768 FXUS21 KWNC 071914 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT September 07 2025 SYNOPSIS: The favored development of strong mid-level high pressure across central North America next week is anticipated to result in a pattern change from week-1, with widespread above-normal temperatures and possible extreme heat conditions emerging from the central contiguous U.S. (CONUS) to the Appalachians during week-2. The abnormally warm temperatures, combined with antecedent dryness and below normal precipitation favored during the next two weeks support a Rapid Onset Drought risk over parts of Mississippi and Alabama. Lingering mid-level low pressure forecast over the western third of the CONUS early in the period favors increased chances for episodes of high winds over the West Coast and parts of the Interior West. A stalled frontal boundary remains forecast across the southeastern U.S. bringing an increased likelihood for enhanced precipitation and high winds. Surface low pressure is forecast to bring increased chances of elevated wind speeds to southern and southeastern coastal Alaska. HAZARDS Slight risk of extreme heat across portions of the eastern Great Plains, Middle and Lower Mississippi, Tennessee, and Ohio Valleys, and southern Great Lakes, Mon-Thu, Sep 15-18. Slight risk of high winds over the West Coast and portions of the Interior West, Mon-Wed, Sep 15-17. Slight risk of heavy precipitation across the Florida Peninsula and coastal Southeast, Mon-Fri, Sep 15-19. Slight risk of high winds across the central and eastern Gulf Coast, and the lower Eastern Seaboard, Mon-Fri, Sep 15-19. Slight risk of high winds across coastal portions of southern and southeastern Alaska, Mon-Fri, Sep 15-19. Rapid Onset Drought risk across portions of the Lower Mississippi and Tennessee Valleys. DETAILED SUMMARY FOR WEDNESDAY SEPTEMBER 10 - SUNDAY SEPTEMBER 14: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR MONDAY SEPTEMBER 15 - SUNDAY SEPTEMBER 21: By the outset of the period, there is good agreement between the 0z GEFS and ECWMF ensemble mean 500-hPa height forecasts, featuring a strongly amplified ridge over central Canada, with a pair of mean troughs, one situated over the Gulf of Alaska and the other oriented along the eastern Seaboard of the U.S. Consistent with previous guidance, the ECMWF continues to be most bullish with the Canadian ridge, favoring a mean height anomaly center in excess of 240 dam along and to the west of Hudson Bay late in week-1 and persisting into early week-2. In response to this predicted mid-level pattern, above-normal temperatures are predominantly favored for much of the central CONUS, with end of summer heat conditions possible for many parts of the Midwest. With this blocking ridge expected to have some staying power in the longer leads, guidance continues to favor a deamplification of the Gulf of Alaska trough with above-normal heights building back over western North America. Though, the GEFS and ECMWF differ somewhat on this evolution, as the latter model shows hints of continued shortwave troughing which could bring additional shots of enhanced onshore flow and unsettled weather into the northwestern CONUS. Based on the latest raw and calibrated temperature tools, a slight risk of extreme heat remains posted across parts of the eastern Plains, Mississippi, Tennessee and Ohio Valleys, and southern Great Lakes, valid through Sep 18. Within the highlighted region, Probabilistic Extremes Tools (PETs) indicate at least a 20% chance for maximum daytime temperatures exceeding the 90th climatological percentile, with increased chances for temperatures exceeding 90 (95) degrees F over the Middle Mississippi and Ohio Valleys (Lower Mississippi and Tennessee Valleys). Although these PET heat signals extend into the Upper Midwest closer to the Canadian ridge, daytime temperatures are not expected to reach hazardous thresholds due to lowering sun angles at these higher latitudes with the autumn equinox soon approaching. Relative to yesterday, the GEFS PET has become more robust with the heat signals (upwards of 40% chances for exceeding the 90th percentile) supportive of a higher risk designation, however the uncalibrated GEFS and ECMWF PET are comparatively milder with the heat potential. In addition to the above-normal temperatures, the combination of favored below-normal precipitation over this part of the country supports a Rapid Onset Drought (ROD) risk, and remains posted mainly over portions of Mississippi and Alabama. Within this area, many local areas are registering less than half their normal rainfall accumulation over the past 60 days. Tied to shortwave