Extended Forecast Discussion NWS Weather Prediction Center College Park MD 431 PM EDT Tue Sep 21 2021 Valid 12Z Fri Sep 24 2021 - 12Z Tue Sep 28 2021 ...Heavy rainfall will slowly exit the Northeast/New England Friday into 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... A pair of interacting closed upper level lows evolving over the Great Lakes and Northeast later this week through the weekend will carry a pair of fronts through the area. The lead system will support a wrapped-up front with possible surface wave development off the Carolinas late Fri/early Sat that may lift northward toward the Gulf of Maine by early Sunday. Trailing upper low will support a secondary cold front eventually reinforcing the lead boundary and helping to move it out of the Northeast. Guidance has wavered markedly on how to handle this interaction/dumbelling but the latest two cycles were closer to agreement. With such a meridional flow, considerable moisture will flow northward out of the sub-tropics in a narrow axis into New England Friday into Saturday as the front slowly moves eastward. Some solutions like the 00Z UKMET were especially bullish and, though possible, will be sensitive to the east-west sharpness of the upper trough and degree of tilt back to the NNW. Elsewhere, model agreement was near average outside the Southwest. There, the 00Z/06Z GFS runs were slightly/much farther east, respectively, than the ECMWF-led consensus and were not preferred. Into the PacNW, the 00Z UKMET and then the Canadian departed from the ensemble consensus with the timing/depth of the trough south of the Gulf of Alaska though the multi-day ensemble trend was for a sharper trough (much like the East). The WPC preference was mainly a non-GFS blend (though it was reasonable in the East); favored the 00Z ECMWF through the period with some contribution from the CMC/UKMET. Trended toward an increasing weight of the 00Z ECMWF ensemble mean and Canadian mean. 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. Cooler, fall-like temperatures will prevail across the central/eastern CONUS Friday into the weekend in the wake of the cold frontal passages. Highs 5-10F below normal are expected while nighttime lows up to 10F below normal are likely, particularly across the Gulf Coast region with dew points into the 50s/60s. Temperatures will moderate this weekend into next week. Meanwhile, upper level ridging will promote above normal readings for the interior western CONUS to the Plains with highs/lows about 5F to as much as 10-15F above normal. Temperatures will trend cooler next week in the Pacific Northwest as a lead and then more substantial trailing cold front move into the area. Fracasso/Taylor Hazards: - Heavy rain across portions of northern New England, Fri-Sat, Sep 24-Sep 25. - Flooding possible across portions of lower Michigan. - Flooding occurring or imminent across portions of the Tennessee Valley. 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