Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Aug 01 2023 Valid 12Z Fri Aug 04 2023 - 12Z Tue Aug 08 2023 ...Heatwave to persist over the South... ...Excessive rain/flash flood threat to focus over the north-central Great Plains Friday into Saturday... ...Overview... Deepened monsoonal moisture and upper system energy working through the Rockies on the western to northern periphery of an amplified upper ridge will favor a slow shift of a main convective rainfall focus into the north-central Plains. Meanwhile, ample northern stream upper troughing from the Canadian Prairies will dig southeast to the Great Lakes Friday through this weekend which along with upper energies working from the Rockies into the Plains will act to suppress the central CONUS upper ridge axis south to the southern tier of states where above normal temperatures/heatwaves persist. Anomalously hot weather will also spread back into the desert Southwest. Downstream, shortwaves rounding the top of the ridge in the northwest flow will spark strong to severe thunderstorms, and locally heavy rain, into later this week from the Mid-South to the Southern Appalachians/Southeast. Activity should also increase up to the Northeast around Friday as the new northern stream trough energy combines with an Atlantic moisture plume and terrain ahead of an organized eastern Canadian surface low and trailing front to bring drier air/moderated temperatures to parts of the central to eastern U.S. in the wake of passage. ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a blend of best clustered guidance from the 12 UTC ECMWF/UKMET and the 12 UTC ECENS/18 UTC GEFS ensemble means along with the 01 UTC National Blend of Models through this forecast period. The latest 00 UTC GFS/Canadian have trended a tad more in line with this blend compared to their prior run. Local convective focus run to run continuity has been less than stellar despite a reasonably established mid-larger scale summertime pattern, but more organized threat areas have tended to show signals in guidance into medium range time scales. ...Weather/Hazards Highlights... The north-central Plains will see a focus for diurnally driven convection Friday into Saturday as the upper ridge axis centered over the High Plains weakens/suppresses south and northern tier flow becomes more zonal. Deep monsoonal moisture and slow moving/organized embedded energies rounding this ridge will produce heavy rain. Recent guidance agrees on an eastward shift to the heavy rain focus onto the Plains with a plan to upgrade to a new Day 4 ERO Slight Risk for excessive rainfall from eastern MT into the central Dakotas. The more zonal flow for Saturday/Day 5 allows the heavy rain focus to expand east/southeast across the Dakotas into MN/IA with general location uncertainty lending an ERO Marginal Risk area. Farther downstream and around the upper ridge, expect areas of thunderstorms to shift out from the Mid-South/TN Valley to the southern Appalachians/Southeast on Friday. A heavy rain/flash flooding threat from organized convective systems in the northwesterly flow over lingering fronts warrants a Marginal Risk for excessive rainfall. Meanwhile, approach of an amplified northern stream upper trough and leading low/frontal system should tap a deepened Atlantic moisture plume over the Northeast. Plan to maintain an associated Friday/Day 5 Marginal Risk for excessive rainfall over New England, especially terrain and eastern Long Island to address the threat. The ongoing heatwave should settle a bit south over this weekend, closer to the southern border and along the Gulf Coast, where high temperatures will be 5 to 10 degrees above normal. Heat indices will rise to 110-115F in some areas, particularly the lower Mississippi River Valley. Record high maximum and especially minimum temperatures are widely forecast in these regions. Otherwise, welcome post-frontal cooler and drier air spread down over the central and eastern CONUS this weekend. 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