Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Sep 04 2023 Valid 12Z Thu Sep 07 2023 - 12Z Mon Sep 11 2023 ...Impressive heat will moderate in the East late week but persist across the south-central U.S and Southwest... ...Guidance Evaluation/Predictability Assessment... Recent model guidance continues to show good agreement with the large scale pattern consisting of mean troughing across the East, an upper high/ridge reorienting itself a bit westward with time into the southwestern U.S., and impulses coming into the Pacific 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. Thus the WPC forecast used a multi-model deterministic blend favoring the GFS and ECMWF early in the period, gradually including and increasing the proportion of the GEFS and EC ensemble means with time to half by day 7. This maintained good continuity from the previous forecast. ...Weather/Hazards Highlights... The emergence of an amplified upper trough through the Midwest into Thursday/Friday will act to suppress upper ridging and high heat across the East, after one more potentially record-breaking day on Thursday. Upper troughing will settle over the East through the weekend 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 drifts a bit west 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 will be 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. For this forecast cycle, added a Marginal Risk into the ERO for Friday for parts of the Northeast/Mid-Atlantic as convection looks to have a relatively better focus near/ahead of the front compared to Thursday. The Marginal Risk encompasses lower FFGs in Pennsylvania into the Northeast but also stretches south into the DC/Baltimore and Richmond metro areas. Though antecedent conditions on the southern side are dry, urban areas could see a flash flooding risk. Rain chances should gradually decrease 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 into the Plains, which will continue to be monitored for some potentially heavy totals. Tate/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