Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Thu Aug 03 2023 Valid 12Z Sun Aug 06 2023 - 12Z Thu Aug 10 2023 ...Heatwave to persist over the South and work back into the Southwest... ...Excessive rain/flash flood threat for the Upper Midwest/Great Lakes Sunday and into the Northeast early next week... ...Overview... A monsoonal moisture connection out from the West and a substantial Gulf moisture and instability feed with deepened low/frontal approach will fuel an excessive rainfall threat whose focus is slated to reach the Upper Midwest/Great Lakes Sunday before ejecting across the Northeast early next week. An initially closed upper system energy will slide eastward overtop a main upper ridge as kicked by additional northern stream upper troughing from the Canadian Prairies. This scenario also acts to suppress a central U.S. upper ridge axis to the U.S. southern tier as their heatwave persists and also spreads back into the Southwest. ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a blend of the 12 UTC ECMWF/ECMWF ensemble mean and 01 UTC National Blend of Models. The 12 UTC ECMWF and to a lesser extent the 12 UTC ECMWF ensemble mean offer a solution of the slower side of the full model and ensemble forecast envelope for translation of what is proving to be a significant/slow moving and initially closed system out from the West to the northern Plains and by this period into the Midwest/Great Lakes onward to eastern Canada. The blend maintains that trend and a stronger surface system and heavy rainfall threat signal through the weekend into next week. 00 UTC model and ensemble guidance has overall trended favorably toward this preferred solution, bolstering forecast confidence. ...Weather/Hazards Highlights... The very wet pattern for the north-central U.S. and main heavy convective rain focus should slowly shift to the Upper Midwest and Great Lakes by Sunday as the upper ridge axis centered over the High Plains weakens/suppresses south and northern tier flow flattens. A lingering monsoonal moisture connection and slow moving and deepened surface low/frontal system and upper system energies combine with a continued and pronounced Gulf moisture and intability feed to fuel an excessive rain threat. An upgrade to a Slight Risk for excessive rainfall has been outlined over portions of an increasingly wet Upper Midwest for Sunday, centered on Wisconsin, along with a surrounding Marginal Risk area also spread downstream across the Great Lakes Sunday and with translation for parts of the Northeast Monday where still favorable ingredients combine with locally increased terrain lift. South of the main low, a trailing front will spread drier air/moderated temperatures across parts of the central to eastern U.S. this weekend. Showers and thunderstorms will also tap pooled moisture near the elongated front from the weekend into next week and as with additional low pressure/frontal system and secondary surges down across the eastern/southeastern to the central U.S., with showers/storms also trailing back across the Rockies to monitor given potential moisture feed and local terrain enhancement. Meanwhile, an ongoing heatwave should settle a bit south over this weekend to the south of the trailing front over the South 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. 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