Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 AM EDT Thu Aug 12 2021 Valid 12Z Sun Aug 15 2021 - 12Z Thu Aug 19 2021 ...Tropical Storm Fred Heavy Rain Threat from Florida this weekend to the Southeast/Appalachians/Mid-Atlantic early-mid next week... ...Pattern Overview... Models and ensembles show amplified troughing moving into the Northwest on Sunday shifting slowly eastward early next week. Meanwhile, an upper level trough will move through the Northeast Sunday-Monday, with heights beginning to rise again across much of the East by later in the medium range period. Tropical Storm Fred will likely affect Florida this weekend bringing heavy rainfall, and then also into parts of the Southeast, southern to central Appalachians and Mid-Atlantic early-mid next week as tropical moisture feeds inland and interacts with a stalled leading frontal zone draped to the north of Fred and terrain. ...Guidance/Predictability Evaluation... The NHC tracks Tropical Storm Fred over the far eastern Gulf of Mexico as the medium range period begins on Sunday before lifting inland into the Southeast and up the Appalachians early next week as a gradually weakening but very wet system both along/east of the track and in advance with deep moisture feed into a draped front. Fred offers a maritime and inland heavy rainfall/runoff threat. There remains some considerable spread regarding Fred's track through the eastern Gulf and into the Southeast. Please refer to NHC guidance regarding the details and official forecast track. Elsewhere, there's fairly good agreement to suffice a deterministic blend for days 3-4 including the latest runs of the ECMWF, GFS, and CMC. The second half of the period there remains some uncertainty around the amplified Western U.S. troughing. The GFS is more amplified and brings a closed low into the northern Rockies, while the ECMWF and CMC are a little weaker and both lift the closed low feature well into central Canada. Given the late period timing, opted to lean more on the ECENS/GEFS ensemble means which were more agreeable. Axis wise, there's decent agreement, but it's the detail and dynamics differences which make the QPF forecast across the northern Rockies/northern High Plains very uncertain. A blend towards the ensemble means also supports the growing forecast spread surrounding Fred after it makes landfall likely near the Florida Panhandle. ...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. 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