Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 PM EDT Thu Sep 07 2023 Valid 12Z Sun Sep 10 2023 - 12Z Thu Sep 14 2023 ...Hazardous Heat from the south-central U.S to the Southwest to moderate next week... ...Guidance Evaluation/Predictability Assessment... On the large scale, the guidance continues to show good agreement on the overall pattern across the U.S. which features upper ridging over the South and West getting suppressed by a couple of systems moving through the northern U.S. A shortwave into the Great Lakes early period may become a closed low around mid-next week. The GFS and ECMWF are fairly consistent with each other and the ensembles, but the CMC is displaced noticeably to the north and was not included in the late period blend today. This system should amplify troughing over the East later next week, also allowing some ridging to build back in across the West as a couple weaker shortwaves move through well north in Canada. The WPC medium range suite for today used a composite blend of the deterministic models days 3-5/Sunday-Tuesday. After Tuesday, increased usage of the ensemble means eventually reaching a 50/50 blend with the GFS and ECMWF due to greater uncertainties in the later periods. This approach maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... The WPC Day 4/5 Excessive Rainfall Outlooks (EROs) denote marginal risk areas from the northern Mid-Atlantic across northern New England Sunday into Monday as convection looks to have a relatively better focus along a front and favorable right entrance region upper jet support. Guidance has also trended to a more organized main frontal wave to wrap in Atlantic moisture. The "Marginal" risks encompass relatively lower FFGs and includes favored terrain from the Poconos and Catskills to the Adirondacks and the Green/White mountains, however much of the Northeast has been very dry recently which may limit the overall flood/flash flood threat. Rain chances should gradually decrease after Monday along the Eastern Seaboard as the wavy fronts push east off the coast but the next system to move in could bring another threat for heavy rainfall, though still with plenty of uncertainty surrounding the details. Upstream, an amplified system will skirt the Pacific Northwest Sunday to support modest coastal showers. Underneath, a series of less defined southern stream shortwaves will work inland across the West Coast and Intermountain West in a pattern with limited precipitation this weekend and next week. This energy will work downstream to the north-central Rockies and dig into the Plains to support more widespread and locally strong to severe convection, where some heavy rain totals are likely under favorable upper difluence and potential cell training. The WPC Day 4/Sunday and Day 5/Monday EROs denote marginal risk areas spread from the central to southern Plains. Antecedent conditions are very dry across this region, and this precludes an embedded slight risk at this time, despite some fairly decent model QPFs. Hazardous late summer heat will linger across the south-central U.S. and the Southwest under a potent upper ridge into Monday. High dewpoints and humidity especially near the western Gulf Coast will lead to dangerous heat indices as high as 115 degrees with temperatures elevated at 10-15 degrees above normal. Expect widespread record high minimum and maximum temperatures will be set. Upper-level energy trending southward will act to reduce ridge strength and begin to bring temperatures to still hot but more reasonable/normal levels onward next week. Santorelli 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 forecast, 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