Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Sun Aug 20 2023 Valid 12Z Wed Aug 23 2023 - 12Z Sun Aug 27 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... The highly impressive upper ridge that is expected to peak at or slightly above 600dm early this week is forecast to start decaying 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 through next weekend. 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 cooler temperatures are also likely for much of the interior Western U.S., along with scattered to numerous showers and storms with continued 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 Friday, although the GFS continues to portray extremely hot temperatures across the Central U.S. 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 potential closed low evolution. Ensemble means accounted for about 35% of the model blend by next Sunday, 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 likely continuing into next weekend. There should be at least scattered coverage of convection across the southern to central Rockies, and this warrants a Marginal Risk area for both Wednesday and Thursday for this region. Scattered to numerous showers and storms are also expected across the Rio Grande Valley on Wednesday associated with the remnants of the tropical wave moving inland from the western Gulf, and a Marginal Risk area is also planned here as well. 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 both Wednesday and Thursday, so a Marginal Risk is planned for this potential. This will then potentially affect the Northeast U.S. 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 about Thursday with the massive upper ridge in place and suppressing cloud development. Widespread 100+ degree high temperatures are expected from southern South Dakota/southern Minnesota and points southward, and some overnight lows may stay above 80 degrees, especially for larger urban areas. These readings across the Midwest are in some cases up to 20 degrees above late August averages, with the core of the anomalies centered mainly over Iowa and portions of adjacent states on Wednesday/Thursday. The good news here is a strong cold front will herald the arrival of a much more pleasant airmass in time for the weekend for all of the Midwest and Ohio Valley with afternoon highs returning to the 70s and low 80s in most areas. Meanwhile, the intense heat is expected to persist unabated across Texas and the central Gulf Coast region through the end of the week and into the weekend 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 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