Extended Forecast Discussion NWS Weather Prediction Center College Park MD 232 PM 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 early next week for the Gulf Coast states... ...Synoptic Overview... 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 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 bring cooler temperatures, with showers and storms likely ahead of the cold front. 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 latest model guidance remains in good enough agreement for a general deterministic model blend days 3-5. After this, there are some differences in timing of the cold front off the East Coast and details with broad troughing across the Eastern U.S.. Eventually, Tropical Storm Franklin should get wrapped up into this troughing early next week, though should remain far enough away from the Northeast coast limiting any sensible weather impacts besides high surf. Out West, a deep upper low should meander off the Northwest coast through at least Monday. There is disagreement in the guidance on when the low may start to shift inland, related to strength of a blocky upper ridge over western Canada. The WPC blend for the late period trended towards a 50/50 blend of the ensemble mean guidance and deterministic guidance to help mitigate these differences. ...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 highlighted. 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 Marginal Risk area for the 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. Much welcomed relief is anticipated for the Midwest and Ohio Valley though 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 leading to heat indices well into the 100s. 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 more comfortable airmass across the region. Santorelli/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