Extended Forecast Discussion NWS Weather Prediction Center College Park MD 223 AM EDT Thu Sep 23 2021 Valid 12Z Sun Sep 26 2021 - 12Z Thu Sep 30 2021 ...Overview... Upper ridging over far eastern Canada and initially over south-central Canada/Northern Plains will favor troughing/closed upper low over the Great Lakes/Northeast and a small closed low over the Southwest. Another trough and cold front over the northeast Pacific on Sunday will track southeastward early next week toward British Columbia and the Pacific Northwest, and encompass much of the Intermountain West by midweek. This favors amplification of the downstream ridge over the Midwest and western Great Lakes. ...Guidance Evaluation/Preferences... There are still some differences in the models with the rather complex interaction of several upper level features over the Great Lakes/southern Canada late this weekend, with the CMC most amplified and the GFS quickest to lift the trough out. Part of this is tied to the movement of a Southwestern upper low that will likely only slowly move eastward as it eventually becomes absorbed by the next trough over the Rockies by mid week. A blended solution continued to serve as a good starting point in the forecast process. For the western U.S., a pattern change is expected to evolve as the upper ridge initially over the region is shunted eastward across the Plains as a strong Pacific storm system reaches the Pacific Northwest by Tuesday. Recent GFS runs have been ahead of the ensemble means, particularly the 18Z GFS. The 12Z and 00Z runs were a bit closer to the model consensus and thus more weighting was applied to the 12Z versus 18Z run for fronts/pressures, along with some of the ECMWF and the EC/GEFS mean. The CMC remains more amplified across the eastern U.S. for the end of the forecast period. ...Weather and Potential Hazards... Rainfall from the exiting storm system in the short range forecast period should be primarily gone by early Sunday, with perhaps some lingering showers across Maine. Meanwhile, a stationary/stalled frontal boundary draped across the Florida peninsula will bring daily chances of diurnally-driven thunderstorms. Return flow moisture will bring increasing chances of storms to Texas by the middle of next week, where some models indicate some heavier amounts. For the Pacific Northwest, some orographically enhanced rainfall is likely for western Washington and Oregon for early in the week as the next Pacific storm system moves inland, and high elevation snow for the Cascades after the cold front moves through. Much of the rest of the nation will be relatively dry, though some isolated showers and storms will be possible across portions of the Desert Southwest and the Four Corners region associated with the weak closed upper level low late this weekend into early next week. Near normal temperatures will prevail across the eastern third of the nation this weekend in the wake of the cold front passage, along with pleasant humidity levels. Meanwhile, as ridging aloft develops across the central U.S. and troughing develops over the West Coast region, above normal readings are likely across the central/northern Plains with daily highs 10 to 20 degrees above normal Sunday into next week. Meanwhile, the cold front crossing the West Coast region will likely bring below normal readings in its wake, with highs running 5 to 15 degrees below normal. Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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