Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 PM EDT Fri Aug 06 2021 Valid 12Z Mon Aug 09 2021 - 12Z Fri Aug 13 2021 ...Overview... A typical August pattern is expected to evolve over the CONUS during the middle of next week with building heights aloft to the south of a persistent and strong low over central and north-central Canada. Outside of the Great Basin and Northwest, daytime heating should lead to thunderstorms across the Southwest, Midwest, Southeast, Great Lakes, and East due to ample moisture availability. Temperatures will be above average for much of the country with some bouts of excessive heat likely. ...Guidance/Predictability Evaluation... The guidance shows general agreement, with the biggest issues related to how strong the trough mid-continent becomes and how far west its axis ends up, with the 06z GFS uncharacteristically on the west side of the guidance and the 00z Canadian showing a closed mid-level center along the trough axis (which is prone to forecasting stronger mid-level features). Strengthening positive 500 hPa height anomalies near 140W and 60W between 50-55N latitude support a broad trough somewhere in the middle. A blend of the available guidance sorts out this issue effectively. ...Weather Highlights/Hazards... Rain is expected across AZ, the Midwest, Southeast, and East ahead of the main frontal zone, with little overlap expected with severe drought areas in the West and Northern Plains (which would broadly worsen drought conditions). Detail issues in the rainfall forecast abound, particularly in the upper Midwest and around the western and southern side of the Great Lakes concerning the when/where of thunderstorm complexes early in the period. Rainfall could be heavy at times. Temperatures will be about 5-15F cooler than normal in the Pacific Northwest and northern Rockies behind the lead cold front into Monday with significant moderation thereafter; 5-15F warmer than average anomalies reappear as soon as Wednesday which would be sufficient for renewed excessive heat concerns. Ahead of this cold front, temperatures will be 5-15F above normal over the Plains Monday and Tuesday which expands/shifts to the Northeast Monday through Friday, closer to the intensifying 500 hPa height anomaly across eastern Canada. Heat and humidity combined will bring heat indices into the low 100s for many. With little relief expected overnight, excessive heat is possible most notably in and around Arkansas and also parts of the Mid-Atlantic later in the week, particularly within urban areas. Roth/Santorelli 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: - Flooding occurring or imminent across portions of the Southeast. - Excessive heat across portions of California, the Pacific Northwest, and the Northern Great Basin, Wed-Fri, Aug 11-Aug 13. - Excessive heat across portions of the Plains and the Middle/Upper Mississippi Valley, Mon, Aug 9. - Excessive heat across portions of the Lower Mississippi Valley and the Southern Plains, Mon-Fri, Aug 9-Aug 13. - Excessive heat across portions of the Mid-Atlantic and the Northeast, Tue-Fri, Aug 10-Aug 13. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml