Extended Forecast Discussion NWS Weather Prediction Center College Park MD 144 PM 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 ridging moves 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, Friday into early Sunday. After early Sunday, recent guidance has differed among each other and consecutive runs on wave details along the front that settles into the Northern Plains and the degree of troughing expected to persist offshore the Pacific Northwest. The 06Z GFS looked too strong and perhaps a little fast with the upper trough moving into the Northern Plains next Tuesday. Teleconnections with a respectable 500 hPa height anomaly near Idaho early next week favor the 00z Canadian in and near the Pacific Northwest, though its upper level low is probably too deep. It was weighted more than the other deterministic guidance late in the period, though as usual, increasing amounts of the 00z NAEFS and 00z ECMWF ensemble means were used to temper the uncertainty from Sunday into early next week. ...Weather/Hazard Highlights... The 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 subtropical 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 should bring some rainfall to the Pacific Northwest by next Mon-Tue. 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. Roth/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