906 FXUS21 KWNC 091807 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT September 09 2026 SYNOPSIS: Continued mid-level high pressure over the southern and central contiguous U.S. (CONUS) supports much above-normal temperatures from the Great Plains to the Mid-Atlantic early in week-2, though the risk of extreme heat is diminishing as we move closer to the start of autumn. Another round of enhanced tropospheric moisture is favored to overspread the western and central CONUS, potentially bringing heavy precipitation and possible flooding over portions of from the Four Corners eastward to the Ohio Valley and lower Great Lakes. A developing area of mid-level low pressure looks to bring an unseasonably cool air mass east of the Rockies, possibly resulting in first frost and/or freezes to the higher latitudes of upper Midwest and into the Northeast later in the period. Antecedent dryness, combined with persistent above-normal temperatures supports a Rapid Onset Drought (ROD) risk for portions of the Central and Southern Plains, Lower and Middle Mississippi, Ohio and Tennessee Valleys. HAZARDS Slight risk of much below normal temperatures for parts of the Northern Plains, Upper Mississippi Valley, and eastern Great Lakes, Thu-Sun, Sep 17-20. Slight risk of much below normal temperatures for parts of northern New England, Sat-Tue, Sep 19-22. Slight risk of heavy precipitation from the Four Corners region eastward to the Ohio Valley and Great Lakes, Thu-Sat, Sep 17-19. Flooding possible for portions of the Desert Southwest, Great Basin, and Rockies. Rapid Onset Drought (ROD) risk for portions of the Central and Southern Plains and the Middle and Lower Mississippi, Tennessee, and Ohio Valleys. DETAILED SUMMARY FOR SATURDAY SEPTEMBER 12 - WEDNESDAY SEPTEMBER 16: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR THURSDAY SEPTEMBER 17 - WEDNESDAY SEPTEMBER 23: Towards the end of next week, mean 500-hPa height forecasts from the ECMWF and GEFS ensembles remain in fairly good agreement favoring an amplified ridge centered over the Gulf of Alaska with downstream troughing overspreading much of eastern Canada and the northeastern CONUS. Underneath the amplified ridging over Gulf of Alaska early in week-2, ensembles also maintain a positively tilted mean trough extending from the West Coast. This feature, combined with additional Tropical Cyclone (TC) activity in the eastern Pacific, may bring another round of enhanced moisture advection into the western CONUS supportive of heavy precipitation with continued flood risks over the Four Corners. Over time, both the ECMWF and GEFS ensembles favor some progression of the anomalous ridging over western Canada along with a deepening of the mid-level trough over eastern Canada by the middle of week-2. Concurrent with this deepening trough, much of the enhanced subtropical ridging is favored to ease and become more suppressed across the southern tier of the CONUS, allowing for cooler, and more fall-like temperatures mainly east of the Rockies. Such a cooldown is anticipated to be a welcomed relief particularly for areas embattled by persistent heat conditions since August across the Plains and Mississippi Valley, however any cold air advection forced by a high latitude ridge over western Canada looks to bring unseasonably chilly temperatures with possible first frosts and/or freezes for portions of the northern tier of the CONUS during the period. Consistent with the observed height pattern since last month, continued enhanced subtropical ridging also remains featured in the ensemble guidance and looks to reach its peak amplitude late in week-1. This mid-level feature is expected to contribute to a prolonged period of unseasonably late summer warmth, though the prospects for heat conditions are lowered due to decreasing sun angles to limit any extreme daytime temperature maximums. While raw temperature tools depict elevated chances for double digit temperature departures extending from the Southern and Central Plains eastward to the Mid-Atlantic associated with a 595-596dm mean ridge axis in the ensembles, calibrated Probabilistic Extreme Tools (PETs) indicate low chances for daytime temperatures exceeding 98th climatological percentile and daily heat index tools have become less supportive of values exceeding hazard thresholds. Therefore, no corresponding heat hazards are issued, however, those sensitive to heat should continue to exercise caution as temperatures are expected to remain well above normal early in the period before the ridging aloft deamplifies. A Rapid Onset Drought (ROD) risk remains designated for parts of the Southern Plains and Lower Mississippi Valley and adjacent portions of the Central Plains, Ohio, Tennessee and Middle Mississippi Valleys, and Southeast U.S. This area has seen 20-40% of normal precipitation and deficits of 1-2 inches over the past 30 days. Above normal temperatures are forecast for this region over the next two weeks increasing evapotranspiration rates into the third week of September. With troughing situated off the West Coast, with the potential for added tropical moisture from the eastern Pacific, a slight risk of heavy precipitation remains posted (Sep 17-19) over the Four Corners and High Plains, with coverage expanded eastward to include portions of the Mississippi and Ohio Valleys, and the lower Great Lakes in the updated outlook. This expansion is based on an uptick in heavy precipitation signals in both the GEFS and ECMWF PETs, as well as the raw ECMWF ensemble favoring a pronounced increase in precipitable water anomalies (now nearing 200 percent of normal over the Midwest early in week-2). The enhanced tropospheric moisture is favored to round the northern periphery of the subtropical ridge, where both the raw GEFS and ECMWF show increased chances for 3-day precipitation amounts exceeding 1 inch over portions of the highlighted slight risk area. Additionally, given the continued indications of potential heavy precipitation, a flooding possible hazard remains posted for portions of the Four Corners region during the week-2 period. This region is prone to flash flooding and caution should be exercised near canyons and creek beds, and additional rainfall may hamper recovery efforts from recent flooding in the Grand Canyon. Tied to the deepening trough favored over the northeastern CONUS, a slight risk of much below normal temperatures remains posted over the Upper Midwest for Sep 17-20, with a separate cold risk area designated for northern New England valid for Sep 19-22 to capture the evolution of the cold air advection. While the GEFS PET is weaker with the cold potential, the ECMWF PET depicts an expanding area with at least 20-30% chances of minimum temperatures falling below the 15th percentile shifting eastward with time, and uncalibrated GEFS and ECMWF tools show increased chances for minimum temperatures falling below 38 deg F over the Northern Plains, Upper Mississippi Valley and Northeast during the period. While such temperatures are not markedly unusual for this time of year (climatological first freezes occur by late September for the highlighted areas), any frost and/or freezes may be impactful to some vegetation and unsuspecting populations. Across the eastern Pacific, the National Hurricane Center (NHC) is currently monitoring two areas for additional TC development (as of 7am PDT), with 90% and 40% chances of development during the next week, both located approximately between 125W and 110W. While the lower likelihood area may become a factor in regards to the enhancing moisture advection into the CONUS, probabilistic TC genesis and activity tools depict the higher likelihood disturbance maintaining a westerly track steered by a subtropical ridge extending into the Central Pacific. Although there remains high uncertainty, additional tropical related precipitation impacting parts of Hawaii cannot be ruled out during the week-2 period. FORECASTER: Nick Novella $$