Extended Forecast Discussion NWS Weather Prediction Center College Park MD 405 PM EDT Mon Sep 07 2020 Valid 12Z Thu Sep 10 2020 - 12Z Mon Sep 14 2020 ...Overview and Guidance/Predictability Assessment... The latest models and ensembles continue to show above average clustering with respect to an upper low holding back over the Four Corners/central Rockies region early in the period, eventually crossing the Plains Friday-Saturday and then being captured by broad cyclonic flow developing across southern Canada and the northern U.S.. Meanwhile, the upper ridge initially near the West Coast should gradually weaken as it moves eastward, through perhaps strengthen a bit again by day 7/Monday. Overall, an operational model blend from the latest suite of deterministic models (00z/06z) provided a good representation of the latest consensus during the first half of the period. After that time, brought modest a modest percentage of ensemble mean guidance (00z ECENS/06z GEFS) to help mitigate detail differences days 6-7, but held onto some weighting of the 06z GFS/00z ECMWF through the entire period. This approach brings strong high pressure across the Upper Mississippi Valley into New England days 3-5 and southward progression of a weakening frontal boundary across the South/East. Ejection of the Rockies upper low leads to a separate wave/frontal system by Saturday, while another northern stream front drops into the northern tier of the U.S. east of the Rockies. The guidance continues to favor potential for one or more weak waves along the trailing part of this front. ...Weather/Hazard Highlights... As the upper low tracks northeastward from the Rockies expect some areas of moderate to heavy rainfall over portions of the Plains/Midwest and possibly extending northeastward into the Great Lakes late this week into the weekend. Two separate cold fronts, the first of which may weaken to a trough that persists along the western Gulf Coast for a while, may enhance rainfall at times over parts of Texas during the period. A dissipating warm front and an approaching Atlantic tropical wave/trough may produce potentially heavy rainfall along parts of the East Coast, with the best potential along the coast of the Carolinas/southern Mid-Atlantic. The cold front pushing southward over the East late this week may provide another focus for rainfall, while stronger high pressure to the north of the front should keep moisture suppressed farther south than expected in some earlier forecasts. Departure of the high and continued progression of the Plains/Midwest frontal system after early Sat should help to pull moisture northward over much of the East during the weekend. Some areas of relatively heavy rain are possible but with low confidence for specifics at this time. Expect record cold to continue into Thu for parts of the central-southern Rockies/Plains, with lows up to 10-20F below normal and highs up to 20-30F below normal. Then temperatures over the central U.S. will moderate with time. The western U.S. will see a gradual expansion of above normal temperatures, initially focused over the Pacific Northwest, as an upper ridge moves into the region. Some areas in the Northwest may see highs up to 10-20F above normal into late week while readings will tend to be 5-15F above normal within the broader area of western warmth Sat-Mon. High pressure crossing the northeastern quadrant of the lower 48 will bring a brief period of cooler weather along its path. Otherwise clouds/rain farther south late in the week, and over more of the East thereafter, should promote temperatures that are above normal for lows but within a few degrees on either side of normal for highs. Rausch/Santorelli 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 Central Plains, and the Upper/Middle Mississippi Valley, Fri, Sep 11. - Heavy rain across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Sun, Sep 13. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Much above normal temperatures across portions of California, the Great Basin, the Pacific Northwest, and the Northern Rockies, Thu-Mon, Sep 10-Sep 14. - Much below normal temperatures across portions of the Plains, the Rockies, the Great Basin, the Great Lakes, the Upper/Middle Mississippi Valley, and the Southwest, Thu-Fri, Sep 10-Sep 11. 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