Extended Forecast Discussion NWS Weather Prediction Center College Park MD 313 PM EDT Thu Aug 12 2021 Valid 12Z Sun Aug 15 2021 - 12Z Thu Aug 19 2021 ...Excessive heat moderates in the interior West as Tropical Storm Fred is forecast to track near the west coast of Florida this weekend and into the Southeast early-mid next week... ...Pattern Overview... The strong upper ridge anchored in the western U.S. will likely give way to an upper trough moving into the Pacific Northwest and western Canada early next week. Meanwhile, Tropical Storm Fred is forecast to track north-northwest just off the west coast of Florida this weekend, and then into the Southeast early to the middle of next week ahead of a weak upper trough dipping into the Great Lakes/Ohio Valley. Heavy rain can be expected to spread north across the Southeast and into the Appalachians but with uncertainty as to how far north the rain could reach toward the end of the week. ...Guidance/Predictability Evaluation... Models and ensembles show good agreement on bringing an upper troughing eastward into the Pacific Northwest early next week. There is a slight tendency for the models to push the trough faster across the northern Plains midweek, bringing a period of rain/thunderstorms followed by much cooler temperatures into the region. Meanwhile, models have been relatively consistent in tracking Tropical Storm Fred up the eastern Gulf of Mexico near/off the west coast Florida this weekend into Monday. Thereafter, the ECMWF has been favoring a fast northward track up the interior sections of the East Coast while other models take Fred farther north at a slower pace ahead of a relatively weak upper trough dipping into the Great Lakes/Ohio Valley. However, the latest (12Z) guidance have decidedly shifted the landfall location farther west and takes the remnants of Fred across the Deep South. It remains to be seen whether this model trend will continue or not. 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. Less QPF from the GFS/GEFS was incorporated into the blend to account for the assumed northward track into the East Coast. ...Weather Highlights/Hazards... The ongoing heat wave over the Northwest during the short range should finally break by the medium range period. Sunday temperatures should still be above normal, but not nearly as extreme as previous days. By Monday, troughing looks to quickly move into the region so temperatures should actually rebound below normal across the Northwest. Some portions of the Northern Rockies could be 10-15+ degrees below normal next Tuesday into Wednesday. Above to much above normal temperatures will shift eastward Sunday and Monday into parts of the Northern Plains as well. Heavy rainfall and gusty winds across Florida associated with Tropical Storm Fred will be ongoing on Sunday, especially along the western Florida Gulf coast. Tropical moisture will likely spread northward even ahead of any landfall by Fred into the Southeast, Appalachians, and Mid-Atlantic, interacting with a leading stalled frontal boundary and favorable terrain. Exact locations of heaviest rainfall depends on the eventual track of Fred and may still take another few days to resolve. Elsewhere, as an upper ridge axis in the Western U.S. shifts east with advent of upper troughing over the Northwest, 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 moderate to heavy rain threat emerging over parts of the northern Rockies/high Plains on Tuesday, with organized and enhanced rainfall then focusing along the cold front as it shifts into the Northern Plains Tuesday into Wednesday. Kong/Santorelli 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