Extended Forecast Discussion NWS Weather Prediction Center College Park MD 336 PM EDT Thu Jun 29 2023 Valid 12Z Sun Jul 02 2023 - 12Z Thu Jul 06 2023 ...Major heatwave from Texas through the mid-lower Mississippi Valley to the Southeast to ease next week... ...Hot temperatures from the Desert Southwest to Interior California Sun-Mon... ...Overview... Strong and prolonged heat over Texas/South will start to relent early next week as the pattern takes on a more zonal flow. In a more typical summertime pattern, northern tier frontal systems will progress southeastward and eastward out of the northern Plains and into the East with daily chances of showers/thunderstorms into the Southeast. ...Guidance Evaluation/Predictability Assessment... Ensemble agreement was average to better than averge through the period, and a blend of the recent 00Z/06Z guidance served as a reasonable starting point. The 06Z GFS was on the quicker side with the front into the Great Lakes midweek next week, whereas the multi-day ensemble trend was a bit slower. The 00Z ECMWF clustered best with the agreeable GEFS/ECMWF/Canadian ensemble consensus. ...Weather/Hazards Highlights... A system will move eastward through the Ohio Valley Sun-Mon with a surge in moisture from the south through the warm sector. Rainfall could be locally heavy per some of the guidance, and have maintained a Marginal Risk for Excessive Rainfall there for Sunday, and into the northern Mid-Atlantic on Monday as the system pushes to the coast where it has been recently quite wet. Upstream, enhanced instability and convective precipitation chances will increase and shift southeastward across the north-central Rockies/Plains to the mid-upper Mississippi Valley through early to mid next week ahead of the next main frontal system set to drop in from Canada. Locally heavy rain is probable but with limited confidence in placement. Excessive heat to start the weekend from southern/southeast Texas into the Lower Mississippi Valley and Southeast/Florida will briefly expand northeastward into the Carolinas through Monday before a general downward trend towards more typical mid-summer averages. The heat will continue to be oppressive with widespread high temperatures in the mid-90s to low 100s which, when combined with the humidity, will lead to heat indices in the 110-115 degree range (and locally even a bit higher). Excessive summer heat will also overspread portions of the West this weekend, with the central California Valleys seeing highs of 105-110 degrees, upwards of 10-15 degrees above average. The Desert Southwest is forecast to see high temperatures into the 110s (5 to 10 degrees above average) from Las Vegas into the CO River Valley eastward to Phoenix. Elsewhere, above average high temperatures in the upper 80s to mid 90s along the northern tier of the central U.S. this weekend will shift eastward into the Upper Midwest/Great Lakes region ahead of a cold front, which will bring much cooler temperatures in the 70s to portions of the central/northern High Plains for the July 4 holiday. This area of below normal temperatures will slide eastward into the Plains by next Thursday. Fracasso/Schichtel 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 Hazards: - Heavy rain across portions of the Great Lakes, the Central Appalachians, and the Ohio Valley, Sun, Jul 2. - Flooding possible across portions of the Northern Plains. - Flooding likely across portions of the Northern Plains. - Hazardous heat across portions of the Central Great Basin, California, and the Southwest, Sun-Mon, Jul 2-Jul 3. - Hazardous heat across portions of the Lower Mississippi Valley, the Tennessee Valley, California, the Southeast, the Southern Appalachians, and the Southern Plains, Sun, Jul 2. - Hazardous heat across portions of the Lower Mississippi Valley and the Southern Plains, Sun-Thu, Jul 2-Jul 6. - Hazardous heat across portions of the Southeast and the Mid-Atlantic, Sun-Tue, Jul 2-Jul 4. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental 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