Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Aug 14 2021 Valid 12Z Tue Aug 17 2021 - 12Z Sat Aug 21 2021 ...Heavy Rain Threat with Fred to focus over the Southeast/South-Central Appalachians early next week... ...Tropical Storm Grace to threaten southern Florida and the eastern Gulf of Mexico later next week... ...Monsoonal moisture from the Southwest/Great Basin leads into emerging Heavy Rain Threat over the north-central U.S. mid-later next week... ...Pattern Overview... Upper level troughing is forecast to move through the Pacific Northwest early next week then advance toward the northern Plains and western Great Lakes later next week, bringing some relief from the high heat and humidity to portions of the northern tier in a wet pattern. Meanwhile, focus is on Fred which is forecast by NHC to re-organize and make landfall as a tropical storm early early next week into the east-central Gulf Coast and then spread heavy rainfall inland toward the eastern Tennessee/Ohio Valleys and especially favored terrain of the south-central Appalachians. Finally, toward the latter half of next week, attention turns to T.S. Grace which is forecast to track to southern Florida and onward into the eastern Gulf of Mexico. ...Guidance/Predictability Evaluation... The WPC medium range product suite was primarily derived from a blend of best clustered guidance from the NHC, WPC continuity, the 00 UTC ECMWF/ECMWF ensemble mean and the 13 UTC National Blend of Models. Nudged blend weights to the ECMWF ensemble mean days 5-7 consistent with slowly growing forecast spread. ...Weather Highlights/Hazards... 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. There remains some uncertainty in the heaviest rainfall axis however across the southern Appalachians, Deep South, and TN/OH Valleys as some westward trends in the track as well as intensity issues could affect the potential rainfall impacts. For Tropical Depression Seven, while there is much above normal uncertainty in the track/intensity forecast, the system may approach portions of southern Florida late next week. Follow the advisories from the NHC on both of these systems for the latest forecast information. Elsewhere, monsoonal moisture activity over the Desert Southwest/Great Basin will again bring a localized flash flood risk. Moisture should stream northward and northeastward ahead of the incoming Northwest to north-central U.S. upper trough, with a potential for moderate to heavy rainfall/convection emerging from the northern Rockies/high Plains to the north-central Plains and mid-upper MS Valley mid-later next week, with organized and enhanced rainfall focusing along a developing frontal system. Toward the latter half of next week, attention may once again be drawn to southern Florida where Tropical Storm Grace is forecast approaching Florida and the eastern Gulf of Mexico. Taylor/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, 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