Extended Forecast Discussion NWS Weather Prediction Center College Park MD 425 PM 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... Today's guidance continues to advertise a general evolution toward less amplified and progressive flow across most of southern Canada and northern parts of the lower 48. The one significant point of contention is with energy on the back side of the upper trough crossing eastern Canada and the Great Lakes/Northeast. The 00Z ECMWF, and ECMWF mean in a more open fashion, use this energy to form a slow trough near the East Coast with embedded upper low near New England. There have been some stray hints of troughing off the East Coast/Canadian Maritimes and the new 12Z GFS has adjusted toward more Atlantic troughing (12Z UKMET similar to the end of its run early Mon) but thus far the evidence points toward a somewhat less extreme solution than the 00Z ECMWF. The new 12Z ECMWF maintains its general evolution but at least is not as deep with its upper low. One point of interest for the ECMWF scenario is that the upper low would ultimately bring Tropical Storm (soon to be Hurricane) Teddy into the Canadian Maritimes, and in fact close to Maine in the 12Z run. Depending on the progression of Sally into late week, the eastern U.S. differences aloft may have some influence on the remaining surface reflection as it interacts with the cold front ahead of strong high pressure building into the eastern half of the country. The 15Z National Hurricane Center advisory for Sally shows a track between the slow 00Z ECMWF/ECMWF mean and fast GFS runs into Fri. Preference for an intermediate solution aloft over the East thereafter would maintain the compromise approach for Sally's surface reflection and moisture before departure into the Atlantic. Note that the new 12Z ECMWF has adjusted toward faster eastward progression. Farther upstream there is some run-to-run spread/variability for specifics but consensus has good continuity with the upper low progressing inland from near the Pacific Northwest coast, eventually bringing a front into the Northern Plains/Upper Mississippi Valley. Upstream energy may bring another front into the extreme Northwest during the first half of next week. The updated forecast accounting for guidance available through the 06Z cycle made use of an early blend of 00Z/06Z operational models with edits to account for the official forecast of Sally, followed by a steadily increasing trend toward mostly 06Z GEFS/00Z ECMWF means. ...Overview and Weather/Hazard Highlights... The heavy rainfall threat from Sally will exit across the southern Appalachians and Carolinas Friday as the system likely transitions to a post-tropical frontal wave. Exactly when/how Sally interacts with the strong front approaching from the north at the start of the period, and details of eastern U.S. upper trough energy, will ultimately determine the timing/magnitude/coverage of rainfall. There continues to be a fair amount of spread with the current forecast reflecting an intermediate solution. For the rest of the period expect strong high pressure over the East to confine moisture mainly to Florida and across the northern-western Gulf of Mexico plus the Gulf Coast. Some moderate/heavy rainfall within this area may reach fairly close to the central/western parts of the Gulf Coast. Meanwhile, an amplified upper trough/embedded low arrival from the Pacific will support 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 Sunday into next week. The strong high pressure settling over the East will bring a period of well below normal temperatures to the region late this week into early next week, with gradual moderation from west to east. Best potential of double-digit anomalies for morning lows will be from the Great Lakes into the Northeast. Some locations in the Mid-Atlantic/Southeast could see as many as three or four days in a row of highs 10-15F below normal. The upper low moving inland from the Pacific Northwest coast will bring a brief period of highs 5-10F below normal from the Northwest/portions of California into the northern Rockies late this week into the weekend. Ahead of this system the warmest anomalies will be over the northern-central Rockies and High Plains which should be 10-15F above normal Fri (Sat as well over central areas). Slightly less extreme warmth will likely continue into next week over the Northern Plains while the Southwest should remain several degrees above normal through the period. 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 Hazards: - Heavy rain across portions of the Southeast and the Mid-Atlantic, Fri, Sep 18. - Flooding possible across portions of the Southeast, the Lower Mississippi Valley, and the Tennessee Valley. - Flooding likely across portions of the Southeast and the Lower Mississippi Valley. - Much above normal temperatures across portions of the Central Great Basin, the Central/Northern Rockies, and the Central/Northern Plains, Fri, Sep 18. - Much above normal temperatures across portions of the Central Rockies and the Central/Northern Plains, Fri-Sat, Sep 18-Sep 19. - Much below normal temperatures across portions of the Great Lakes, the Mid-Atlantic, the Northeast, the Central Appalachians, and the Ohio Valley, Sat-Mon, Sep 19-Sep 21. - Much below normal temperatures across portions of the Great Lakes, Fri-Sat, Sep 18-Sep 19. 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