Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Sep 14 2020 Valid 12Z Thu Sep 17 2020 - 12Z Mon Sep 21 2020 ...Sally Excessive Rains Tennessee Valley & Southern Appalachians/Mid-Atlantic Thu-Fri... ...Welcome late summer rains for the Northwest, northern California & northern Rockies... ...Guidance/Predictability Assessment... The WPC medium range product suite was primarily derived from the NHC forecast for Sally and for the rest of the nation a composite of the increasingly well clustered guidance from the latest GFS/ECMWF/Canadian, GEFS/ECENS means, and the National Blend of Models. ...Overview and Weather/Hazard Highlights... With Sally's forecast track farther inland into the interior Southeast after landfall, the threat of heavy rainfall has increased focus across the Tennessee Valley through the southern Appalachians and southern Mid-Atlantic. The timing of the heavy rain should be from Wednesday night through Friday morning. Sally is expected to become extratropical over the interior Southeast as it merges with a stationary front there. A longwave trough forecast to dip into the eastern U.S. should then push the system relatively quickly off to the east and into the Atlantic by the weekend. Meanwhile, a complex evolution over the northeastern Pacific will yield a amplified upper trough/low slated to slowly bring some welcome rainfall and cooler temperatures to the fire stricken Northwest and northern California by midweek then increasingly inland to the northern Intermountain West/Rockies into the weekend in a wet late summer pattern. Schichtel Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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