Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Mon Sep 04 2023 Valid 12Z Thu Sep 07 2023 - 12Z Mon Sep 11 2023 ...Impressive heat to shift from the East/South to the south-central U.S and Southwest... ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a broad composite blend of best clustered overall guidance from the 18 UTC GFS/GEFS mean and the 12 UTC ECMWF/ECMWF ensemble mean along with the compatible 01 UTC National Blend of Models in a pattern with seemingly above normal mid-larger scale predictability. This blend tends to best mitigate widespread smaller scale warm season guidance differences generally consistent with system predictability and maintains good WPC continuity. A composite of guidance from the latest 00 UTC guidance cycle overall remains in line. ...Pattern Overview and 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. 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/Southwest. Hot late summer temperatures lingering across the East and Southeast into Thursday will accordingly refocus to the south-central U.S. and the Southwest through the weekend into next week with the repositioning of the upper ridge. Temperature anomalies upwards to the 10-15 degrees above normal range offers potential for widespread record values (both daily records and possibly some monthly records) in what has proven to be a sweltering summer. Meanwhile in this pattern, expect a main organized/mainly moderate overall precipitation focus will be associated with the upper troughing over the East with fueling moisture and instability into a leading surface low/frontal system later week into the weekend. Some localized heavy convective downpours will also occur in scattered/periodic convection back to/across southern to central states near a wavy trailing front in a pattern with favorable upper difluence and periodic impulse passages. This occurs as an uncertain series of shortwaves work from the Pacific through the West/Northwest and inland across the Intermountain West/Rockies though the period, albeit in a pattern with limited precipitation but localized convection that may become modestly organized this weekend across the northern Rockies before working into the central U.S. states. While no WPC Excessive Rainfall areas have been issued for the Day 4/5 period Thursday/Friday at this time given uncertainties at these longer time frames for summer convection, there is a guidance trend suggesting a few slower training cells may offer a threat for some runoff issues over the Mid-South and vicinity. 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