Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Wed Jun 28 2023 Valid 12Z Sat Jul 01 2023 - 12Z Wed Jul 05 2023 ...Major heatwave expands from Texas through the mid-lower Mississippi Valley and Southeast... ...Heatwave develops over Desert Southwest... ...Excessive rainfall threat focus from the Ohio Valley through the Northeast this weekend... ...Pattern Overview and Guidance Evaluation/Predictability Assessment... The upcoming weather pattern for the first week of July will likely be highlighted by an upper ridge slated to further expand across the southern tier of the nation to extend a major heatwave while a main strong to severe convection/rainfall focusing upper trough and surface system propagates across the central Plains through the Ohio Valley and onward over the holiday weekend. Another upper ridge will amplify and heat up the Southwest/West as well. The WPC medium range product suite was primarily derived from the weekend into early next week from a blend of reasonably well clustered mid-larger scale guidance from the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with the 01 UTC National Blend of Models. This solution has good overall ensemble support, albeit with ample local convective focus differneces. Replaced the UKMET/GFS with GEFS/ECMWF ensemble means by the 4th amid growing forecast spread. This replacement also removes from the equation less supported GFS tropical interactions into the Gulf of Mexico. Overall, WPC product continuity is fairly well maintained with the blending process tending to mitigate smaller scale guidance differences consistent with individual system predictability. Latest 00 UTC guidance remains broadly in line through the holiday. ...Weather/Hazards Highlights... The main ejection of upper trough energies out from the Rockies/central Plains into the holiday weekend is expected to focus showers and strong to severe storm chances out from the central Plains through the mid-Mississippi and Ohio/Tennessee Valleys to the Appalachians/northern Mid-Atlantic/Northeast along with a threat for locally heavy rainfall and runoff issues. "Marginal Risks" of excessive rainfall are planned given instability/moisture pooling and potential for repeat/training of cells mainly with a developing wave and frontal system. Anticedent rainfall and additional influence from northern stream energy are also in the mix. A "Slight Risk" area has also been introduced over portions of the Ohio Valley for Saturday given high pooled precipitable water values. The overall convection and heavy rain focus should shift southeast and become less organized into early next week with wave and trailing front progression then eventual stalling. 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 as well as across portions of the Southeast/Florida. The heat will be oppressive with widespread high temperatures exceeding 100 degrees and heat indices as high as 120 degrees over the south-central U.S.. 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. 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