Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Mon Sep 14 2020 Valid 12Z Thu Sep 17 2020 - 12Z Mon Sep 21 2020 ...Sally Excessive Rains for parts of the Southeast and Southern Appalachians/Mid-Atlantic Thu-Fri... ...Welcome late summer rains for the Northwest/northern California and northern Rockies... ...Guidance/Predictability Assessment... Models and means advertise a gradual evolution toward less amplified and more progressive flow aloft with time across the northern CONUS/southern Canada and vicinity, as an East Pacific upper low moves inland and opens up while a trough (initially anchored by a deep upper low in Canada) crosses the northeastern quadrant of the lower 48. Trends over the past couple days have been toward faster progression of these features, more so in the ECMWF/ECMWF mean. However the 12Z ECMWF has adjusted slower to negate some of these trends. Trends before the 12Z ECMWF's arrival had led to the strong surface high pressure initially over southern Canada and the northern U.S. building into the East a little more quickly--contributing to a recent trend for less northwestward extent of the rainfall shield associated with Sally which has been recently upgraded to hurricane status. Check the latest National Hurricane Center products for further info regarding Sally. A blend among the 00Z operational models/06Z GFS early (with some editing to accommodate Sally's official track) followed by a transition to the 06Z GFS/00Z ECMWF and their respective ensemble means provided a reasonable starting point for depicting significant features. The updated forecast reflected some of the aforementioned faster adjustments through the 00Z/06Z cycles but otherwise continuity is good. ...Overview and Weather/Hazard Highlights... The heavy rainfall threat with Sally should extend into Thu-Fri when portions of the Southeast and southern Appalachians/Mid-Atlantic are the most likely areas to see significant rain. By that time there are still important differences for track and timing, constraining confidence in the specifics. The likely merging of Sally with a front initially over the South and then interaction with a stronger front approaching from the northwest add to the complexity of the forecast. The upper trough supporting the second front should push any remaining reflection of Sally into the Atlantic after Fri. One or more leading waves may track just off the East Coast as well. Meanwhile a complex evolution currently over the northeastern Pacific will yield an amplified upper trough/embedded low whose approach/arrival will bring some welcome rainfall and cooler temperatures to the fire stricken Northwest and northern California by mid-late week then increasingly inland to the northern Intermountain West/Rockies into the 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 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 5-10F below normal across portions of the West late this week, displacing plus 5-15F anomalies forecast over the Interior West/Rockies on Thu. Locations over the Southwest may remain several degrees above normal for the rest of the period though, with similar anomalies extending into the central High Plains into the weekend. Rausch/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