Extended Forecast Discussion NWS Weather Prediction Center College Park MD 510 PM EDT Mon Jul 31 2023 Valid 12Z Thu Aug 03 2023 - 12Z Mon Aug 07 2023 ...Southern heatwave persists/shifts farther south this weekend... ...Heavy Rain/flash flood threat focuses over the north-central Great Plains Thursday into Friday... ...Overview... Troughing from the Canadian Prairies digs southeast to the Great Lakes Friday through this weekend which suppresses 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. Prior to that, deepened monsoonal moisture working on the western periphery of the ridge will favor a slow shift of a main convective rainfall focus into the north-central Plains. Meanwhile, shortwaves rounding the top of the ridge in the northwest flow will spark strong to severe thunderstorms, and locally heavy rain, past midweek across the Mid-South/Southeast/Southern Appalachians. Activity should also increase up to the Northeast around Friday as new northern stream trough energy combines with an Atlantic moisture plume and terrain in advance of an organized eastern Canadian surface low and trailing front that should will 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 00 UTC ECMWF/CMC/UKMET and 06Z GFS with inclusion of the 00Z ECENS/06Z GEFS ensemble means for Days 5/6. Applied greater weighting in the blend to the ECMWF/ECMWF ensemble mean that have recently showed better run to run continuity. Since then, the 12Z consensus is stronger on the southern suppression of the mid-level ridge over the central CONUS with typical summer time differences between guidance on impulses that drive mesoscale convection. ...Weather/Hazards Highlights... The north-central Plains will see a focus for diurnally driven convection Thursday into Friday 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. 12Z guidance is in agreement on an eastward shift to the heavy rain focus onto the Plains with the Day 4 Slight Risk for excessive rainfall now over the eastern WY/MT border into the central Dakotas. The more zonal flow for Friday/Day 5 allows the heavy rain focus to expand east/southeast into central MN/IA with general location uncertainty and most of this area being dry continuing to warrant a Marginal Risk for Day 5. Farther downstream and around the upper ridge, expect areas of thunderstorms to shift from the Mid-Mississippi Valley through the TN Valley to the southern Appalachians both Thursday and Friday. A heavy rain/flash flooding threat from organized convective systems in the northwesterly flow warrants expanded Marginal Risks for excessive rainfall across these areas. 12Z guidance is also farther east with a northern stream upper trough taps a deepened Atlantic moisture plume over the Northeast. The Day 5 Marginal Risk for excessive rainfall is shunted farther east over New England and Long Island as a result. The ongoing heatwave should settle 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 cooler air spread over the northern and central CONUS this weekend behind a cold front. Jackson Hazards: - Heavy rain across portions of the northern Plains, Thu-Fri, Aug 3-Aug 4. - Heavy rain across the central High Plains into portions of the southern Plains, Sat-Sun, Aug 5-Aug 6. - Hazardous heat across the south-central Plains including the Ozarks, Thu-Sat, Aug 3-Aug 5. - Hazardous heat across the southern tier of the mainland U.S. including the interior Desert Southwest, and into the mid-Mississippi Valley, as well as the western half of Florida, Thu-Mon, Aug 3-Aug 7. - Hazardous heat across much of the Southeast including the eastern portion of Florida, Sat-Mon, Aug 5-Aug 7. 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