Extended Forecast Discussion NWS Weather Prediction Center College Park MD 231 AM EDT Mon Jun 26 2023 Valid 12Z Thu Jun 29 2023 - 12Z Mon Jul 03 2023 ...Major extended heat wave to focus from Texas to the mid-lower Mississippi Valley... ...Excessive rainfall threat across the Northeast and the central U.S. late week... ...Overview with Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of well clustered mid-larger scale guidance from the 18 UTC GFS/GEFS mean, the 12 UTC ECMWF/UKMET/Canadian and ECMWF ensemble mean along with the compatible 01 UTC National Blend of Models. Forecast confidence is bolstered at these scales. WPC product continuity is also reasonably well maintained in this fashion and the forecast remains in line with a composite of newer 00 UTC cycle guidance. The broad blending of compatible guidance tends to mitigate smaller scale differences consistent with predictability. ...Weather/Hazards Highlights... An amplified trough slowly progressing through the Northeast 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-moving system. A WPC Excessive Rainfall Outlook (ERO) "Marginal Risk" area is planned to linger over the Northeast Thursday into early Friday 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/vicinity along with locally heavy rainfall. This lends issuance of a "Marginal Risk" of excessive rainfall for Thursday and Friday given height falls/instability and potential for repeat/training of cells. Later, the main focus for showers and thunderstorms with locally heavy downpours should shift to the mid-Mississippi, Ohio and Tennessee Valleys before sinking increasingly into the South and broadly eastward across the Eastern Seaboard into this weekend with approach of an organizing low and lead/trailing frontal system, but details for local focus are less certain. Excessive heat will continue to plague the south-central U.S. through the holiday weekend with expansion north and east across 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. Temperature records are in danger of being broken during this significant heat wave. The Desert Southwest will also experience temperatures well over 100 degrees, but that is not really anomalous for this time of year. Florida will also see hot to record high values in this pattern. 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