troughing that is favored to encroach the western CONUS early in week-2, both the GEFS and ECMWF PETs indicate increased chances for winds speeds exceeding the 85th percentile, and a slight risk of high winds remains posted for the West Coast, and into portions of the Interior West. This slight wind risk is extended through Sep 17 in the updated outlook before positive mid-level height departures generally begin to overspread the western CONUS by the middle of week-2. Any additional episodes of high winds could prolong the risk for wildfire development due to very low soil moisture values being registered for many parts of the West. Associated with more onshore flow favored in the ECMWF height forecasts, this is likely contributing to elevated (>40%) precipitation signals in the ECMWF PET for 3-day amounts exceeding the 85th percentile across many parts of the northwestern CONUS. By comparison, the GEFS PET remains much more muted with this potential, and given that these signals in percentile space are more the result of low precipitation climatologies in the northwestern CONUS, with actual amounts favored to remain below hazardous thresholds, no corresponding precipitation hazards are issued. If anything, any rainfall above climatology would provide needed relief to many areas in the northwestern CONUS that are experiencing moderate to extreme drought conditions based on the latest U.S. drought monitor. Over the southeastern CONUS, WPC days 3-7 surface prognosis maps depict a stalled frontal boundary located over the Gulf of America and into the western Atlantic, which looks to be the focus for enhanced and possibly heavy precipitation during week-1. This surface feature is expected to persist well into week-2, with the ECMWF (GEFS) PETs continuing to favor a more southward (northward) solution of the heavy precipitation potential early in week-2. By the middle of week-2, the uncalibrated and calibrated ECMWF tools begin to shift the precipitation risk back into Florida, becoming better aligned with the GEFS PET with 3-day amounts exceeding the 85th percentile and 1 inch. Therefore, a slight risk of heavy precipitation remains posted mainly over the Florida peninsula, and now valid through Sep 19. Due to continuous enhanced precipitation over the state during the next two weeks, any heavy rainfall may trigger isolated flash flooding during the week-2 period. The wind PETs also continued to support a broader slight risk area of high winds from the central Gulf to the lower Mid-Atlantic. Consistent with previous thinking, surface low formation along the frontal boundary cannot be ruled out, which if realized, could bring a renewed threat of rainfall and high winds later in week-2. Since yesterday, a tropical disturbance (Invest 91L) located over the Main Development Region (MDR) has dissipated and the National Hurricane Center is indicating no other areas in the tropical Atlantic for potential development as of 1:00 PM EDT. In the eastern Pacific, the ECMWF probabilistic tropical cyclogenesis tool continues to favor additional tropical cyclone development to the south and west of Mexico, which could possibly trigger a Gulf of California surge event into the Desert Southwest in week-2. The ECWMF favors an enhancement of mid-tropospheric moisture into the lower Four Corners, along with increased chances (20-30%) for 3-day precipitation amounts exceeding the 85th percentile through the middle of week-2. However, the GEFS remains largely unsupportive of a heavy precipitation risk, resulting in no precipitation related hazards, though this potential will continue to be monitored in upcoming outlooks. The mean troughing favored over the Gulf of Alaska, and associated mean surface low pressure is expected to bring enhanced precipitation with potentially hazardous high winds to parts of south-central and southeastern coastal Alaska during week-2. Consistent with previous guidance, the ECMWF PET depicts elevated chances (40%-60%) of wind speeds exceeding the 85th climatological percentile and 40-mph across southern and southeastern coastal Alaska, whereas the GEFS PET remains more limited with this potential. While a moderate risk was considered for the outlook, only a slight risk of high winds remains designed for this part of Alaska for Sep 15-19 due to weaker mean surface pressure favored and troughing aloft compared to yesterday. A slight risk of heavy precipitation was also considered for inclusion based on increased signals in the PETs, however there is less confidence that actual totals will exceed hazardous thresholds and this potential will be reassessed in upcoming outlooks. FORECASTER: Nick Novella $$