Extended Forecast Discussion NWS Weather Prediction Center College Park MD 219 AM EDT Tue Sep 21 2021 Valid 12Z Fri Sep 24 2021 - 12Z Tue Sep 28 2021 ...Heavy rainfall expected to continue pushing through the Northeast and New England Friday into Saturday... ...Weather Pattern Overview and Guidance Evaluation/Preferences... A closed upper level low evolving over the Great Lakes and Northeast later this week will lift northward into Hudson Bay region as another shortwave trough is forecast to drop south over the Great Lakes this weekend into early next week then closing off. Overall, troughing over the central/eastern U.S. and ridging over the Southwest U.S. is expected to be the main upper level pattern through the period. There is substantial model uncertainty with respect to the evolution of the closed upper low over the Great Lakes on Day 3/4 and how the next system evolves this weekend as well. The 18Z GFS was a considerable outlier with its flatter 500 mb pattern and subsequent faster/progressive solution. This wasn't supported by the ensemble means or other deterministic guidance which favors a slower solution with a deeper/pronounced closed upper low on day 3. As a result, much of the 18Z GFS wasn't included in the WPC blend preference. An early look at the 00Z suggests some improvement in agreement toward the a solution for later this week and weekend however. The slower solution does favor a stubbornly slow cold front passage across the Northeast/New England which allows for additional moisture to lift northward ahead of the front. Combined with the favorable forcing for ascent, streaks of heavier precipitation will be likely on day 3/4 across portions of New England. The secondary upper level low that approaches late in the weekend will likely keep unsettled weather in place. Elsewhere, model agreement is average to above average with just minor timing differences and model biases seen with an approaching system for the Pacific Northwest days 6/7. A cutoff low over southern California has some model differences again where the GFS is a bit too fast. Its impacts on potential precipitation for the Southwest U.S. are notable where the GFS is much drier compared to the slower ECMWF/CMC which allows for moisture to be drawn northward longer. The WPC blend preference was mainly a non-GFS blend, favoring the 12Z ECMWF/CMC/UKMET and followed later in the periods with mostly ECENS/NAEFS ensemble means. 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 country will be relatively dry, minus for potential showers and storms across California and Nevada 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 U.S. Friday into weekend in the wake of the cold front passages. Highs 5-10 below normal are expected while nighttime lows up to 10F below normal are likely, particularly across the Gulf Coast region. Meanwhile, upper level ridging will promote above normal readings for the western U.S. with highs/lows 5F to as much as 10F above normal. 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