Extended Forecast Discussion NWS Weather Prediction Center College Park MD 314 PM 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... Overview of the forecast model guidance during the current medium range period (Friday Aug 4 - Tuesday Aug 8) depicts mean troughing over the Northeast south of a Hudson Bay upper-low, the stagnant upper-level high over the south-central/southern tier of the U.S., and upper-level flow trending more zonal to the north. Initial trends through the middle of the period show the trough over the Northeast beginning to lift to the northeast with shortwave energy in the northern stream dropping southward over central portions of the country, helping to amplify general mean troughing along and east of the Rockies into the Plains/Midwest. This also begins to displace the upper-high and generally higher heights over the southern tier further southward and shifting mean ridging to the east and west, especially over the Southwest and up the West Coast into Canada where the pattern amplifies in response to the deepening trough over the central U.S. The pattern looks to amplify more broadly across the country at the end of the forecast period as the trough shifts eastward towards the East Coast and heights continue to rise over portions of the West, with the troughing deeper in the 00Z/06Z GFS and 00Z CMC compared to the 00Z ECMWF, and the 00Z ECens and 00Z GEFS means offering an expected compromise between the deterministic guidance. An upper-level wave dropping south from the northeastern Pacific may impinge on the mean upper-ridging over the West Coast into the Pacific Northwest at the end of the period, indicated most by the 00Z CMC and less so by the 06Z GFS, 00Z GFS, and 00Z ECMWF, respectively. Generally good agreement in the guidance early in the period leads to an updated WPC forecast of a composite guidance blend, with a lesser contribution from the 00Z UKMET whose details diverged a bit overall compared to the closer clustering of the other guidance, particularly with respect to the extent of the upper-high over the southern tier. A contribution from the 00Z ECens and GEFS means is added mid-period as differences in smaller-scale shortwave energy over the central U.S. increase and the forecast-time limited UKMET is ramped down. The blend trends towards a slight emphasis on the 00Z ECMWF/00Z ECens late period, which show the greatest compromise in the overall amplification of the pattern, specifically with respect to mean troughing over the eastern half of the U.S. The contribution from the 00Z CMC is reduced the most as it shows more significant amplification in comparison. ...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 initially becomes more zonal before transitioning to northwesterly. Deep monsoonal moisture and slow moving/organized embedded energies rounding this ridge will produce heavy rain. Recent guidance had suggested an eastward shift to the heavy rain focus onto the Plains, and a Day 4 ERO Slight Risk for excessive rainfall has been outlined from eastern MT into the central Dakotas where the highest ensemble probabilities indicate the potential for locally heavy rainfall despite lingering uncertainties in the deterministic guidance on QPF placement. However, it is possible given notable PW anomalies 1.5-2 standard deviations above the mean across the region and a westward trend with some higher QPF in the guidance that a larger Slight Risk area extending westward into Montana may be necessary if confidence in rainfall amounts increases, but drier antecedent conditions may ultimately keep any flood risk low. 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 and less current consensus on heavier rainfall amounts leading to a ERO Marginal Risk area for now. 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 over more sensitive areas of the upper Tennessee Valley. Meanwhile, the approach of the more amplified northern stream upper trough and leading low/frontal system should tap a deepened Atlantic moisture plume over the Northeast leading to widespread heavy rainfall on Friday. This has resulted in a Day 5 Marginal Risk for excessive rainfall over New England, with the bulk of the heaviest rainfall looking to stay over less sensitive coastal regions compared to the more sensitive interior. The ongoing heatwave should settle a bit south over this weekend, closer to the southern border and along the Gulf Coast as upper-troughing shifts southward and the upper-high flattens out. High temperatures will be 5 to 10 degrees above normal with high humidity leading to heat indices in the 110-115F range in some areas, particularly the lower Mississippi River Valley. Record high maximum and especially minimum temperatures are widely forecast in these regions. Additionally, after a brief drop in temperatures over the Desert Southwest, highs will once again warm into the 110s. Some above average temperatures will also spread up the West Coast following a bit of upper-ridge amplification, with highs in the central California valleys climbing into the mid-100s into the weekend and early next week. Otherwise, welcome post-frontal cooler and drier air spread down over the central and eastern CONUS this weekend. Putnam/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