


Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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661 FXUS21 KWNC 171900 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT October 17 2025 SYNOPSIS: Strong low pressure at the surface and mid-levels is favored to shift eastward from the northeastern Pacific and bring an elevated risk of hazardous winds, heavy precipitation, and high elevation heavy snowfall across many parts of the West Coast and Interior West throughout the forecast period. The persistence of above-normal and potentially heavy precipitation may lead to flooding across parts of the Northwest. Tied to the evolution of the mid-level troughing, surface low development is favored in the lee of Rockies, increasing chances for heavy precipitation across the central contiguous U.S. (CONUS) during the middle of the period. Additional mid-level low pressure favored over the Great Lakes late next week looks to promote enhanced northwesterly flow and cooler minimum temperatures where frost and/or first autumn freezes may adversely impact vulnerable vegetation east of the Mississippi. HAZARDS High risk of heavy precipitation for parts of northern and central California and southern Oregon, Sat-Sun, Oct 25-26. High risk of high elevation heavy snow over the Sierra Nevada and Klamath mountains, Sat-Sun, Oct 25-26. Moderate risk of heavy precipitation from the Bay Area of California northward to southwestern Oregon, Sat-Mon, Oct 25-27. Moderate risk of high elevation heavy snow over portions of the Sierra Nevada, Klamath, and southern Cascade Mountains, Sat-Mon, Oct 25-27. Moderate risk of heavy precipitation over the Pacific Northwest, Sat-Wed, Oct 25-29. Moderate risk of high elevation heavy snow over the Cascades, Sat-Wed, Oct 25-29. Moderate risk of high winds from the Bay Area of California northward to the Pacific Northwest, Sat-Wed, Oct 25-29. Moderate risk of high elevation heavy snow for portions of the northern Rockies and northern Intermountain, Sat-Mon, Oct 25-27. Moderate risk of high winds over parts of the Great Basin, Rockies, Southwest, and the Great Plains, Sat-Tue, Oct 25-28. Slight risk of heavy precipitation for portions of California and the Pacific Northwest, Sat-Fri, Oct 25-31. Slight risk of heavy snow for the Sierra Nevada, Klamath, and Cascade Mountains, Sat-Fri, Oct 25-31. Slight risk of heavy snow for portions of Northern Intermountain, Northern and Central Rockies, Sat-Wed, Oct 25-29. Slight risk of high winds for much of the western half of the CONUS, Sat-Fri, Oct 25-31. Slight risk of much below normal temperatures for portions of the Great Lakes, Ohio and Tennessee Valleys, Appalachians, and Mid-Atlantic, Sat, Oct 25. Slight risk of heavy precipitation for portions of the Plains and the Mississippi, Tennessee and Ohio Valleys, Mon-Wed, Oct 27-29. Slight risk of high winds for the southern Mainland and southeast Alaska, Sat-Mon, Oct 25-27. Flooding possible across parts of the Pacific Northwest and northern California. DETAILED SUMMARY FOR MONDAY OCTOBER 20 - FRIDAY OCTOBER 24: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR SATURDAY OCTOBER 25 - FRIDAY OCTOBER 31: The week-2 hazards perspective remains on track today with much of the outlook focusing on potential Atmospheric River (AR) activity producing multiple weather related hazards of elevated risk throughout the western CONUS. Early in week-2, good agreement exists among the dynamical model ensemble mean 500-hPa height forecasts featuring a potent 500-hPa trough encroaching the West Coast, with a 850-hPa flow pattern favorable for ample incoming Pacific moisture to supply locally heavy precipitation. Some differences between the 0z GEFS and 0z ECMWF ensembles are in the details of the upper-level features, with the former favoring stronger and deeper negatively tilted trough axis, along with a stronger jet protruding into California. As a result, there is the potential for more AR activity further south in California in the ECMWF compared to the GEFS, however both models continue to well agree in regards to the timing of this potential event with the highest AR related risks occurring on days 8 and 9 (Oct 25-26). Thereafter, these risks are anticipated to retreat northward into the Pacific Northwest and ease by days 10 and 11 over California as the trough axis shifts further inland and mid-level heights begin to rise over the southwestern CONUS. During this time, however, ensembles continue to point to additional troughing offshore in the eastern Pacific, which looks to maintain above-normal precipitation for the northwestern CONUS and potentially bring a renewed threat of heavy precipitation later in October. Based on Integrated Vapor Transport (IVT) tools, as well as the latest raw and calibrated precipitation guidance, high risks for heavy precipitation and high elevation heavy snow