Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 PM 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... The medium range period will begin Tuesday rather amplified in the upper levels, with a trough axis near the West Coast, a deep trough in the East, and ridging in between. The eastern trough is forecast to develop a closed upper low around Wednesday and spin over the Atlantic along with its associated surface low, likely far enough east not to impact the Northeast U.S. significantly from midweek onward. The trough in the West will move eastward as the week progresses, and it is becoming more likely it will split into two separate streams, with southern energy digging into Mexico and northern stream energy sliding across the Northern Plains and Great Lakes. The latter may eventually combine with a shortwave in central Canada to form a closed low over eastern Canada next weekend. The southern stream energy and moisture flowing into a frontal system could produce locally heavy rain across the south-central U.S. on Wednesday into Thanksgiving Day. ...Guidance/Predictability Assessment... 00Z/06Z model agreement was fairly good with the departing trough across the East, with some lingering questions regarding timing when the closed mid-upper low develops and placement of surface lows. The bulk of model guidance agrees that these features will mainly cause marine and southeast Canada threats, with the system far enough from the Northeast U.S. during the medium range period not to cause significant impacts. In the West, guidance continues to show plenty of run to run variability regarding placement and strength of shortwaves moving through the mean trough, which leads to differences in timing and progression of the separation into northern and southern streams during the latter half of the week. The 00Z/06Z model cycle generally flipped back to a solution with more separation of streams, with many deterministic models (though not the 00Z GFS) and even the 06Z GEFS mean and 00Z EC mean at times indicating a southern stream closed low over northern Mexico that lingers there Friday and Saturday. The 00Z deterministic ECMWF showed this 500 mb closed low farthest west, along with a good portion of the EC ensemble members, pushing the 00Z EC ensemble mean trough/low farther west compared to CMC and GEFS ensemble solutions that are not as slow to move the southern stream trough/low. The stream separation does cause sensible weather differences as well--the cold front ahead of the northern stream trough could move faster with more separation, and QPF in these solutions is lower over the south-central U.S., possibly because the pattern holds back more Pacific moisture. The updated WPC forecast did follow the trend toward a closed low and attempted to take a middle ground solution in placement between the slower EC suite and the faster guidance. Given the variability between models and from run to run, there is still considerable uncertainty with this evolution. Currently incoming 12Z guidance remains split, with the ECMWF and GFS still showing considerable stream separation (with the new 12Z EC a bit faster than the 00Z run and the 12Z GFS aligned with its 06Z run), while the CMC shows a more phased solution. Overall the WPC fronts/pressures began with a blend of the deterministic guidance early in the period, lessening the proportion of the slow 00Z ECMWF as the period progressed, splitting the difference with the faster 00Z and slower 06Z GFS, while increasing the proportion of the 06Z GEFS and 00Z EC ensemble means. ...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 Northeast Coast may bring periods of breezy/gusty winds to parts of New England and combined with already chilly temperatures could make for some unpleasant wind chills during at least the beginning of next week. Some light to perhaps locally moderate precipitation is expected to spread across the Rockies around Tuesday-Wednesday as a cold front moves across. Then by midweek, the 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, with amounts currently expected to lessen by Friday as the front moves offshore. Though this update lowered forecast rainfall amounts per the latest model consensus, stay tuned for forecast updates as totals could increase again, and the rain could have impacts for a couple of the busiest travel days of the year. 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, which could impact travel as well. 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 Thanksgiving, before modestly above normal temperatures may begin to spread across the West and High Plains by later in the week. Tate/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