Extended Forecast Discussion NWS Weather Prediction Center College Park MD 309 PM EDT Thu Aug 05 2021 Valid 12Z Sun Aug 08 2021 - 12Z Thu Aug 12 2021 ...Overview... Typical late summer pattern expected to evolve over the CONUS during the middle of next week with building heights aloft. The main storm tracks will skirt the northern Rockies/Plains into the Upper Midwest/Great Lakes regions with the rest of the country likely to see a drier weather regime. Temperatures will side toward above normal for much of the country with some bouts of excessive heat likely in the typical August summer pattern. ...Guidance/Predictability Evaluation... The 00Z/06Z deterministic models showed fairly good consistency and agreement with the ensembles. Early on in the period, shortwave troughing will drop through the Pacific Northwest and Northern Rockies, opening up as it approaches the Northern Plains by the middle of the week. Here the 06Z GFS in particular was much faster with the wave, also flatter, while the ECMWF was slower and more amplified. The consensus leaned toward the UKMET solution and that agreed well with continuity as well. For the rest of the forecast package, the ensembles all point toward weak but general troughing over the Great Lakes/Upper Midwest by the middle/end of the next week, but anomalies don't look extreme at this point. As a result, the latter portions of the forecast largely leaned on the ECENS/GEFS means, which offered a reasonable solution for now that transitioned well with continuity. ...Weather Highlights/Hazards... Rainfall will expand across the Corn Belt and Great Lakes Sunday as a shortwave lifts toward Wisconsin. Some rainfall could be locally heavy. As the system lifts into southern Ontario/Quebec, rainfall coverage and intensity should wane some before perhaps picking up again next Tue-Thu along the tail-end of the lead boundary and in advance of another Canadian front, in addition to afternoon convection over the Tennessee and Ohio Valleys. Another shortwave pushing into the Northern Rockies on Monday could bring some locally heavy rainfall to parts of northern/northwest Montana. Temperatures will be about 5-15 degrees cooler than normal in the Pacific Northwest and northern Rockies behind the lead cold front this weekend into Monday with a moderation thereafter. Ahead of this cold front, temperatures will be 5 to 15 degrees above normal over the Plains Sun-Mon and expanding to the East Coast Mon-Thu. A ridge over the Southeast Monday will maintain temperatures into the 90s with heat indices into the low 100s as dew points rise into the 60s and low 70s. Temperatures into the 80s and 90s will reach into the Northeast/ Mid-Atlantic ahead of the Great Lakes trough which is about 5-10 degrees above normal. Fracasso/Taylor 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 Middle Mississippi Valley, the Great Lakes, and the Upper Mississippi Valley, Sun-Mon, Aug 8-Aug 9. - Heavy rain across portions of the Southwest, Sun, Aug 8 and Thu, Aug12. - Flooding possible across portions of the Southeast. - Flooding likely across portions of the Southeast. - Excessive heat across portions of the Pacific Northwest and the Northern Great Basin, Wed-Thu, Aug 11-Aug 12. - Excessive heat across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, and the Ohio Valley, Sun-Tue, Aug 8-Aug 10. - Excessive heat across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southeast, the Southern Plains, and the Ohio Valley, Sun-Wed, Aug 8-Aug 11. - Excessive heat across portions of the Mid-Atlantic and the Northeast, Tue-Thu, Aug 10-Aug 12. 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