Extended Forecast Discussion NWS Weather Prediction Center College Park MD 357 PM 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... 00/06z guidance captured the evolution of the southern stream ridge reasonably well during the medium range period. The 06z GFS diverged from the rest of the guidance when depicting the northern stream troughing through early next week. That being said, the GFS had better run-to-run consistency than the ECMWF, therefore our general model blend was weighted toward the 06 GFS through day 4. The gmb also consisted of the 00z EC/CMC/UKMET. The 06z GEFS and 00z ECE were introduced on day 5 while the 00z UK was removed due to its overamplification of the shortwave trough over the Midwest/Upper Great Lakes. The deterministic 00z CMC was replaced by the 00z CMCE on day 6 and the ensembles make up a majority of the blend through the end of the period. The last several runs of the deterministic GFS and EC have signalled a potentially tropical closed low in the eastern Pacific near Baja California. ...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 southeast Montana and 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. Kebede/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 Hazards: - Heavy rain across portions of the Northern Plains, Sat, Aug 5. - Severe weather across portions of the Central Plains and the Middle Mississippi Valley, Sat, Aug 5. - Flooding possible across portions of the Middle Mississippi Valley and the Ohio Valley. - Hazardous heat across portions of the Lower Mississippi Valley, the Southern Rockies, California, the Southeast, the Southern Plains, and the Southwest, Sat-Wed, Aug 5-Aug 9. - Hazardous heat across portions of California, Sun, Aug 6. - Hazardous heat across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, the Southeast, and the Southern Plains, Sat-Sun, Aug 5-Aug 6. - Hazardous heat across portions of the Southeast and the Mid-Atlantic, Mon-Tue, Aug 7-Aug 8. - Hazardous heat across portions of mainland Alaska, Sat-Mon, Aug 5-Aug 7. - Hazardous heat across portions of mainland Alaska, Sun, Aug 6. 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