Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Tue Sep 08 2020 Valid 12Z Fri Sep 11 2020 - 12Z Tue Sep 15 2020 ...Overview and Guidance/Predictability Assessment... Continue to expect the large scale pattern to trend closer to what might be expected for late summer/early fall, after an upper low tracking out of Wyoming crosses the Northern Plains late this week and is then sheared/whisked away by progressive southern Canada/northern U.S. flow. Varying degrees of ridging aloft will prevail over the Southeast and into southern Texas while a West Coast ridge will move into the western U.S. Behind the western ridge there will be a gradually opening East Pacific upper low whose trough should approach/reach the West Coast during the first half of next week. A consensus blend of the 12Z global models/18Z GFS represented significant features well for the first half of the period, Fri into early Sun. There has been good continuity after guidance finally converged for the early period northern tier upper low 24-36 hours ago. After early Sun recent guidance has differed among each other and consecutive runs on wave details along the front that settles into the Northern Plains, which might be expected given the low predictability of supporting shortwaves embedded within progressive and low amplitude mean flow. The 12Z CMC looked too fast with the upper trough reaching the West Coast (corrected in the new 00Z run) but over the past day the GFS/GEFS mean/ECMWF mean have all trended a bit faster toward earlier ECMWF runs. Specifics become increasingly uncertain late in the period due to possible involvement of Gulf of Alaska energy. Ensemble members also showed widening spread over the eastern Pacific and western U.S. by Mon and especially Tue. Decreasing confidence in details favored a transition to a model/mean blend by days 6-7 Mon-Tue, with total 18Z GEFS/12Z ECMWF mean input reaching 40-60 percent. ...Weather/Hazard Highlights... Best potential for highest rainfall totals during the five-day period should be from the Carolinas into parts of the Southeast. Two separate fronts settling into the region for a time will provide a focus for rain and thunderstorms while a wave/trough originating from the tropical Atlantic and reaching the Southeast Coast early in the period may contribute some added moisture. Occasional rainfall of varying intensity will also extend through the Gulf Coast region into parts of Texas. The upper low ejecting across the Northern Plains late this week will bring a brief period of enhanced rainfall to portions of the Plains/Midwest and possibly into the Great Lakes. Flow ahead of the associated frontal system (and Canadian/northern tier front right behind it) will pull moisture northward across the eastern states during the weekend. These fronts will separate two periods of dry and pleasant weather supported by fairly strong surface highs crossing the Great Lakes and Northeast. The upper trough heading toward the West Coast late in the period may bring some rainfall to the Pacific Northwest by next Mon-Tue. However confidence in specifics is low, as guidance ranges from limited coverage and light amounts to broader coverage of significant totals for so early in the season. The upper ridge moving into the West will spread above normal temperatures across the region and eventually into the Northern Plains. Highest anomalies in the plus 10-15F range are most likely to progress from the Pacific Northwest into the Northern Plains. The Northwest will see a cooling trend late in the period with the approach of the Eastern Pacific trough. The Rockies and Plains will see moderating temperatures after the record cold in the short-range time frame. Some locations may still see 10-15F below normal readings on Fri but from the weekend onward the central-southern Rockies will trend toward normal while central/southern portions of the Plains should see lingering single-digit negative anomalies. Clouds/rain over the southeastern quadrant of the lower 48 will favor lows that are above normal and highs within a few degrees on either side of normal. The northeastern quadrant will see more variable temperatures with the progression of features. Rausch 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