remain issued for parts of northern and central California and southern Oregon for Oct 25-26. Within these highlighted areas, both the GEFS and ECMWF Probabilistic Extreme Tools (PETs) indicate at least a 50% chance of 3-day amounts exceeding the 85th climatological percentile, with the uncalibrated ECMWF favoring similar chances for amounts exceeding an inch (two inches) on day 8 (days 8-10). In deference to the ECMWF and model trends, these high risk areas are expanded in coverage southward to approximately the Bay Area of central California. Across the Pacific Northwest, there are comparably lower probabilities in percentile space in the PETs, though raw tools show 30-40% for 3-day amounts exceeding two inches persisting through the middle of week-2, which is also consistent with IVT guidance. To better account for the evolution of the heavy precipitation risk across the West Coast, moderate risk areas of heavy precipitation and high elevation heavy snow remain issued for portions of California and Oregon for Oct 25-27, with a separate pair of moderate risk areas for heavy precipitation and high elevation heavy snow focused over the Pacific Northwest, both valid through Oct 29. Although soils are not currently oversaturated over parts of the West Coast, a flooding possible hazard remains posted as this area may be more responsive to flooding given the elevated precipitation risks, and with increased precipitation amounts favored during week-1 to help prime conditions. At the base of the amplified troughing, a moderate risk for high winds also remains issued (Oct 25-29) where PETs depict higher chances (30-40%) for wind speeds exceeding the 85th percentile through the middle of week-2. Broader slight risk areas for heavy precipitation and heavy snow are still posted for the West Coast, and remain valid for the entirety of week-2 due to the potential for anomalous troughing reloading later in October. With much of the anomalous mid-level troughing shifting eastward next weekend, enhanced Pacific moisture being advected over high elevation areas of the Interior West supports the potential for heavy snowfall over portions of the Rockies and the Northern Intermountain region. Based on raw snow tools and the Snow Water Equivalent (SWE) PET, a moderate risk of heavy snow remains issued, and is now valid through Oct 27, with a broader slight risk area extending further southward into the Four Corners, valid through Oct 29. A separate moderate risk area for high winds is also posted (Oct 25-28) over the higher elevations of the Interior West, with its coverage also expanded southward into the Southwest and Southern Rockies in the updated outlook where PETs and raw tools favor higher signals for wind speeds exceeding the 85th percentile with gusts of over 34 knots. A broad slight risk area of high winds is posted for nearly the western half of the CONUS to capture the wind potential associated with the amplified mid-level pattern favored during week-2. Early in week-2, anomalous troughing favored over the Great Lakes looks to bring more northwesterly flow and a round of anomalously cold temperatures into portions of the eastern U.S. While both GEFS and ECMWF PETs are weak in percentile space (less than 20% chances for minimum temperatures falling below the 15th percentile) associated with this cooler air, there continues to be support for sub 40 degree F minimum temperatures as far south as the Tennessee Valley. Because many areas across the eastern CONUS have not yet experienced their first autumn freezes and/or frost, infringing cold temperatures could impact vulnerable vegetation, and a slight risk of much below normal temperatures is highlighted for Oct 25 before temperatures are expected to moderate later in the period. Towards the middle of week-2, models are showing stronger signals for shortwave energy being pinched off over the Great Plains associated with the building heights over the southwestern CONUS, leading to surface low development in the lee of the Rockies. The ECMWF is more pronounced with this realization, and given the mean surface high pressure (associated with the aforementioned cold air early in week-2) that is favored to the east to bring moist return flow from the Gulf, the precipitation response is becoming increasingly reflected in the raw and calibrated tools. Therefore, a slight risk of heavy precipitation is posted over portions of the eastern Plains, and Mississippi, Tennessee, and Ohio Valleys for Oct 27-29. Over Alaska, a slight risk of high winds is posted (Oct 25-27) with its coverage remaining focused over the southern Mainland and the Southeast. While much of the stronger mid-level troughing has shifted southward to affect the western CONUS, PETs still maintain increased signals for wind speeds exceeding the 85th percentile over the region. FORECASTER: Nick Novella $$