Extended Forecast Discussion NWS Weather Prediction Center College Park MD 219 PM EST Fri Nov 19 2021 Valid 12Z Mon Nov 22 2021 - 12Z Fri Nov 26 2021 ...Departing east coast system to produce heavy rainfall in eastern Maine Monday night and anomalously cold temperatures in the Southeast through early next week... ...Increasing confidence in Thanksgiving/Black Friday heavy rainfall event for portions of the Southern Plains and Lower Mississippi Valley... ...Overview and Guidance/Predictability Assessment... An amplifying trough in the eastern U.S. will see out a pair of coastal surface low pressure systems into the Canadian maritime at the beginning of our medium range period (Monday and Tuesday). Upper-level ridging will shift into the central U.S. before a northern Pacific trough phases with a cut-off low west of the California coast, and amplifies over the Southwest on Tuesday. The new trough may split back into separate streams heading into midweek as the southern energy digs into Mexico and the northern wave rides the upper jet across the northern/central tier of the country. The latest guidance is in agreement with the timing of a departing upper-level trough in the East on Monday-Tuesday. Some run-to-run variability exists between GFS members as they struggle with deciding whether to generate a closed upper-low or not. The 00z CMC attempts to retrograde a pair of closed upper-lows over the Northeast by Thanksgiving, which is out of step with the other deterministics. The ensemble means, save for the Canadian, handle the timing and intensity of the departing system fairly well. Unfortunately, this doesn't translate well to the surface for the GFS as its 06z run attempts to drag the surface low back toward the New England coast by Thursday while the 00z run keeps it out to sea with the rest of the models. Models have trended stronger with the Pacific energy approaching the West Coast on Monday. We begin to get more pronounced spread by Wednesday as deterministics like the 06z GFS and 00z UKMET begin to deviate from the enemble means by slowing and strengthening the upper-low. All of the operational models have trended toward a deepening closed low feature over northern Mexico by Thanksgiving day. That being said, the ensemble means appear to be our best bet for determining the 500mb pattern over the Southwest and Great Plains toward the tail end of the medium range period. Medium range progs used a general deterministic model blend with heavier weighting toward the 06z GFS and 00z UKMET on days 3 and 4. By day 5 the 06z GFS weighting was reduced in favor of its 00z predecessor due to the 06z's trouble resolving the Atlantic low, and ensemble means were added to the blend. This blend ratio continues through days 6 & 7. ...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 eastern 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 10-20 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 which may impact sensitive soils. 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, before shifting focus to the central Gulf coast. 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 on Thursday, with temperatures generally near or slightly above average across the West upper level ridging attemps to build over the region. Temperatures will drop again for this region by Friday as the cold front associated with the low pressure system propagates through. Kebede/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