Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Sat Aug 19 2023 Valid 12Z Tue Aug 22 2023 - 12Z Sat Aug 26 2023 ***Major heat wave for the Central U.S. and extending to the Gulf Coast region for much of next week with numerous record highs likely*** ...Synoptic Overview... An impressive 600dm upper ridge will be anchored in place across the Midwest for the beginning to middle of next week with widespread excessive heat and humidity accompanying it. This ridge will likely start to decay some going into the end of the week and then gets displaced towards the south as shortwave energy impinges upon it from the north, but the heat and humidity will likely continue to make weather headlines for much of the south-central states. Across the Northeast U.S., an upper level trough is expected to be in place for most of the week and this will shield the region from the excessive heat that is expected across the Central U.S., and an increase of moisture across the northern Gulf will fuel more widespread coverage of showers and storms across southern Texas going into Tuesday. Showers are also expected from the Pacific Northwest to the Northern Rockies through mid-week. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite indicates very good overall synoptic scale agreement across the forecast domain for the middle of the week, with above average confidence with the huge upper ridge over the Central U.S. early in the week, with multiple models indicating a rare 600dm contour over Iowa and Missouri. The main differences noted later in the week involve the timing and magnitude of shortwaves traversing around the northern periphery of the ridge, with the CMC/ECMWF slower with the shortwave passages. The CMC becomes a little to the west of the model consensus with the core of the upper high by next Saturday, but the 00Z run is in better alignment compared to the 12Z that was not favored beyond Thursday. The GFS temperatures with the upcoming heatwave are likely too high across much of the Midwest and even across parts of the Great Lakes and Ohio Valley, with the ECMWF and ensemble mean temperatures closer to what is expected to evolve. The WPC blend for the fronts/pressures incorporated a nearly deterministic model blend through Wednesday, and then reducing the CMC and slowly transitioning to about 40-50% of the 18Z GEFS/12Z ECENS ensemble means for Friday and Saturday. ...Weather/Hazards Highlights... The good news here is the heaviest rain associated with the remnants of Hilary should be over by Tuesday morning with just some scattered showers and storms remaining in its wake across the Desert Southwest and the Intermountain West. However, there will likely be a more concentrated corridor of moisture convergence from eastern Oregon to northern Idaho where the models are depicting local rainfall maxima around an inch on Tuesday, and given expected rainfall prior to this time, a Slight Risk area is planned for Day 4 across this region. Across the Gulf Coast region, a tropical wave moving underneath the strong upper ridge is expected to advect westward across the Gulf of Mexico and inland across southern Texas, supporting the development of widespread showers and thunderstorms on Tuesday, and then across northern Mexico by Wednesday and beyond. A Marginal Risk area is planned for this region as a result with 1-2 inches of rainfall possible south of the Interstate 10 corridor. There is also a Marginal Risk area planned for portions of the Mid-Atlantic on Day 5 with a potential MCS tracking southeast from the Great Lakes, although confidence on its evolution is limited for now. Elsewhere, very little in the way of rain is expected across a wide expanse of the Central U.S. as the ridge severely restricts the development of any showers or storms. A major late summer heatwave is expected across much of the Midwest and extending across the Central Plains through about Thursday with the massive upper ridge in place and suppressing cloud development. Widespread 100+ degree high temperatures are expected from southern South Dakota and points southward, and some overnight lows may stay above 80 degrees, especially for larger urban areas. These readings across the Midwest are on the order of 10-20 degrees above late August averages, with the core of the anomalies centered mainly over Iowa and portions of adjacent states through mid-week. There will likely be some abatement to the late summer heatwave across the Midwest by Friday and into next weekend as the ridge is suppressed to the south. Meanwhile, the intense heat is expected to persist unabated across Texas and the Deep South states through the end of the forecast period with highs running 10-15 degrees above average, with many locations across the Deep South likely having their warmest August on record. Numerous daily records and perhaps some monthly record highs are expected. The opposite holds true from California to interior portions of the Pacific Northwest with temperatures running 10 to 20 degrees below average owing to expansive cloud cover and widespread rainfall, perhaps setting some record low max temperatures across this region on Tuesday. Temperatures slowly moderate going into the second half of the week across this region and return closer to late August averages by next weekend. Much of the Northeast U.S. should also avoid the brutal heat with multiple shortwave passages reinforcing a comfortable airmass across the region. Hamrick 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 Hazards: - Heavy rain across portions of the northwestern U.S. from eastern Oregon into central Idaho, Mon, Aug 21. - Heavy rain across portions of southern Texas, Tue-Wed, Aug 22-Aug 23. - Hazardous heat across much of the mid-section of the country including the lower and mid-Mississippi Valley, portions of the Ohio Valley, the Southeast, and into northern Florida, Mon-Fri, Aug 21-Aug 25. - Hazardous heat across the inner coastal plain of the Carolinas, Tue, Aug 22 and Fri, Aug 25. 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