Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Mon Aug 21 2023 Valid 12Z Thu Aug 24 2023 - 12Z Mon Aug 28 2023 ***Major heat wave for the Central U.S. and extending to the Gulf Coast region with numerous record highs likely*** ...Synoptic Overview... The monster upper ridge that will be anchored in place across the Midwest states through mid-week will start decaying some going into the end of the work week, and then gets displaced towards the south and then southwest as a strong trough amplifies across the Great Lakes and then the Northeast. The record breaking heat and accompanying humidity will likely continue to make weather headlines for much of Texas and interior portions of the Gulf Coast region going into the weekend as the heatwave here will have more staying power. Except for a hot day Friday, much of the Mid-Atlantic, Northeast, and Great Lakes region should avoid this heatwave as an upper level trough governs the overall weather pattern with a cold front arriving in time for the weekend. Showers and storms will likely continue for much of the Rockies and portions of the Intermountain West with monsoonal moisture in place. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite is in excellent agreement across the continental U.S. through the end of the week with above average forecast confidence on the synoptic scale, and this also includes less model spread with the shortwave trough crossing southern Canada that will amplify into a larger scale trough across the Northeast U.S. by the weekend. Therefore a multi-deterministic model blend suffices as a starting point in the forecast process for fronts/pressures through early Saturday, although the GFS continues to portray extremely hot temperatures across the Midwest on Thursday that are unlikely to verify so the ECMWF/NBM/ECENS served as a better proxy for temperatures. Looking ahead to the weekend, the model guidance is still in very good agreement, with the greatest differences noted across the eastern Pacific regarding closed upper low evolution, and timing of the next trough dropping south from the Hudson Bay region. Ensemble means accounted for about 40% of the model blend by next Monday, whilst still keeping sizable portions of the CMC/GFS/ECMWF solutions. ...Weather/Hazards Highlights... The region with the greatest coverage of showers and thunderstorms is expected to be across the Four Corners region in response to monsoonal moisture in place on the backside of the upper ridge. Southerly low-mid level flow will ensure a continued transport of moisture from western Mexico through the end of the week and continuing into next weekend. There should be at least scattered to occasional widespread coverage of convection across the southern to central Rockies, and this warrants a Marginal Risk area for both Thursday and Friday for this region. An area of enhanced QPF is noted in the latest guidance across central Utah on Thursday, so a Slight Risk is planned here, and a Slight Risk for Friday is also planned for portions of eastern Wyoming and Colorado where models are likely picking up on a potential MCS developing. Across the Eastern U.S., a couple of convective complexes embedded within northwest flow aloft around the ridge may affect portions of the Great Lakes and extending across northern portions of the Mid-Atlantic for Thursday, so a Marginal Risk is planned for this potential. This will then potentially affect New England going into Friday with locally heavy rainfall ahead of the approaching cold front. A major late summer heatwave will continue to make all kinds of weather headlines across the Midwest states through Thursday with the massive upper ridge in place and suppressing cloud development, followed by much welcomed relief as a strong cold front heralds the arrival of more seasonal temperatures. Widespread 100+ degree high temperatures are expected from Nebraska and Iowa and points southward, with oppressive dewpoints above 75 degrees in many cases. These readings across the Midwest equate to 15-20 degrees above average, with the core of the anomalies from Iowa to northern Indiana on Thursday. Meanwhile, the intense heat is expected to persist unabated across Texas and the central Gulf Coast region through about Saturday before a gradual moderating trend commences. Highs are expected to be 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 all-time record highs are expected, particularly across portions of Mississippi and Louisiana. The opposite holds true across much of the interior Western U.S. with temperatures running 5 to 15 degrees below average owing to greater cloud cover and showers through the end of the week. Temperatures slowly moderate going into the weekend across this region and return closer to late August averages. 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 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