276 FXUS02 KWBC 150657 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Sat Aug 15 2026 Valid 12Z Tue Aug 18 2026 - 12Z Sat Aug 22 2026 ...Long-duration heat wave continues across the South/Southeast... ...Midwest heavy rain/flooding threat renews Tuesday... ...Overview... Southern tier upper ridging will be well established as the medium range period begins Tuesday and continue to produce hazardous heat. To the north of the ridge, relatively active flow will produce rounds of rain and thunderstorms across the northern tier. Rainfall amounts are forecast to peak in the Midwest Tuesday with a flash flooding threat. Shortwaves should gradually deepen troughing across the eastern U.S. into late week and slowly ease the magnitude and scope of the heat in the Southeast, while maintaining rounds of rain/thunderstorms there. However, the strong upper ridge/high will keep a hold on the Four Corners states and High Plains, growing north across the Intermountain West. Some monsoonal moisture should stream north under the ridge for scattered convection. ...Guidance/Predictability Assessment... Recent model guidance is generally agreeable with the pattern described above. The most questionable aspect of the pattern will be with shortwaves moving through the northern tier, particularly as they serve to deepen the trough over the eastern half of the U.S., and how they affect frontal positions and rainfall potential. After the 18Z GFS seemed to be an outlier with the slow timing and strength of a shortwave moving through the central Plains and vicinity by Thursday, the 00Z GFS has trended more like other guidance, i.e. shallow and more progressive. So the 00Z model cycle shows a better consensus, and a multi-model blend of the dynamical and AI guidance worked well for the pattern. Models are also agreeable with the upper ridge growing northward across the West into late week. But this will be somewhat dependent on a less certain eastern Pacific trough approaching the West Coast. 00Z model guidance has slowed a bit with the trough just reaching the West by early Saturday. The 00Z GFS trough is considerably weaker than other guidance with that trough, as it favors a stronger upper low well northwest in the northeast Pacific instead, unlike the non-NCEP guidance. Meanwhile the 00Z AIGFS shows the trough moving into the West faster than consensus. So as the medium range period progressed, favored increasing proportions of the ensemble means and less GFS-based guidance. ...Weather/Hazards Highlights... A strong upper ridge remaining positioned over the southern U.S. will maintain hot conditions across the southern Plains through the Lower Mississippi Valley, Tennessee Valley, and Southeast. Afternoon temperatures will commonly reach the upper 90s to lower 100s across the interior South, while high humidity and persistent elevated dewpoints will produce oppressive heat indices of 105-115 degrees. Record-breaking warm overnight temperatures will further limit recovery, particularly where several consecutive days of heat occur. Widespread Major to Extreme HeatRisk conditions should persist into much of next week, and several consecutive days of high heat stress can lead to heat- related illnesses. There is some gradual minor relief in sight for eastern areas late next week/weekend due to the trough aloft and weak cold fronts, but the south-central U.S. and back toward the Southwest will likely continue to see dangerous heat. See WPC's Key Messages for more information. Shortwave energy moving through the northern tier will carry frontal systems that could focus moisture for rounds of showers and thunderstorms next week. One strong shortwave and cold front are forecast to bring increasing rain amounts to the Upper Midwest in particular by Tuesday. A Slight Risk remains in place in the Day 4/Tuesday Excessive Rainfall Outlook for strong thunderstorms/MCS activity that could cause flash flooding. Rain and thunderstorm chances will push across the Great Lakes to Northeast back across the Ohio Valley and central Plains by Wednesday with an isolated flash flood threat. Farther south, a weak cold front should provide a focus for scattered thunderstorms across the Southeast Tuesday- Wednesday in the hot and unstable airmass. By late week, this activity may all combine into chances for scattered showers/storms across the Eastern Seaboard, and west across the Mid-South and Mississippi Valley with the tail end of the front. Meanwhile, some monsoonal moisture should bring scattered, diurnally driven convection to the Southwest especially by Wednesday and beyond. Moisture may continue spreading north across the Intermountain West by Friday, allowing for rain and thunderstorm chances there. 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 $$