Extended Forecast Discussion NWS Weather Prediction Center College Park MD 323 PM EDT Mon Aug 21 2023 Valid 12Z Thu Aug 24 2023 - 12Z Mon Aug 28 2023 ...Major heat wave continues Thursday in the Midwest with numerous record highs likely, easing there late week but persisting across the Gulf Coast states... ...Overview... The monster upper ridge that will be anchored in place across the Midwest states through midweek will start decaying some going into the end of the workweek, and then gets displaced towards the south and then southwest as a strong trough amplifies across the Great Lakes and then the Northeast. The areal coverage of excessive and record-breaking heat will shrink with it, becoming more limited to the Gulf Coast states by the weekend after a hot day in the Midwest Thursday and as far north as the Ohio Valley Friday. The deep trough should allow for the Great Lakes region to the Mid-Atlantic/Northeast to be overall cooler than average and will also push a front or two across the eastern half of the U.S. with showers and storms ahead of it. Cooler temperatures and widespread rain and storms with potential for flash flooding are likely across the Intermountain West and Rockies with monsoonal moisture in place late this week into the weekend. ...Guidance Evaluation/Predictability Assessment... Model guidance remains in good agreement that the anomalously strong upper high will hold on through Thursday, but then a shortwave tracking near the Canada/U.S. border east-southeastward should help suppress it on the northeast side while developing amplified troughing atop the Northeast by the weekend. With reasonably high confidence in this pattern, a multi-model blend of the deterministic guidance from the 06Z GFS and 00Z ECMWF/CMC/UKMET was used for the forecast early in the period (except for temperatures and dewpoints, as the GFS continues to show way too much boundary layer mixing, producing too high temperatures and too low dewpoints, which is a known issue). Then models have been waffling some with the ridge-trough interface around the Midwest by early next week. Some guidance including the 00Z GFS maintained more ridging, but the larger cluster of guidance including the 00Z ECMWF and CMC and the 06Z GFS brought in additional reinforcing shortwave energy, so the latter was favored. The 00Z GFS also seemed displaced with an eastern Pacific upper low that was becoming more agreeable with the other guidance, including the incoming 12Z guidance. But other than that run, the multi-model blend was able to be maintained, with minor incorporation of the EC and GEFS means with time to smooth out generally small 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 precipitable water anomalies well above the 90th percentile. Moisture levels of this magnitude and scattered to occasionally widespread coverage of convection warrants large Marginal Risks in the EROs for both Thursday and Friday, with an embedded Slight Risk in Utah Thursday where enhanced QPF falls over sensitive areas like slot canyons, and a Slight Risk Friday into portions of Colorado and Wyoming where there is MCS potential. 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. A Marginal Risk is in place for this potential, and embedded Slight Risks cannot be ruled out in future cycles if models agree better on placement of the QPF and if it is in sensitive areas. By Friday the front and rain should push into New England. The Gray and Caribou Maine Weather Forecast Offices preferred not to have any Marginal Risk at this point in the forecast, but this will continue to be monitored as there is good dynamics for ascent in place, but instability could be a limiting factor. Scattered convection is possible farther south along and ahead of the cold front, through the central Appalachians to Mid-Atlantic Friday and the Carolinas and Southeast over the weekend. A major late summer heat wave 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 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. Tate/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