Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Wed Aug 23 2023 Valid 12Z Sat Aug 26 2023 - 12Z Wed Aug 30 2023 ...Relief from the extreme heat expected to spread farther south into the Gulf Coast states beginning this weekend as heavy rain threat emerges over the central High Plains and the Carolinas... ...Synoptic Overview... Welcomed relief from the extreme heat is in store for the north-central U.S. by the weekend as the ongoing major late summer heatwave abates and 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 likely. The upper trough that is forecast to amplify over the northeastern quadrant of the country next week not only will be responsible for bringing the cooler temperatures but also a threat of heavy rain for the central High Plains this weekend, followed by the Carolinas and possibly farther up the East Coast during the first half of next week. ...Guidance Evaluation/Predictability Assessment... Overall, the 06Z and 12Z model guidance suite is in relatively good agreement across the mainland U.S. through much of the medium-range period. The most noticeable differences are found across the Great Lakes where there appears to be a tendency for the models to amplify a shortwave digging into the longwave trough downstream. The more amplified trough appears to slow down the eastward and southward progression of the front across the eastern U.S. early next week, leading to a better chance for the associated showers and thunderstorms to linger over the East Coast especially over the Carolinas. The better-defined troughing also helps to bring a better chance of precipitation into the Northeast by midweek. Over the central Plains, the ECMWF and CMC favor more precipitation ahead of a shortwave trough compared with the GFS. On the other hand, the GFS favors more precipitation over the Carolinas early next week relative to the drier solution given by the ECMWF. The WPC medium-range forecast package was based on a composite blend of 40% from the 06Z GFS/ GEFS, 40% from the 00Z ECMWF/EC mean, and 20% from the CMC/CMC mean, with a higher percentage from the ensemble means toward the end of the forecast period. This solution maintains good continuity with the previous WPC forecast but with a slower progression of the front across the eastern U.S. next week. ...Weather/Hazards Highlights... Showers and thunderstorms are expected to be prevalent across much of Colorado and extending into portions of the Central Plains this weekend as a couple of convective complexes are likely to develop, although the heaviest rainfall will likely happen before Saturday. A Marginal Risk area is in effect for both weekend days from central Colorado to adjacent portions of Nebraska and Kansas, with the potential for future upgrades to Slight Risks once models converge on placement of QPF. Across the eastern U.S., as a cold front intercepts a moist and unstable airmass across the Carolinas, scattered to numerous showers and storms are expected to develop. Given a slower eastward progression of the front in this forecast cycle, a Marginal Risk area is in effect for this region for this weekend. There is growing model signals to sustain an area of moderate to heavy rain across the Carolinas into early next week, possibly spreading farther up the East Coast as an upper trough approaches. In addition, tropical convection appears to stream toward Florida from the southwest as the National Hurricane Center is monitoring the potential for a tropical system to develop over the eastern Gulf of Mexico next week. In the mean time, mainly dry conditions can be expected across most of the Midwest and the Tennessee River Valley. There is some good news to report about the Central U.S. heatwave going into the weekend. A pattern change will lead to the big upper ridge decaying some as the northern stream flow across the Great Lakes and New England weakens it. This will result in a cold front slowly settling southward across the Deep South and then the Gulf Coast region by early next week. Although the heatwave will have concluded across the Midwest, it will still be quite hot and uncomfortable from Texas to South Carolina this weekend, with widespread 100-105 degree highs expected on Saturday south of the Interstate 40 corridor, and then the triple digits become more confined to the Interstate 20 corridor and points south on Sunday. This will likely establish additional daily record highs and probably secure the hottest August on record for many Gulf Coast and Deep South cities. The heat abates across the Gulf Coast going into Tuesday and Wednesday with mainly 90s for highs expected for these same areas. Unfortunately heat builds again across the lower elevations of the Desert Southwest with some 110s likely by the weekend. Elsewhere, temperatures will be pleasant across much of the northeastern quadrant of the nation with highs mainly in the 70s and 80s and comfortable humidity levels. Kong/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