Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM 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 and ensemble guidance overall continues to show good agreement with the mid-large scale pattern consisting of mean troughing across the East, an upper high/ridge reorienting itself a bit westward with time from the south-central U.S. into the southwestern U.S., and impulses coming into the Northwest and tracking eastward to the north of the ridge. Meanwhile there remain some differences in the details of shortwave and frontal positions that are certainly within typical spread for the medium range period. The largest differences arise with timing and placement of an upper trough or low over the eastern Pacific into the West by early next week that will take additional time to resolve, but in general the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian solutions seem mostly compatible over the lower 48 and vicinity Friday and Saturday where a composite seems to offer a good forecast basis well supported by ensembles, the 01 UTC National Blend of Models (NBM) and WPC continuity. 12 UTC ECMWF detail was a bit better match with GEFS/NAEFS/ECMWF ensemble means and NBM Sunday to Tuesday, so that blend was used, but the latest 00 UTC ECMWF shows less than stellar run to run continuity at these longer time frames. ...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. Temperature anomalies may be strongest through late week with some temperatures 15-20 degrees above normal in the southern Plains, well into the 100s. High dewpoints and humidity especially near the western Gulf Coast will lead to dangerous heat indices. Above normal temperatures by about 10-15 degrees should persist into the weekend and early next week across the south-central U.S. and into the Southwest. Widespread 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. 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. 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. 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, which will continue to be monitored for some potentially heavy totals. 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