Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Aug 02 2023 Valid 12Z Sat Aug 05 2023 - 12Z Wed Aug 09 2023 ...Heatwave to persist over the South... ...Excessive rain/flash flood threat to focus over the northern Plains into Saturday... ...Overview... It remains the case that 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 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. ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was mainly derived from a blend of best clustered guidance from the 18 UTC GFS and especially the best pattern matching 12 UTC ECMWF/ECMWF ensemble mean along with the 01 UTC National Blend of Models through this forecast period. Prefer to lean increasingly on the ECMWF ensemble mean over time, consistent with forecast spread growth. The latest 00 UTC UKMET/Canadian have again trended a tad more in line with this blend compared to their prior runs. 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 well into medium range time scales. WPC product continuity is decently maintained. ...Weather/Hazards Highlights... The very wet pattern for the north-central Plains with heavy convective rains will persist into Saturday before transitioning as the upper ridge axis centered over the High Plains weakens/suppresses south and northern tier flow slowly becomes more zonal before transitioning to northwesterly. Deep monsoonal moisture and slow moving/organized embedded to efficient closed energies rounding this ridge will produce more heavy rain. A Day 4 ERO Slight Risk for excessive rainfall has been outlined across parts of the Dakotas where the highest ensemble probabilities indicate the potential for locally heavy rainfall despite lingering uncertainties in the deterministic guidance on max QPF placement. This area is also expected to have quite a bit a heavy rain leading into this period to significantly wetten soils and streamflows. However, it is possible given notable PW anomalies 1.5-2 standard deviations above the mean. The more zonal flow that develops later Saturday/Day 5 should allow the remaining heavy rain focus to slide to the Upper Midwest and vicinity with general location uncertainty and less current consensus on heavier rainfall amounts leading to a ERO Marginal Risk area. 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, An organized eastern Canadian surface low and trailing front will bring drier air/moderated temperatures across parts of the central to eastern U.S. this weekend. Expect temporary moderate moisture/rainfall rebounding early next week with a next main low pressure/frontal system and secondary surges across the central to eastern U.S., with some additional showers/storms trailing back to the Rockies. 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