Extended Forecast Discussion NWS Weather Prediction Center College Park MD 428 PM EDT Fri Aug 13 2021 Valid 12Z Mon Aug 16 2021 - 12Z Fri Aug 20 2021 ...Heavy rain associated with Tropical Storm Fred expected to spread further inland into the Southeast early next week... ...Potential Tropical Cyclone Seven could approach Florida during the latter half of next week... ...Pattern Overview... An upper level trough will likely move through the Northwest during the early part of next week before reaching the northern Plains later next week, heralding the end of the ongoing heat wave across the region. Meanwhile, Tropical Storm Fred is forecast to make landfall on the eastern Gulf Coast early next week and spread heavy rainfall inland across the Southeast U.S. into the southern Appalachians. Toward the latter half of next week, Potential Tropical Cyclone Seven is forecast to intensify into a tropical storm and approach Florida. ...Guidance/Predictability Evaluation... Models and ensemble means are in very good overall agreement on the pattern change expected across the north-central U.S. through the beginning of the week. However, deterministic models have become more divergent toward the end of next week regarding whether a low pressure wave will develop and intensify over the northern Plains. A general model compromise with increasing weights towards the ensemble means on Days 6 and 7 yielded a solution fairly close to continuity. This is also true regarding the remnants of Fred across the Southeast with the ECMWF still favors more QPF pushing north into the southern Appalachians. In contrast, the GFS/GEFS continues to advertise much less QPF across the Southeast given a much weaker remnant circulation of Fred in the forecasts. An intermediate solution was adapted as a compromise given past performances of these two models on tropical systems. The WPC medium-range package was based on a blend of the 00Z ECMWF/EC mean, 06Z GFS/GEFS, and some contributions from the 00Z CMC/CMC mean with more weights from the ensemble means for Days 6 and 7. ...Weather Highlights/Hazards... The ongoing heat wave over the Northwest is expected to be over by Monday with temperatures returning to near normal levels. Some portions of the Northern Rockies and adjacent portions of the northern Plains could be 10 to 20 degrees below normal next Tuesday into Wednesday. High are expected to be above normal ahead of the cold front across the northern Plains on Monday, followed by more pleasant readings by midweek. Tropical moisture is expected to lift northward across interior portions of the Southeast and southern Appalachians after Fred makes landfall, potentially interacting with a stalled frontal boundary and terrain enhancement. Exact locations of heaviest rainfall depends on the eventual track of Fred, and this is subject to additional changes in future forecasts. Elsewhere, monsoon activity over the Desert Southwest should increase and spread northward, again bringing a localized flash flood risk. Moisture should stream northward ahead of the incoming trough into the Northwest, with a potential for moderate to heavy rainfall emerging over parts of the northern Rockies/high Plains on Tuesday, with organized and enhanced rainfall then focusing along the cold front as it advances across the Northern Plains Tuesday into Wednesday. Toward the latter half of next week, attention may once again be drawn to Florida where Potential Tropical Cyclone Seven over the tropical Atlantic is forecast to intensify into a tropical storm before approaching the Sunshine State. Kong/Hamrick 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 Central Great Basin, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Ohio Valley, and the Southwest, Mon-Wed, Aug 16-Aug 18. - Heavy rain across portions of the Upper Mississippi Valley and the Northern Plains, Wed-Thu, Aug 18-Aug 19. - Heavy rain across portions of the Central Plains, the Southeast, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Thu-Fri, Aug 19-Aug 20. - Heavy rain across portions of the Southeast, Mon, Aug 16. - Flooding possible across portions of the Southeast. - Flooding occurring or imminent across portions of the Southeast. - Excessive heat across portions of the Central Great Basin, California, the Northern Plains, and the Southwest, Mon, Aug 16. - Excessive heat across portions of the Central Plains and the Northern Plains, Mon-Tue, Aug 16-Aug 17. 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