Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 AM EDT Sat Aug 14 2021 Valid 12Z Tue Aug 17 2021 - 12Z Sat Aug 21 2021 ...Heavy rain associated with Tropical Depression Fred expected to spread further inland into the Southeast early next week... ...Tropical Depression Seven could approach Florida during the latter half of 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. Meanwhile, focus is on Tropical Depression Fred which is forecast to make landfall as a tropical storm early next week across the central Gulf Coast and then spread its heavy rainfall inland toward the Tennessee and Ohio Valleys. Finally, toward the latter half of next week, attention turns to Tropical Depression Seven which may intensify into a tropical storm and approach southern Florida, though there remains considerable uncertainty with that system's track and intensity forecast. ...Guidance/Predictability Evaluation... The deterministic and ensemble guidance overall is in reasonably good agreement for the synoptic pattern for the medium range period. For the troughing advancing through the Northwest, northern Rockies, and northern Plains, the main model outlier that was not considered was the 18Z GFS which cuts off the upper level energy over the central/northern Rockies while the rest of the deterministic guidance and ensemble support maintains a progressive wave through the western Great Lakes. With this upper level troughing pushing through, surface low pressure is forecast to develop over the Plains and lift toward the Great Lakes in the latter portions of the period. Some of the typical timing biases show up in the deterministic models so the blend leaned on the ECENS/GEFS means to help mitigate those biases and provide some continuity from the previous forecast. Regarding the southern U.S. and tropical features, a ECMWF/GFS blend was favored for Fred as a good proxy for the official NHC track through there remains some spread in the track guidance as the system enters the central/eastern Gulf. For Tropical Depression Seven, there remains considerable uncertainty in the strength and intensity forecast for days 5-7. Overall, the WPC medium-range package was based on a blend of the 12 ECMWF/ECENS mean, 12Z GFS/GEFS mean, and some contribution from the 12Z CMC with more weights from the ensemble means for Days 6 and 7. ...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 is expected to increase, 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 southern Florida where Tropical Depression Seven is forecast to intensify into a tropical storm before approaching the Sunshine State, though there remains some considerable uncertainty in the eventual track/intensity of that system. 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 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