Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Wed Sep 09 2020 Valid 12Z Sat Sep 12 2020 - 12Z Wed Sep 16 2020 ...Overview and Guidance/Predictability Assessment... Latest guidance continues to show the rapid departure of a Northern Plains-Upper Midwest upper low/trough (the upper low now near the Four Corners) after early Sat in response to progressive southern Canada/northern U.S. flow, with the overall trough continuing across eastern Canada and Northeast U.S. by early next week. Upper ridging will tend to cover the Southeast and Gulf Coast region while a separate ridge progresses across the West and into the Plains, either weakening in favor of a diffuse trough/shear axis or eventually merging with the Southeast ridge. An opening East Pacific upper low and likely another bundle of energy dropping down from the Gulf of Alaska or nearby will comprise an overall trough expected to approach and reach the West Coast. Models and ensemble means have been agreeable and consistent for features over the eastern half of the lower 48 aside from small scale/low-predictability details. This leaves the upper trough reaching the West Coast as well as late-period southern Canada flow the primary forecast uncertainties. Trends over the past day have generally been leading toward more involvement of energy from near the Gulf of Alaska, which ECMWF runs were a little earlier to advertise. This has led to a deeper overall trough in the ensemble means and the GFS. Individual model runs and ensemble members still show a lot of spread for exactly how the energy from the initial upper low and the Gulf of Alaska will interact--whether forming one shortwave or the northern energy helping to kick out the southern feature for example. Thus confidence in specifics is still rather low, favoring a conservative forecast that trends the early-period operational model blend more toward the 18Z GEFS/12Z ECMWF ensemble means by days 6-7 Tue-Wed. In varying ways GFS runs over the past 12 hours have been somewhat on the aggressive side with bringing height falls across the Northwest while CMC runs have been more eager to close off an upper low near the West Coast versus other solutions. Even with the uncertainty for the Pacific energy and southern Canada flow there is decent clustering toward the emergence of surface low pressure along the U.S.-Canadian border east of the Rockies by day 6 Tue, though the increasing detail spread aloft does cause some divergence for wave/frontal specifics by midweek. ...Weather/Hazard Highlights... Highest five-day rainfall totals still appear to be most likely from the Carolinas through parts of the Southeast to the Gulf Coast region, with an early dissipating front and another front arriving early next week helping to focus rain/thunderstorms. Subtropical Atlantic moisture may also contribute to enhanced rainfall. Other locations near the Gulf Coast into Texas may see periods of rain with varying intensity as well. The system tracking out of the Plains/Midwest will bring an episode of rainfall with some locally moderate to heavy activity possible from near the Mississippi Valley through the Northeast/Mid-Atlantic during the weekend, followed by drier conditions as trailing high pressure builds in. Specifics remain rather uncertain but there is gradually increasing confidence that the upper trough heading toward the West Coast should bring some rainfall to the Pacific Northwest by the first half of next week with at least some moderate totals possible. The upper ridge moving across the West and into the Plains will bring above normal temperatures with it during Sat-Tue. Expect the greatest anomalies to be plus 10-15F or so, progressing from the Northwest and northern Great Basin into the Northern Plains. The upper trough nearing the West Coast should lead to a cooling trend toward near normal highs. The southern half of the Rockies should gradually warm up from moderately below normal to slightly above normal readings, while the southern half of the Plains will likely stay slightly below normal for highs. Above normal low temperatures over the southern two-thirds of the East on Sat will expand north/northeast Sun-Mon ahead of the frontal system tracking out of the Plains/Midwest, followed by a cooler trend as strong high pressure builds across the Great Lakes/Northeast. Clouds ahead of the front and then the trailing high will keep highs over the East within a few degrees of normal most of the time. 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