Extended Forecast Discussion NWS Weather Prediction Center College Park MD 229 AM 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 over the Labrador Sea and 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 the Pacific Northwest. This favors a mostly dry pattern once the lead eastern system moves into Atlantic Canada and generally moderating temperatures. ...Guidance Evaluation/Preferences... Upper level troughing will progress through the Great Lakes and Northeast Saturday and Sunday with a couple vort maxes pushing through. This will act to sweep a pair of cold fronts cleanly off the coast later Sunday into Monday. Meanwhile upstream heights building over the central/northern Plains will continue into early next week ahead of a digging shortwave trough for the Pacific Northwest Tuesday/Wednesday. The fast flow supports relatively dry conditions across much of the CONUS aside from increasing moisture for the Pacific Northwest by the middle of next week. For the Northeast/New England, guidance has finally come together on the timing and depth of the upper low features as they push through the region. Earlier differences between the GFS and other models have lessened such that a consensus approach can be used there. For the Northwest early to mid next week, considerable uncertainty in how the upper pattern evolves as the energy moves onshore toward the Intermountain West by the middle of next week. The GFS is fast and flat while the ECMWF slows and close off the upper low over the interior West (likely tied to downstream slowing/closing off of another low over the Great Lakes). Given the much above normal spread, higher amounts of the ECENS/GEFS means were used. 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 Friday into 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 while return flow moisture will bring increasing chances of storms to Texas by the middle of 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 with daily highs 10 to 15 degrees above normal Sunday into next week across the central/northern Plains. 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. Taylor/Fracasso 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