Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 AM EDT Sun Aug 15 2021 Valid 12Z Wed Aug 18 2021 - 12Z Sun Aug 22 2021 ...Heavy Rain Threat with Fred to focus over the Southeast/South-Central Appalachians early next week... ...Tropical Storm Grace may threaten southern Florida and move into the 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... An upper level shortwave trough will work through the northern Rockies and northern Plains early in the forecast period then lift well north into Canada as upper level ridging off the Southeast U.S. asserts itself westward over the southern U.S. by next weekend. This will favor synoptic storm tracks across the Plains and Great Lakes. Attention focuses also on two tropical systems - Fred as it moves inland across the Southeast and South-Central Appalachians and Tropical Storm Grace as it nears southern Florida late in the week and then could move into the Gulf of Mexico by next weekend. Both systems will pose a heavy rainfall threat. ...Guidance/Predictability Evaluation... The latest model guidance suite was reasonably clustered for the Northwest to northern Plains shortwave trough, though there remains some uncertainty in how much if any energy splits into a closed low over the Intermountain West. For now, the WPC blend favored a more progressive/flatter system that leaned on the ensemble means by day 5 onward. By the end of the period, signals are that the upper pattern takes on a more zonal direction with several impulses quickly moving through the flow - overall uncertainty/spread is higher than normal for day 6 and day 7. Other areas of uncertainties are associated with the tropics - particularly for Grace as it emerges into the Gulf of Mexico late in the week and next weekend. The strength/position of the upper level ridging off the Southeast U.S. may shunt the system further west/southwest but the ECENS/GEFS members show a wide spread, with the GFS being the southern solution scenario while the ECENS, including its deterministic run, show a weaker ridge that allows the system to curve northward toward the central Gulf Coast. For now, following the latest NHC advisory, a compromise of the two scenarios was used with the caveat that model spread and uncertainty is higher than normal for the system. ...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 are the favored locations for several inches of rainfall that may lead to flash flooding. For Tropical Storm Grace, heavy rainfall hazards may impact the southern Florida peninsula and then the system may emerge into the Gulf of Mexico by next weekend. It's too early to tell where the system goes beyond that. Follow the latest NHC advisories for the most updated 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. 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