Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Mon Sep 07 2020 Valid 12Z Thu Sep 10 2020 - 12Z Mon Sep 14 2020 ...Overview and Guidance/Predictability Assessment... With the latest models and ensembles (including new 00Z runs of the GFS/ECMWF/UKMET/CMC) now strongly clustering around the idea of an upper low holding back over the Four Corners region/central Rockies around Wed-Thu, hopefully guidance is providing something close to its final answer for an evolution that has proven to be extremely challenging over recent days. Current consensus now shows the upper low/trough crossing the Plains Fri-Sat and then being captured by broad cyclonic flow that develops across southern Canada and the northern U.S. The latter theme has been fairly consistent though with very different timing depending on the model. Meanwhile the upper ridge initially near the West Coast will gradually weaken as it moves into the West, though perhaps strengthen a bit by day 7 Mon. Western Atlantic/eastern U.S. upper ridging should also weaken somewhat, especially during the first half of the period. Overall an operational model blend from the 12Z/18Z cycles provided a good representation of the latest consensus during the first half of the period. The GFS was a little southeast of the majority with the upper low late this week and the CMC was a bit weak. By days 6-7 Sun-Mon the forecast incorporated 40-60 percent total of the 18Z GEFS/12Z ECMWF means while holding onto minority weight of the 18Z GFS/12Z ECMWF into day 7. Refinement of the pattern aloft has led to stronger Upper Mississippi Valley through New England high pressure Thu-Sat and a corresponding southward trend for the front to its south, while ejection of the Rockies upper low leads to a separate wave/frontal system by Sat. A blended approach has held up well for the developing northern stream cyclonic flow that brings a front into the northern tier of the U.S. east of the Rockies. Thus far guidance has been signaling potential for one or more weak waves along the trailing part of the front. The favored blend leans away from the 12Z ECMWF/CMC toward the end of the period as they are fastest with bringing the remains of an East Pacific upper low toward the West Coast. ...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 significant 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 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