Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Tue Aug 22 2023 Valid 12Z Fri Aug 25 2023 - 12Z Tue Aug 29 2023 ***Improving conditions for the Midwest but the heat and humidity continues into the weekend for the Gulf Coast states*** ...Synoptic Overview... Finally some welcomed changes are coming for the north-central U.S. by Friday and into the weekend as the major late summer heatwave abates as the huge upper ridge is suppressed to the south. The areal coverage of excessive and record-breaking heat will shrink with it, becoming more limited to the Gulf Coast states by the weekend where additional daily record highs are possible. The deep trough developing near the Great Lakes and then across the Northeast should produce a pleasant cool down, and will also push a cold front or two across the eastern half of the U.S. with showers and storms ahead of the boundaries. Cooler temperatures and widespread rain and storms with potential for flash flooding are expected across portions of the Intermountain West and Rockies with monsoonal moisture in place late this week into the weekend. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite is in excellent agreement across the continental U.S. through Saturday with above average forecast confidence on the synoptic scale, and this also includes less model spread with the evolving closed upper off the Pacific Northwest coast. Therefore a multi-deterministic model blend suffices as a starting point in the forecast process for fronts/pressures through Saturday night. Looking ahead to Sunday and early next week, the model guidance is still in very good agreement, with the greatest differences across the Midwest and Great Lakes regarding the timing and placement of a second trough dropping south from Canada, with the CMC indicating more of an upper ridge extending across the northern Plains and the GFS indicating more of a broad trough axis. Ensemble means accounted for about 30-40% of the model blend by early next week, 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 states 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 the weekend, with a large Marginal Risk in the ERO for Friday and a Slight Risk into portions of Colorado and Wyoming where there are increasing chances of MCS development. Across the eastern U.S. on Friday, the front and associated rain should reach eastern New England where a Marginal Risk area is planned. Scattered convection is becoming more likely farther south along and ahead of the cold front on Saturday across portions of the Carolinas and into the Hampton Roads area in Virginia, and there is enough moisture and instability here to merit a planned Marginal Risk area for the new Day 5 outlook. A major late summer heat wave will continue to make all kinds of weather headlines across the Gulf Coast states through the end of the week and even into the weekend with the massive upper ridge in place and suppressing cloud development. Much welcomed relief is anticipated for the Midwest and Ohio Valley as a strong cold front heralds the arrival of more seasonal temperatures. Widespread 100+ degree high temperatures are expected from Texas to southern Georgia, with oppressive dewpoints above 75 degrees in many cases. 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 10 degrees below average owing to greater cloud cover and showers through the end of the week. Temperatures slowly moderate going into the weekend across that 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