898 FXUS21 KWNC 181811 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT August 18 2026 SYNOPSIS: Dynamical models favor a potentially major and long duration extreme heat event over the southern tier of the contiguous U.S. (CONUS), with the greatest risk focused over the Southern and Central Plains underneath a potent mid-level high pressure center next week. This mid-level high pressure center is favored to shift westward with time, bringing increased risks for extreme heat across the Southern High Plains, Rockies, and Desert Southwest. Meanwhile, due to antecedent dryness, above-normal temperatures and below-normal rainfall forecast during next two weeks supports a Rapid Onset Drought (ROD) risk for portions of the Central and Southern Plains and Middle and Lower Mississippi Valley. HAZARDS High risk of extreme heat for parts of the Desert Southwest, Southern Rockies, Southern and Central Plains, and adjacent areas of the Mississippi Valley, Wed-Fri, Aug 26-28. Moderate risk of extreme heat for much of the Desert Southwest, Rockies, Central and Southern Plains, parts of the Middle and Lower Mississippi Valley, Wed-Sun, Aug 26-30. Slight risk of extreme heat for portions of the Southwest, Great Basin, Rockies, Central and Southern Plains, Middle and Lower Mississippi and Tennessee Valleys, and Southeastern CONUS, Wed-Tue, Aug 26-Sep 1. Slight risk of heavy precipitation for the coastal Mid-Atlantic and Southeast, Wed-Fri, Aug 26-28. Rapid Onset Drought (ROD) risk for portions of the Central and Southern Plains and the Middle and Lower Mississippi Valley. DETAILED SUMMARY FOR FRIDAY AUGUST 21 - TUESDAY AUGUST 25: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR WEDNESDAY AUGUST 26 - TUESDAY SEPTEMBER 01: Since yesterday, the hazards perspective remains on track as there continues to be elevated chances for extreme heat conditions persisting into the week-2 period throughout the southern tier of the CONUS. Dynamical models remain in good agreement and are bullish with an amplified subtropical ridge center retrograding from the Southern Plains into the eastern Four Corners region, where models maintain some signal for 500-hPa heights exceeding 597 dm throughout much of the period. One notable change since yesterday concerns the GEFS, which has been consistently weaker with the subtropical ridging in relation to the ECMWF, now favors a mean solution that is nearly equal in strength early in the period. Given this, and the persistence of strong 500-hPa height signals in the guidance, there is higher outlook confidence for a long duration and potentially stronger heat event unfolding next week. In addition to the subtropical ridging, a strong ridge axis is also featured in the ensembles extending northward into Canada, supportive of above-normal temperatures and possible extreme heat conditions emerging over portions of the Interior West, Plains, and Mississippi Valley next week. Bookended by a pair of mean troughs over the eastern Pacific and eastern CONUS, the 500-hPa mean height forecasts reveal more of an Omega block presentation, implying the potential for stable pattern with little respite to the predominant ridging and summertime warmth for much of central North America later in August and possibly into early September. In proximity to the amplified subtropical ridge center, a high risk of extreme heat remains posted and is slightly expanded to include additional portions of the Southern Plains, and westward into New Mexico and southern Arizona, now valid through day 10 (Aug 28). These adjustments are based on stronger and more expansive temperature anomalies that are favored to persist into late next week, as well as the persistence of elevated maximum temperature signals in the Probabilistic Extreme Tools (PETs). Within the highlighted high risk area, calibrated heat tools continue to highlight >60% chances for daytime temperatures and/or heat index values exceeding the 90th percentile, where there is good model support for daytime temperatures above 100 deg F (except for higher elevations) with heat index values peaking above 110 deg F where higher dewpoints are expected. Surrounding the high risk area, a moderate risk of extreme heat remains posted (Aug 26-30) based on lesser, but increased chances in the temperature and heat tools exceeding the upper climatological percentiles. The moderate risk area is inclusive of locations in the National Blend of Models which remain forecast to approach or break daytime highs and nighttime lows, and where probabilistic guidance indicates elevated chances for double digit daytime temperature departures at least through the middle of week-2 before ensembles weaken the mean ridge center (below 594 dm). A broad slight risk area of extreme heat remains posted for all of week-2 which includes much of the southern tier of the CONUS, with its northern extent capped to the Central Rockies and Central Plains. Despite the better support for anomalously warm temperatures over the northern tier of the CONUS, there is less confidence for temperatures reaching hazardous thresholds due to decreasing sun angles over the higher latitudes during late August, which is also supported in the heat index guidance depicting a limited potential for values exceeding 100 deg F. The extensions of both the high and moderate risks of extreme heat later in August are also supported by feedback associated with rapidly drying soils and drought being registered within the highlighted regions. The hot and dry conditions favored are also expected to aggregate fuels and bring increased chances for elevated fire weather conditions during the period. In addition, a Rapid Onset Drought (ROD) risk also 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 expected during the next two weeks. Much of this ROD area is within the moderate risk of extreme heat as increased evapotranspiration rates look to accelerate surface moisture depletion. For precipitation, the highest chances for above-normal precipitation reside along and ahead of the mean trough axes favored over the portions of the western and eastern CONUS. Relative to yesterday, daily ensembles and PETs show some increased support for a heavy precipitation risk over coastal portions of the Mid-Atlantic and the Southeast associated with stalled frontal activity. Although raw tools show the highest amounts remaining offshore, both the GEFS and ECMWF PETs are in agreement featuring at least 20% chances for 3-day amounts exceeding the 85th percentile and 1 inch. Therefore, a slight risk of heavy precipitation is issued from coastal South Carolina to the DelMarVa region for Aug 26-28. Across the Desert Southwest, the aforementioned subtropical ridge center is forecast to be in a less favorable position for the advection of monsoonal moisture. However, an uptick in tropical cyclone activity is expected over the eastern Pacific in the coming weeks, where any transient tropical disturbances nearing the Gulf of California could trigger a gulf surge event. Even with this potential, precipitation tools remain rather muted at this time resulting in no precipitation related hazards. Despite the current lack of support in the tools, isolated areas of heavy rain remain possible, and this region is prone to flash flooding even with relatively low rainfall totals along dry creek beds (arroyos) and in canyons. Caution should always be exercised in such areas if thunderstorm activity is initiated nearby. Also, some convective activity may result in little or no rainfall while still being capable of generating lightning, which poses a risk of wildfire initiation. In addition to the eastern Pacific, additional tropical cyclone formation is also favored in the central Pacific during the next two weeks which is highlighted in the National Hurricane Center (NHC) Tropical Weather Outlook (TWO) for week-1 (https://www.nhc.noaa.gov/gtwo.php?basin=cpac&fdays=7), as well as in the latest Global Tropics Hazards (GTH) Outlook for week-2 (https://cpc.ncep.noaa.gov/products/precip/CWlink/ghaz/index.php). Should any development occur with a track in the vicinity of the Hawaiian, additional rainfall is likely to exacerbate saturated ground conditions following the passage of Hurricane Lala to the south of the Big Island this past week, as well as possibly bring other adverse impacts to the state. FORECASTER: Nick Novella $$