Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Fri Nov 19 2021 Valid 12Z Mon Nov 22 2021 - 12Z Fri Nov 26 2021 ...Potential continues for a significant storm to affect the East next week with possible moderate to heavy rainfall across portions of the Southern Plains/Lower Mississippi Valley around or on Thanksgiving Day... ...Overview and Guidance/Predictability Assessment... Amplification of an Eastern U.S. upper trough will be underway as the medium range period starts Monday, pushing a cold front off the East Coast by Tuesday as low pressure then lingers near/off the Canadian Maritimes the remainder of the week. Out West, a southern stream upper low initially southwest of California should combine with a Pacific shortwave to yield amplified troughing, and potentially embedded compact closed low, to drop southward along the California coast and into northern Mexico by the end of the week. Northern stream energy may deepen as it tracks near the U.S./Canadian border and into the Great Lakes by Friday. Models and ensembles show reasonable agreement on the evolution of the East Coast trough/closed low but continue to exhibit some smaller scale differences in intensity and location of surface low pressure lingering near the Canadian Maritimes later in the week. 12z (yesterday) CMC is displaced a bit west of the latest ECMWF and GFS runs, which also have support from the ensemble means. Out West, the latest guidance has trended deeper/stronger with Pacific Northwest energy which may form a closed low dropping southward along the California coast and into the Northern Mexico during the early to middle part of next week. By Thursday though, run to run variability increases significantly regarding the evolution and of this southern stream energy across northern Mexico and potentially into parts of the High Plains. The latest run of the ECMWF is the deepest/slowest piece of guidance, while the GFS is weaker and a little more progressive. Ensemble solutions vary widely but suggest that both the ECMWF and GFS are within the realm of possibilities. The ensemble means right now seem to be the best starting point, with a more middle ground solution. The overnight medium range progs used a deterministic model blend for days 3-5 (with slightly more weighting towards the ECMWF and GFS). After this, the forecast trended quickly towards the ensemble means, with some modest contributions still from the deterministic solutions for added system definition. Overall, this left a reasonably consistent starting point with the previous WPC forecast. ...Weather/Hazards Highlights... The Northeast low pressure will bring a cold front into and through the East on Monday with much of the heaviest precipitation heading off the East Coast. Moderate to heavy rainfall should continue into parts of New England though with the heaviest totals (in excess of an inch) across far northeast Maine on Monday aided by strong Atlantic inflow. Cold air filtering in behind the system should support at least light accumulating snow downwind of the lower Great Lakes as well as along westward-facing terrain from the central Appalachians and northeastward. There is still enough uncertainty in system evolution to allow for other possibilities regarding precipitation type and amounts, even into parts of far northern New England. Deepening low pressure should allow for a period of breezy/gusty winds across much of the East Monday and Tuesday and possibly into the middle and latter parts of the week across the Northeast as low pressure lingers offshore. Daytime highs and morning lows Monday and Tuesday could be 1o-20 degrees below normal from portions of the Ohio Valley/Midwest into the Mid-Atlantic and the Southeast. Near to below freezing morning lows could extend rather far into parts of the South. Especially for northern locations, this combination of cold temperatures and gusty winds could make for some unpleasant wind chills during the upcoming Holiday week. The Pacific Northwest into northern Rockies should see mostly light to perhaps locally moderate precipitation during the first half of next week with a shortwave/cold front arriving from the Pacific. The combination of this energy aloft and the opening/progression of the upper low southwest of California may spread some moisture into other parts of the West by Tuesday/Wednesday, but still with modest confidence for precipitation coverage/amounts. By mid week, the western front should reach far enough into the Plains to begin interacting with Gulf moisture to produce an expanding area of moderate to possibly heavy rainfall from eastern Texas into parts of the Lower Mississippi Valley for Wednesday and especially into Thanksgiving day. Above normal temperatures ahead of the western front will march from the Rockies to the Plains Monday-Wednesday with anomalies +10 to +15F possible. Modestly above normal temperatures are possible into the Southern Plains and Midwest late week, with temperatures generally near or slightly above average across the West upper level ridging attemps to build over the region. Santorelli 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