Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sat Nov 20 2021 Valid 12Z Tue Nov 23 2021 - 12Z Sat Nov 27 2021 ...Potential heavy rainfall for portions of the Southern Plains and Lower Mississippi Valley may impact travel for the Thanksgiving holiday... ...Overview and Guidance/Predictability Assessment... An amplifying trough over the East Coast to start the period on Tuesday will eventually close off an upper low off the Northeast Coast which may linger for a couple of days before finally weakening and drifting northward. Meanwhile, a Western U.S. trough may split into two seperate streams with southern energy digging into Mexico/the southern Plains and northern stream energy sliding across the Northern Plains and Great Lakes. This energy may eventually combine with a shortwave in central Canada to form a possible closed low over eastern Canada next weekend. Models continue to show good agreement on the departing trough across the East with lingering questions mainly surrounding how close to the coast the low lingers (CMC closest, GFS farthest away) or how quickly the energy phases with a shortwave/closed low into eastern Canada late period. The ensemble means provided a reasonable middle ground for the second half of the week. The guidance continues to show plenty of run to run variability regarding shortwave energy into the West Coast on Tuesday which should phase with closed low energy southwest of California. There is good agreement that this new trough should eventually split into northern/southern stream energy. Previous runs of the deterministic models indicated potential for a southern stream closed upper low to form and linger over northern Mexico, but the most recent runs have somewhat backed off on this scenario now indicating a bit more progressive of a system, but still impactful nonetheless. There are still a number of ensemble members indicating a closed low, so the potential is still there. By days 6-7, there is a great deal of uncertainty regarding strength and timing of this system but the ensemble means seem to handle the uncertainty well and provided a good starting point for the forecast. The northern stream energy across the northern Plains into the Great Lakes is handled relatively well by the deterministic solutions the first half of the period. Thereafter, some minor timing differences begin to emerge, but nothing out of the ordinary for this time range. The WPC progs used a majority blend of the deterministic solutions for days 3-5, with more emphasis towards the ensmelbe means days 6-7 to help mitigate the forecast uncertainties. This approach maintains good continuity with the previous WPC forecast for days 3-6. ...Weather/Hazards Highlights... The cold front associated with the Northeast low pressure should be well off the Atlantic coast by Tuesday with sprawling high pressure in its wake across much of the eastern third of the nation. As a result, daytime highs and morning lows Tuesday and Wednesday could be 10-20 degrees below normal from portions of the Tennessee/Ohio Valley into the Mid-Atlantic and the Southeast. Near to below freezing morning lows could extend rather far into parts of the South, resulting in the potential for the first frost or freeze for some locations, most certainly impacting sensitive vegetation. Low pressure lingering off the Northeat Coast may bring periods of breezy/gusty winds to parts of New England and combined with already chilly temperatures coule make for some unpleasant wind chills during at least the beginning of next 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 associated cold 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 on Friday. Some models show the potential for several inches of rain. Additional shortwave energy into the Pacific Northwest may also bring another round of moderate to locally heavy coastal rain and mountain snows on Thursday and Friday. For both regions, the south and Pacific Northwest, this rainfall could impact travel for the busy Thanksgiving holiday. Above normal temperatures ahead of the western front will across the Plains Tuesday-Wednesday with anomalies +10 to +20F possible. Much of the country looks to be generally near normal on Thursday, though modestly above normal temperatures may begin to spread across the West by later in the week. 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