Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Wed Aug 23 2023 Valid 12Z Sat Aug 26 2023 - 12Z Wed Aug 30 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 the weekend as the 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 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 along with showers and storms with potential for some flash flooding are expected across portions of the Rockies and western High Plains with monsoonal moisture in place into the weekend. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite is in excellent agreement across the continental U.S. through Sunday 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 for the weekend. Looking ahead to the first half of next week, the model guidance is still in overall decent agreement, although the CMC is faster with bringing the upper low inland across the northern Rockies and indicates a stronger shortwave off the Southeast coast on Wednesday. The greatest differences are over south-central Canada, where the GFS is stronger with a trough just north of the Great Lakes. Ensemble means accounted for about 30-40% of the model blend by next Wednesday, whilst still keeping sizable portions of the CMC/GFS/ECMWF solutions. ...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 planned 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., the cold front will intercept a moist and unstable airmass across the Carolinas with scattered to numerous showers and storms expected to develop, and a Marginal Risk area is planned for this region as well. Unsettled weather conditions are likely to continue across portions of the Southeast U.S. going into early next week, with convection increasing across much of Florida. 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. 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