551 FXUS21 KWNC 181801 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT October 18 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. An enhancement of atmospheric moisture is also favored over the southwestern U.S., supporting an increased risk of heavy precipitation mainly over the lower Four Corners. Meanwhile, renewed surface low development is favored over the western Atlantic following a coastal low 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, Mon-Sun, Oct 26-Nov 1. Slight risk of high elevation heavy snow for the Sierra Nevada Mountains, Mon-Sun, Oct 26-Nov 1. Slight risk of heavy precipitation for parts of the Desert Southwest, Great Basin, Southern Rockies, and Southern High Plains, Mon-Wed, Oct 26-28. Slight risk of high winds for portions of the Mid-Atlantic and the Southeast, Mon-Thu, Oct 25-29. DETAILED SUMMARY FOR WEDNESDAY OCTOBER 21 - SUNDAY OCTOBER 25: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR MONDAY OCTOBER 26 - SUNDAY NOVEMBER 01: Based on the latest ensemble guidance, dynamical model 500-hPa height anomaly forecasts have come into better agreement today in regards to the development of an amplified mid-level trough off the West Coast after next weekend. A comparably more amplified mean solution is still apparent in the ECENS ensemble, which features a more expansive coverage of negative height departures over the Pacific Northwest and California, supportive of enhanced Pacific onshore flow by the start of the period. In addition to the West, a secondary source of incoming enhanced atmospheric moisture is being advertised in the precipitable water anomaly guidance across the lower Four Corners and Southern Plains, which appears to be associated with additional tropical cyclone activity in the eastern Pacific. Taken together, much of CONUS west of the Mississippi is primed for above-normal precipitation, with the greatest risk of hazardous weather in proximity to the amplified mean trough nearing the West Coast. Meanwhile across the eastern U.S., ensembles continue to advertise a secondary surface low formation to the north of the Bahamas in the wake of a coastal low initially favored during week-1. While the heaviest precipitation amounts look to remain offshore in the western Atlantic, continued troughing and cyclonic surface flow is likely to bring a period of unsettled weather across the lower eastern Seaboard at least through early week-2. With enhanced onshore flow favored across the West Coast, a slight risk of heavy precipitation remains posted over portions of central and northern California, and adjacent southern Oregon. Based on the calibrated ECENS Probabilistic Extremes Tool (PET), a moderate risk of heavy precipitation has been considered given elevated chances (>40%) for 3-day amounts exceeding the 85th percentile and 1 inch (locally, as much as 20% chances for amounts exceeding 2 inches). However, forecast confidence for this higher hazard designation is tempered by the drier solutions found in the uncalibrated ECMWF, as well as the GEFS and Canadian, which are more supportive of a slight risk at this time. 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 PETs. The enhanced onshore flow favored 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 remains posted for all of week-2, where uncalibrated ECENS tools show increased chances for 3-day snowfall amounts exceeding 6 inches with better support from the GEFS Snow Water Equivalent (SWE) PET during week-2. Periods of high winds are also possible at the base and ahead of the mean trough. However, in the absence of strong surface pressure gradients favored, weak signals in the Integrated Vapor Transport (IVT) tools reflective of atmospheric river activity, and continued large ensemble spread in the GEFS height pattern, a corresponding high wind risk is not included in the updated outlook for the time being. This potential will continue to be reassessed in upcoming outlooks this week. Tied to the influx of enhanced moisture across the southwestern CONUS, a slight risk of heavy precipitation is also added over this part of the country in the updated outlook. While the GEFS continues to point to a heavy precipitation threat further east likely tied to lingering frontal activity, this hazard addition is primarily based on the potential for shortwave energy ejecting across the Rockies, where there is also growing support in the ECENS PET for amounts exceeding the 85th percentile and 1 inch, and an increase in the 6-10 day total precipitation anomalies favored over the lower Four Corners in the reforecast consolidation tool relative to yesterday. Across the western Atlantic, ensemble guidance has shown improved resolution in regards to the development of two distinguished areas of surface low pressure during the next two weeks. A coastal low situated off the Mid-Atlantic remains favored to form off a frontal boundary in the next day or so, where the National Hurricane Center (NHC) designates 30% chances (as of 2pm EDT) of this low acquiring subtropical characteristics during the next week. As this low is favored to eventually weaken and be pushed out by rebuilding high pressure over the Northeast, ensembles continue to point to a new cluster of deepening low members further south to the north of Bahamas early in week-2. As a result, a renewal of cyclonic surface low is favored and precipitation looks to become quite enhanced with the availability of tropical moisture off the Eastern Seaboard. Consistent with previous guidance, 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 remains valid through Oct 29 before much of the troughing aloft deamplifies. 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 next weekend. This pattern change looks to bring drier conditions to parts of 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, as well as an uptick in signals in percentile space compared to yesterday, there remains low chances that precipitation amounts and wind speeds exceed seasonal hazard thresholds for Alaska, resulting in no hazards being posted for the state. FORECASTER: Nick Novella $$