Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Tue Sep 05 2023 Valid 12Z Fri Sep 08 2023 - 12Z Tue Sep 12 2023 ...Heatwave to moderate in the East late week, but persist in earnest across the south-central U.S and Southwest... ...Guidance Evaluation/Predictability Assessment... Model guidance is in decent agreement on most of the large scale details through the forecast period. The period begins with a bit of amplification to the pattern with a trough deepening over the eastern U.S. and a ridge over the Southern Plains/Southwest expanding/building a bit west and northward. The main implication of this shift will be for the deepening trough/embedded shortwaves to drive a couple frontal systems to the East and Gulf Coasts. Thereafter, the main surface features look to remain relatively stagnant through most of the period. Some uncertainty arises towards the end of the period early next week as the pattern trends less amplified with the ridge over the Southwest flattening as more northern stream energy begins to drop southward over the Plains. The GFS had been trending a bit more aggressive in developing a stronger closed low over south-central Canada into the Northern Plains/Upper Midwest before the 06Z GFS reversed course and returned towards a more similar solution to the EMCWF/CMC, which bring an open northern stream wave southward. This leads to higher uncertainty with the related chances for storms and potential for heavy rainfall over the Plains during this time frame. The good guidance agreement through most of the period led to a simple consensus blend of the 00Z ECMWF/CMC/UKMET and 06Z GFS for the updated WPC forecast. A contribution from the 00Z ECens/GEFS means is added for the late period as the UKMET is time-limited, but a remaining blend of deterministic guidance is otherwise still sufficient given a reasonable clustering of the solutions. ...Weather/Hazards Highlights... The emergence of an amplified upper trough through the Midwest/Ohio Valley will act to suppress upper ridging and high heat across the East by Friday. Upper troughing will settle over the East through the weekend into early next week as anchored by an upper low over Hudson Bay and as a series of weaker impulses ride over the top then to the lee of the transitioning upper ridge as it strengthens over the South and expands westward into the Southwest. With this ridge in place, hot late summer temperatures will linger across the south-central U.S. and the Southwest through this weekend. Temperature anomalies will be strongest in the Southern Plains late week, with some temperatures 15-20 degrees above normal, well into the 100s. High dewpoints and humidity especially near the western Gulf Coast will lead to dangerous heat indices as high as 115 degrees. Above normal temperatures by about 10-15 degrees should persist into the weekend broadly across the south-central U.S. and into the Southwest. Many daily record high minimum and maximum temperatures are likely to be set, with perhaps even a few monthly records, in what has proven to be a sweltering summer. Upper-level energy trending southward should help to reduce the strength of the ridge and begin to bring temperatures to still hot but more reasonable levels early next week. Meanwhile in this pattern, the East offers a main area for precipitation to focus, given moisture (higher precipitable water values than average in the Northeast at least) and instability pooling ahead of the trough and frontal system. While rain amounts should be mainly moderate, the ingredients will be in place for localized heavier rain totals especially because storms may be slow-moving and relatively parallel to the front, allowing for potential training of storms. The WPC Day 4/5 Excessive Rainfall Outlooks (EROs) denote "Marginal" risk areas for Friday and Saturday from the northern Mid-Atlantic into the Northeast as convection looks to have a relatively better focus near/ahead of the front and favorable right entrance region upper jet support. The Marginal Risk encompasses lower FFGs from Pennsylvania into the Northeast and includes favored terrain of the Poconos and Catskills to the Adirondacks and Green Mountains. Rain chances should gradually decrease into next week along the Eastern Seaboard as the fronts push east off the coast. Farther west, scattered convection is also likely near a wavy trailing front across the Lower Mississippi Valley and vicinity through late week, though the placement of heavier rainfall totals remains uncertain. Additionally to the north, a series of shortwaves looks to work from the Pacific through the West/Northwest and inland across the Intermountain West/Rockies through the period, albeit in a pattern with limited precipitation initially. But by the weekend into early next week, convection may start to become more widespread and better organized in the northern Rockies before working down through the Plains, where some heavy rain totals look plausible but uncertainty remains on timing/location. Putnam/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 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