617 FXUS21 KWNC 101822 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT August 10 2026 SYNOPSIS: A major, long duration period of extreme heat continues to be predicted across portions of the south-central contiguous U.S. (CONUS) and Southeast persisting into week-2 tied to anomalous mid-level high pressure combined with high humidity levels. Model guidance has continued to trend stronger with this event supporting an expansion of the high risk area into more of the Central Plains, Middle Mississippi Valley, and Southeast compared to yesterday. Antecedent dryness, combined with little precipitation favored over the next two weeks supports a Rapid Onset Drought (ROD) risk for portions of the Central and Southern Plains and Middle and Lower Mississippi Valley. Enhanced monsoonal activity continues to be indicated across the Four Corners, although with less certainty with downstream progression across the Midwest as some of the stronger signals have now shifted into week-1. HAZARDS High risk of extreme heat for portions of the Central and Southern Plains, Middle and Lower Mississippi Valley, and Southeast, Tue-Fri, Aug 18-21. Moderate risk of extreme heat for portions of the Central and Southern Plains, Middle and Lower Mississippi and Tennessee Valleys, Southeast, and southern Mid-Atlantic, Tue-Sat, Aug 18-22. Slight risk of extreme heat for portions of the Central and Southern Plains, Middle and Lower Mississippi and Tennessee Valleys, Southeast, and southern Mid-Atlantic, Tue-Mon, Aug 18-24. Slight risk of heavy precipitation for portions of the Four Corners, Tue-Sat, Aug 18-22. Rapid Onset Drought (ROD) risk for portions of the Central and Southern Plains and the Middle and Lower Mississippi Valley. DETAILED SUMMARY FOR THURSDAY AUGUST 13 - MONDAY AUGUST 17: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR TUESDAY AUGUST 18 - MONDAY AUGUST 24: Dynamical model guidance is in good agreement regarding subtropical ridging persisting across the south-central and southeastern U.S. into week-2. The 0z ECMWF, GEFS, and Canadian ensembles depict a 594-596 dm 500-hPa height maximum slowly retrograding from the Southeast to the Southern Plains throughout the period, with its amplitude steady or even increasing slightly. As a result, confidence remains high for a long duration heat event, with many tools indicating the potential for oppressive conditions lingering well into late August. Both the GEFS and ECMWF Probabilistic Extremes Tools (PETs) depict at least a 60 percent chance of maximum temperatures exceeding the 85th climatological percentile at the start of week-2 centered across the Central Gulf Coast states, and remaining above 40 percent for most of the period. The ECMWF PET is generally more east-focused with a greater extension of higher probabilities into the Southeast, with the GEFS favoring higher odds over the Southern Plains and Lower Mississippi Valley tied to quicker ridge retrogression. A large contribution from increased humidity is expected, with a strong signal in the calibrated skill-weighted GEFS-ECMWF heat risk guidance, particularly across the Central and Southern Plains and adjacent Mississippi Valley where probabilities exceed 60 percent for maximum heat index values to exceed the 95th climatological percentile. The high risk of extreme heat is extended into the Central Plains, Middle Mississippi Valley, and the Southeast, where heat index values may exceed 110 deg F, and is valid Aug 18-21. However, the retrogression of the ridge may allow some of the highest heat to shift out of the Southeast by the middle of week-2 consistent with the evolution in the PETs. In addition, the National Blend of Models (NBM) also depicts a number of locations with nighttime lows nearing and/or breaking daily records, where little overnight relief to daytime heat is expected. The moderate risk for extreme heat is now extended through Aug 22 and is also extended slightly northward where there is increased potential for heat index values to exceed 105 deg F. A slight risk surrounds the moderate risk area and continues for all of week-2. The GEFS is more robust in depicting increasing heat expanding into the Northern Plains later in week-2. Since the ECMWF and Canadian ensembles keep the heat across the Central Plains and south, no northward expansion is indicated in today's outlook but trends will continue to be monitored. Troughing initially near the West Coast of the CONUS favors an influx of moisture across much of the western U.S., and enhanced Southwest Monsoon activity continuing over the Four Corners where the GEFS and ECMWF PETs depict at least a 20 percent chance 3-day precipitation exceeds the 85th climatological percentile and a half-inch justifying a slight risk of heavy precipitation, Aug 18-22. This region is prone to flash flooding even with relatively small precipitation accumulations especially along dry creek beds (arroyos) and in canyons. Caution should always be exercised in such areas should thunderstorm activity initiate nearby. Some of this moisture is likely to propagate overtop the ridge axis over the south-central and southeastern CONUS and lead to enhanced frontal or Mesoscale Convective Systems (MCSs) across the Midwest, Great Lakes, and possibly into the Northeast or Mid-Atlantic. However, the PETs are not indicating any enhanced signal for hazardous precipitation, and uncalibrated guidance, particularly from the 0z ECMWF, indicates a stronger chance of a shortwave moving through late in week-1. While it is not out of the question for additional activity during week-2, confidence is no longer high enough to support a slight risk of heavy precipitation over these areas. Toward the middle of week-2, troughing along the West Coast is favored to retreat with rising heights across the West tied to expansion of the ridging over the south-central CONUS. This evolution would decrease the moisture advection into the West, and also promote an increase in above-normal temperatures and possibly extreme heat across portions of the Interior West where the GEFS PET depicts increasing coverage of probabilities greater than 20 percent for maximum temperatures to exceed the 85th climatological percentile. However, due to lower probabilities in the ECMWF PET, no related extreme heat hazards are posted across the West at this time. A Rapid Onset Drought (ROD) risk remains designated across portions of the Central and Southern Plains and Middle and Lower Mississippi Valley. This ROD area continues to register 30-day precipitation deficits greater than 2 inches, with little rainfall forecast during the next two weeks. Much of this ROD area is also included within the moderate risk of extreme heat as the potential for increased evapotranspiration rates also looks to reinforce this risk later in August. Across the eastern Pacific, the National Hurricane Center (NHC) is monitoring three areas located near 140 deg W, 130 deg W, and 115 deg W, with 90 percent, 60 percent, and 10 percent chances of tropical cyclone (TC) development, respectively, during the next seven days (as of 11am PDT). For the western watch area near 140 deg W, both dynamical model and Google AI guidance are indicating a clustering of tracks near or just to the south of the Hawaiian Islands, although potential impacts are shifting more into the late week-1 timeframe. However, the GEFS and ECMWF PETs continue to depict at least a 20 percent chance 3-day precipitation totals exceed the 85th climatological percentile early in week-2 across most of the state, and interests across Hawaii are advised to monitor the latest forecasts from NHC (https://www.nhc.noaa.gov/). FORECASTER: Thomas Collow $$