Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 PM EDT Mon Jun 26 2023 Valid 12Z Thu Jun 29 2023 - 12Z Mon Jul 03 2023 ...Major heat wave expands from Texas through the mid-lower Mississippi Valley rest of this week before pivoting to Southeast this weekend... ...Heavy wave develops over Desert Southwest this weekend... ...Excessive rainfall threat expands from the central Plains through the Ohio Valley late week... ...Overview with Guidance Evaluation/Predictability Assessment... A western U.S. trough begins a slow eastern ejection Thursday, lifting through the Midwest into eastern Canada this weekend while troughing over the Northeast lifts out Friday. The WPC forecast from through Monday was primarily derived well clustered deterministic guidance, the 06Z GFS and the 00Z ECMWF/CMC/UKMET through Saturday and then included more 06Z GEFS and 00Z ECENS for Sunday and Monday. ...Weather/Hazards Highlights... An amplified trough slowly progressing through the Northeast Thursday will wrap Atlantic moisture around a gradually weakening low pressure system to fuel widespread and lingering precipitation. Locally heavy rainfall will be possible with potential training and embedded thunderstorms (especially into favored terrain) in this slow-to-exit system. A Marginal Risk of excessive rain is maintained over New England to the Adirondacks Thursday/Thursday night in an area with antecedent rainfall from a previous slow-moving system. Additional low pressure and enhanced rains with the remnants of Cindy is expected to pass to the east Friday/Saturday toward the Canadian Maritimes. Meanwhile, the ejection of upper trough energies out from the West/Rockies later week is expected to bring scattered showers and strong to severe storm chances out across the central Plains along with locally heavy rainfall Thursday night through Friday night. A Marginal Risk of excessive rainfall is in effect for Thursday/Thursday night for the central High Plains through central Nebraska Thursday and Friday given height falls/instability and potential for repeat/training of cells north of a developing warm front. This frontal wave expands east Friday, warranting an expansion of the Marginal Risk east from the central Plains across the Midwest to the central Appalachians for Friday/Friday night. The heavy rain focus then shifts east and southeast this weekend ahead of the further developing frontal wave. 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. Jackson 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