147 FXUS21 KWNC 151819 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT August 15 2026 SYNOPSIS: A major, long duration period of extreme heat is expected to persist into week-2 across the central CONUS and a large part of the Nation's southern tier. The highest chances for extreme heat cover the Central and Southern Plains and Lower Mississippi Valley, but at least a slight risk covers most of the southern tier of the Nation from the Desert Southwest to the South Atlantic Coast. Anomalous mid-level high pressure forecast near the Four Corners is forecast in concert with an incursion of Gulf moisture, producing the high heat indices. This mid-level high pressure is favored to drift slowly westward or southwestward while deamplifying, ending the extreme heat risk late week-2. Meanwhile, given antecedent dryness, the above-normal temperatures and subnormal rainfall expected over the next two weeks support a Rapid Onset Drought (ROD) risk for portions of the Central and Southern Plains and Middle and Lower Mississippi Valley. HAZARDS Moderate risk of extreme heat for much of the Central and Southern Plains, and parts of the Lower Mississippi and adjacent Tennessee Valleys, Sun-Mon, Aug 23-24. Slight risk of extreme heat for portions of the Southwest, Southern Rockies, Central and Southern Plains, Lower Mississippi and Tennessee Valleys, and Southeastern CONUS, Sun-Thu, Aug 23-27. Rapid Onset Drought (ROD) risk for portions of the Central and Southern Plains and the Middle and Lower Mississippi Valley. DETAILED SUMMARY FOR TUESDAY AUGUST 18 - SATURDAY AUGUST 22: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR SUNDAY AUGUST 23 - SATURDAY AUGUST 29: Dynamical models remain in good agreement regarding subtropical ridging across the south-central to southwestern U.S. through most of week-2. The 0z European ensemble (ECENS), GEFS, and Canadian ensemble (CMCE) means all start the period with a 596 dm 500-hPa height maximum centered near the south-central CONUS or eastern Four Corners region. The ECENS and GEFS are in excellent agreement placing the center of this feature over northern or eastern New Mexico while the CMCE mean depicts this feature over the Red River (South) Valley, but the slightly farther west placement is favored since it is the majority solution, and more consistent with earlier runs. Meanwhile, ensemble means depict a somewhat stronger and more persistent mid-level trough over the northeastern CONUS relative to yesterday. The CMCE mean is strongest with this feature and the ECENS mean the weakest, with the GEFS mean splitting the difference. The GEFS mean, which approximately equals the average of all three ensemble means, is favored with this mid-level trough. Over the course of week-2, the 500-hPa height center remains roughly in place while flattening and deamplifying. This process is expected to increase 500-hPa heights in the Southeast for a day or two early in the period before the mid-level trough to the north is reinforced, causing heights there to drop again, and remain near normal for the last half of week-2. Confidence remains high for a long duration heat event, with many tools indicating the potential for oppressive conditions lingering into late August. But with a stronger, more persistent mid-level trough forecast over the Northeast with an axis southward along the coast, the greatest risk of extreme heat is focused a bit farther west, over the Lower Mississippi Valley and Southern Plains early week-2. A moderate risk of extreme heat is posted there for the first two days of the period (Sun-Mon Aug 23-24). The broadening mid-level ridge, however, increases the chances for at least a brief period of extreme heat across a broad area stretching from the Desert Southwest to the lower South Atlantic Coast until later week-2. By Fri-Sat Aug 28-29, the ensemble means all show the height center of the mid-level ridge remaining over the eastern Four Corners Region or sinking slightly southward into far northern Mexico while the greatest 500-hPa heights drop to around 591 dm, which is expected to end the risk of extreme heat. Both the GEFS and ECENS Probabilistic Extremes Tools (PETs) depict at least a 40 percent chance of maximum temperatures exceeding the 85th climatological percentile early week-2 over the Red River (South) Valley and Southern High Plains. The GEFS PET is far more extensive with its depiction of elevated chances for extreme heat, with odds topping 40 percent from the Central and Southern Rockies through the Desert Southwest, Southern Plains, and Lower Mississippi Valley. Given the strength, position, and size of the mid-level ridge driving the hot weather, the higher likelihoods depicted in the GEFS PET are preferred. After day 9 (Mon Aug 24), the ECENS PET shows the chances for extreme heat dropping below 40 percent throughout the CONUS. This marks the end of the moderate risk for extreme heat in the south-central CONUS, but higher odds in the GEFS PET remain until later in week-2, and a slight risk for extreme heat is maintained across the southern tier of the Nation until the GEFS PET shows chances for extreme heat dropping below 40 percent. This is also the point when the ensemble means all show the mid-level ridge beginning to more quickly deamplify and weaken. Increased humidity is expected to contribute to extreme heat index values from the Southern Plains eastward, 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 in some areas exceed 70 percent for maximum heat index values to exceed 105 deg F. The likelihood of heat index values topping 110 deg F exceed 50 percent in South Texas and the Lower Colorado Valley, and ranges from 20 to 40 percent across Oklahoma, southernmost Kansas, Louisiana, Mississippi, Arkansas, and northern Texas. In addition, the National Blend of Models (NBM) depicts nighttime lows nearing or breaking daily records early week-2 from southern Florida to southern California, resulting in little overnight relief from daytime heat. 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 over the Four Corners, especially the higher elevations of Colorado. However, the trend toward reduced signal strength has continued today, so no hazards are posted in association with potentially heavy monsoon-related convective activity; however, isolated areas of heavy rain remain possible, and this region is prone to flash flooding even with relatively small precipitation totals along dry creek beds (arroyos) and in canyons. Caution should always be exercised in such areas should thunderstorm activity initiate nearby. Also, any convective activity may result in relatively little rainfall while still being capable of generating lightning, which poses a risk of wildfire initiation. 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 (near or under half of normal), with little rainfall expected during the next two weeks. Furthermore, much of this ROD area is within the moderate risk of extreme heat. This could potentially increase evapotranspiration rates and accelerate surface moisture depletion. FORECASTER: Rich Tinker $$