Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Fri Aug 13 2021 Valid 12Z Mon Aug 16 2021 - 12Z Fri Aug 20 2021 Heavy rain likely for portions of the Southeast as Tropical Storm Fred moves northward after landfall ...Pattern Overview... A welcomed pattern change arrives by early next week for the western U.S. as the upper ridge breaks down and a trough builds in across the Pacific Northwest and northern Rockies, heralding the end of the ongoing heat wave. Meanwhile, Tropical Storm Fred will likely be weakening as the storm moves inland across the Southeast U.S., bringing heavy rain along its path. Upper level ridging will likely build back in across the Gulf Coast region by the middle to end of the week. ...Guidance/Predictability Evaluation... Models and ensemble means are in very good overall agreement on the pattern change expected across the north-central U.S. through the beginning of the week. The 12Z/00Z CMC differs some with lower 500mb heights across the Great Lakes and Midwest compared to the UKMET/GFS/ECMWF, and it is slower with the passage of the trough across the Rockies late in the week. It is also worth noting that the GFS is considerably stronger with a potential tropical disturbance near the Bahamas by next Friday, and is not supported by the other guidance. In terms of T.S. Fred, the UKMET is a bit slower than the model consensus, the ECMWF slightly left, and the GFS weaker overall after the system makes landfall. The WPC medium-range package was based on a blend of the 12Z ECMWF/EC mean, 18Z GFS/GEFS, and some contributions from the 12Z UKMET and previous WPC continuity, with greater weighting towards the ensemble means by Thursday and Friday. ...Weather Highlights/Hazards... The ongoing heat wave over the Northwest is expected to be over by Monday with temperatures returning to near normal levels. Some portions of the Northern Rockies and adjacent portions of the northern Plains could be 10 to 20 degrees below normal next Tuesday into Wednesday. High are expected to be above normal ahead of the cold front across the northern Plains on Monday, followed by more pleasant readings by midweek. 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. Exact locations of heaviest rainfall depends on the eventual track of Fred, and this is subject to additional changes in future forecasts. Elsewhere, 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 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. Hamrick 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