Extended Forecast Discussion NWS Weather Prediction Center College Park MD 239 PM EDT Wed Sep 22 2021 Valid 12Z Sat Sep 25 2021 - 12Z Wed Sep 29 2021 ...Heavy rainfall will slowly exit the Northeast/New England Saturday... ...Overview... Upper ridging near the Labrador Sea/Davis Strait and initially off the West Coast into the Great Basin will favor troughing over the Great Lakes/Northeast and a small closed low over the Southwest. Troughing over the North Pacific this weekend will dive southeastward early next week toward British Columbia and the Pacific Northwest. This favors a mostly dry pattern for the middle of the CONUS once as heights build and temperatures moderate. A lead eastern system near New England will move into Atlantic Canada as its upper level support wraps up through central Quebec. ...Guidance Evaluation/Preferences... Models continued to struggle with the rather complex interaction of several upper features over the Great Lakes/southern Canada this weekend, resulting in poor run-to-run consistency for system details for a day 3-4 lead time. Part of this is tied to the movement, or lack thereof, of a Southwestern upper low that will likely only slowly move eastward. Some GFS runs, including the 06Z cycle, have pushed this farther east toward Texas by the weekend which muddied the already uncertain trough forecast along and east of 90W. 00Z GFS run aligned well with the other 00Z guidance overall and used that as a base to start the forecast period. A blended solution continued to act to minimize changes in the guidance that have not been steady. For the Northwest early to mid next week, considerable uncertainty remains in how the upper pattern evolves as the energy moves onshore toward the Intermountain West by the middle of next week. The GFS Multi-day ensemble trend has been quicker, with the ECMWF and Canadian ensembles a bit faster than the GEFS members. Despite building ridging downstream, opted to rely more with the larger/quicker consensus near the ECMWF ensemble mean by the end of the period given the trend. This favored well with continuity while adjusting toward the latest trends in the guidance. ...Weather and Potential Hazards... Expect lingering rain, some of which could be locally heavy, along and ahead of the slow-moving cold front across the Northeast and New England on Saturday. The second system in its wake will bring additional showers and possibly thunderstorms to the Great Lakes region through the weekend. Meanwhile, a stationary/stalled frontal boundary draped across central/south Florida will bring daily chances of diurnally-driven thunderstorms. Much of the rest of the Lower 48 will be relatively dry, though some isolated showers and storms will be possible across portions of CA/NV/AZ associated with the weak closed upper level low late this weekend into early next week. Return flow moisture will bring increasing chances of storms to Texas by the middle of next week, where some models indicate some heavier amounts. Areas of the Pacific Northwest may see periods of modest rain and some highest elevation snow (once cooler temperatures push through) early next week. Cooler, but still near normal, temperatures will prevail across the central/eastern U.S. this weekend in the wake of the cold front passages. Meanwhile, as ridging aloft asserts itself across the central U.S. and troughing develops on each coast, above normal readings are likely across the central/northern Plains with daily highs 10 to 15 degrees above normal Sunday into next week. A front passing through the Northwest early next week will bring below normal readings in its wake with highs 5 to locally 15 degrees below normal. Fracasso/Taylor 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