Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Tue Sep 15 2020 Valid 12Z Fri Sep 18 2020 - 12Z Tue Sep 22 2020 ...Sally Heavy Rains exit across the Southern Appalachians and Carolinas Friday... ...Welcome late summer rains for the Northwest/northern California and northern Rockies... ...Guidance/Predictability Assessment... The WPC medium range guidance suite was originally derived from a composite blend of the 18 UTC GEFS and 12 UTC ECMWF along with the 01 UTC National Blend of Models and with incorporation of the 03 UTC NHC Sally forecast. Earlier model forecast runs spread with embedded systems was less than stellar, so opted to work with the more compatible ensemble means. A continued trend with 00 UTC guidance toward a flatter ejection of extratropical Sally with frontal interaction favored a corresponding QPF adjustment, mainly day 4/Fri. ...Overview and Weather/Hazard Highlights... The heavy rainfall threat will exit across the southern Appalachians and Carolinas Friday as Sally transitions to a post-tropical frontal wave. The likely merging of Sally with a front initially over the South and/or interaction with a stronger front approaching from the north add to the complexity of the forecast, but guidance trends have certainly been increasingly to a flatter track to the east along with the main rainfall axis, especially with the newer 00 UTC models. The upper trough supporting the second front should accordingly push any remaining reflection of Sally into the Atlantic after Fri. Meanwhile, an amplified upper trough/embedded low arrival from the Pacific supports some welcome rainfall and cooler temperatures to the fire stricken Northwest and northern California into late week, then increasingly inland to the northern Intermountain West/Rockies this weekend. Some rainfall may accompany the leading frontal system as it continues east of the Rockies late in the period. Expect an area of well below normal temperatures over the Northern Plains/Upper Midwest to progress and expand into much of the East and South late this week and weekend. Many locations within this area should see at least one day with minus 10-15F anomalies for min and/or max temps. The East Pacific trough/upper low will bring a brief period of highs up to 10F below normal across portions of the West late this week. 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