at this time, though this potential will continue to be closely monitored in the upcoming hazard outlooks. However, given the synoptic setup, a slight risk of high winds remains posted where there continues to be the potential for strengthening surface pressure gradients tied to the troughing aloft. The slight risk is now extended through Oct 29 and expanded further northward in the Mid-Atlantic where the ECENS PET maintains increased chances for wind speeds exceeding the 85th percentile. Over Alaska, 500-hPa troughing and an accompanying surface low is favored to lift out over the Bering Strait, with rising mid-level heights favored over the Aleutians and southern Mainland early in the period. This pattern change looks to bring drier conditions to the southern Mainland and the Southeast, while shifting the enhanced precipitation and potential for elevated wind speeds further north into more of the Mainland. While both the precipitation and wind based PETs reflect this northward shift, actual amounts and wind speeds are not expected to reach hazard thresholds, resulting in no hazards being posted for the state. FORECASTER: Nick Novella $$