Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Tue Jun 27 2023 Valid 12Z Fri Jun 30 2023 - 12Z Tue Jul 04 2023 ...Major heatwave expands from Texas through the mid-lower Mississippi Valley before pivoting to Southeast into this weekend... ...Heatwave develops over Desert Southwest by this weekend... ...Excessive rainfall threat for the central Plains, Midwest and Northeast late week into the holiday weekend... ...Overview with Guidance Evaluation/Predictability Assessment... An upper ridge will expand across the southern tier of the CONUS, while a trough propagates across the Rockies and Great Plains during the medium range. Another upper ridge will move through the West early next week. The WPC model blend consisted of a standard 00z EC/CMC/UK and 06z GFS through day 4 despite a notable difference with respect to the orientation of the southern stream pattern between the last several runs of the GFS and the rest of the models. The differences between the GFS and the rest of the guidance continues on beyond day 4, which is why less weighting is given to it in our blend. The 00z ECE/CMCE are introduced on day 5 and continued through day 7. ...Weather/Hazards Highlights... A main ejection of upper trough energies out from the West/Rockies is expected to spawn scattered showers and strong to severe storm chances out from the central Plains to the mid-Mississippi and lower Ohio Valley along with a threat for locally heavy rainfall and runoff issues into Friday. A "Marginal Risk" of excessive rainfall is planned given height falls/instability and potential for repeat/training of cells mainly along/north of a developing wave and leading warm front. This wave shifts east Saturday with the warm front and southwestward trailing cold front, warranting an overlap/expansion of a "Marginal Risk" area from the mid-Mississippi/Ohio Valleys through the Northeast given wet soils and additional northern stream energy. The pattern will likely warrant future issuance of "Slight Risk" areas and bumped up QPF given pooled high PWs with advent of a stronger local guidance signal focus closer to the event. The overall heavy rain focus will shifts east and southeast and become less organized later weekend into early next week with wave and trailing front progression then eventual frontal stalling and return. Excessive heat will continue to plague the southern Plains and expand north and east this weekend to portions of the central Plains and the mid-lower Mississippi Valley. The heat will be oppressive with widespread high temperatures exceeding 100 degrees and heat indices as high as 120 degrees. Heat indices may even exceed 120 degrees in some locations in South Texas. Tens of high temperature records are in danger of being broken during this significant heat wave. The Desert Southwest will also experience a notable heating this weekend with max temps 5 to 10 degrees above normal which means 110 to 115 degrees max temps for these hot environs south from Las Vegas and west from Phoenix. Kebede/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 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