978 FXUS02 KWBC 191903 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 PM EDT Wed Aug 19 2026 Valid 12Z Sat Aug 22 2026 - 12Z Wed Aug 26 2026 ...Hazardous heat will continue across the Southern Plains and Lower Mississippi Valley... ...Overview... Heatwave to persist across portions of the southern tier this weekend and into the coming week thanks to a persistent upper high. The longest period of hazardous heat will be across the southern Plains and Lower Mississippi Valley. Monsoonal moisture is forecast to stream north under the ridge for scattered convection across the West, while rounds of rain dive through portions of the central U.S. Meanwhile, multiple shortwaves should maintain troughing in the East, leading to rain and thunderstorms across the Eastern Seaboard into the weekend. ...Guidance/Predictability Assessment... The latest suite of guidance has decent agreement with the large scale pattern with the anchoring upper high and troughing in the East. Multiple shortwaves will be passing through the trough during the extended period which brings uncertainty to its depth and evolution. The last few model cycles have shown the GFS solution diverging from the cluster by taking progressing the closed low faster and to the north; the majority/AI solutions favor a position near the Great Lakes/Canada through Monday. Meanwhile, a shortwave trough will be moving inland across the Pacific Northwest Saturday however timing differences increase thereafter. GFS continues to be a bit faster than consensus as the shortwave moves quickly across southern Canada Sunday- Monday and toward the Upper Midwest Tuesday. This shortwave could serve to deepen the trough in the East again by midweek as it dives into the Midwest/Great Lakes, after the trough becomes shallower Monday- Tuesday. The 00Z GFS even creates a closed low over the Upper Great Lakes from this feature, which seems possible, but it is stronger than other models. Another trough/low is anticipated to track from the northeast Pacific toward the Pacific Northeast Coast by midweek. In general guidance is onboard with this however the CMC depicts a notably more progressive track across Canada and the U.S. northern tier than consensus, becoming out of phase with central North American troughing where other models have ridging by Wednesday. The WPC forecast used a multi-model blend early in the period, and then leaned away from the GFS and CMC in favor of the ECMWF, AIFS/AIGFS, and increasing proportions of the ensemble means. ...Weather/Hazards Highlights... Hot temperatures well into the 100s, with oppressive heat indices, are forecast across the south-central U.S. for the weekend and into much of next week under the strong upper high. Record-breaking warm overnight temperatures across the southern tier will further limit recovery from the heat, especially since several consecutive days of heat will occur. Widespread Major to Extreme HeatRisk will persist in the southern Plains and Lower Mississippi Valley. Several consecutive days of high heat stress can lead to heat- related illnesses. The West should also see warm to hot temperatures primarily Saturday when the upper ridge reaches the farthest north. The Southeast should see some minor relief over the weekend as the trough aloft and weak cold fronts lessen the magnitude and scope of the heat, but could gradually warm again as next week progresses. Over the weekend, combining energies and frontal systems will allow for widespread rain and thunderstorms across the East. Rain amounts should generally stay light to moderate, but some heavier amounts are possible in coastal areas of the Mid-Atlantic Saturday, with isolated flash flooding possible. By Sunday rain chances are forecast to focus in the Great Lakes/Appalachians/Northeast as the surface low/frontal systems move through, and some could linger Monday for the Interior Northeast on the backside of the low. Scattered showers and thunderstorms remain possible for Florida through the period. Monsoonal moisture is forecast to bring scattered, diurnally driven showers and thunderstorms to the Intermountain West through the period. Localized flash flooding will be possible, particularly for sensitive areas like burn scars, favored terrain like slot canyons, and urban areas. A round of rain is also possible with a shortwave in the Pacific Northwest and into the Northern Rockies, likely peaking Saturday. Areas of rain and storms are also possible in the central U.S. as shortwaves round the northeast side of the ridge and dive southeast. Placement of these shortwaves and thus the rain chances are uncertain, but generally the northern/central Plains and the Mississippi Valley could be impacted, reaching the Southeast into the workweek. Campbell/Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw $$