Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 AM EST Mon Nov 19 2018 Valid 12Z Thu Nov 22 2018 - 12Z Mon Nov 26 2018 ...Record cold temperatures on Thanksgiving/Black Friday for much of the Northeast/New England... ...Could be the coldest Thanksgiving since 1996 for the DC-PHL-NYC-BOS corridor... ...Overview... Very cold but mostly dry in much of the east while it will be stormy in the western US later this week. A couple of cold fronts will push across the lower 48 and potentially lift up along the East Coast Sat/Sun during a very busy travel time. Above average temperatures in the High Plains will trend back to below average by next week. ...Guidance/Uncertainty Assessment... Multi-model blend proved to be a good starting point for Thu/Fri as the northeastern trough exits and the western trough plows eastward. Differences start to expand in the Southeast by Saturday as low pressure organizes in the northeastern Gulf of Mexico and lifts northeastward along the coast. The ECMWF and most of its and the GEFS' ensembles were generally a bit west of recent GFS runs except for perhaps the FV3 (which was a bit quicker). Given the trend toward more amplification in the medium range vs earlier forecasts, preferred the more robust/west solutions near the 12Z ECMWF/Canadian which will take the low up the coast to about the Mid-Atlantic before turning eastward in response to increased ridging to its northeast. Upstream, pair of fronts will converge and slow into the mid-Mississippi and western Ohio Valleys as the upper trough sharpens. This may take another area of low pressure northeastward but on the west side of the Appalachians next Sun/Mon. Again preferred the slower/sharper solutions unlike the recent quicker/weaker GFS runs. Gave much more weight to the 12Z ECMWF ensemble mean by next Sun/Mon which was more amplified than the GEFS mean. ...Weather Highlights/Threats... Thanksgiving summary (Thu 11/22) - Cold turkey, indeed. Record cold will likely be an unwelcome Thanksgiving guest for many in the Northeast, with high temperatures 15 to near 30 deg F below average and many areas staying well below freezing, while low temperatures may reach the single digits on either side of zero. Wet along the west coast into the Great Basin and Rockies with near average temperatures and higher elevation snow. Milder and dry for the High Plains with 40s/50s common. Rain chance for the Gulf Coast from Texas eastward through much of Florida to the north of a front in the Florida Straits and surface low south of Houston. The entire west coast and much of the interior will see widespread rain/elevation snow to start the period Wednesday. Multi-day rainfall amounts may be in the 3-6 inch range over favored areas and even about an inch to an inch and a half in the Central Valley. Snow levels will waver but generally fall through the period with several feet of snow likely at the highest elevations. A potential low pressure system moving from the Gulf and across the Southeast next weekend would support the potential for heavy precipitation across portions of the Southeast, Mid-Atlantic, and southern/central Appalachians. This may coincide with another very busy travel day on Sunday. Much below average temperatures in the Northeast will slowly ease back toward typical late November values by the weekend (and likely above average min temperatures). Temperatures in the west will be near to above average for overnight lows but near to below average for afternoon highs thanks to an abundance of cloud cover. That will trend back to near average by the end of the period. The Plains and Upper Midwest will see milder temperatures Thursday-Saturday, generally above freezing for afternoon highs, but then colder air will likely move in behind a front this weekend. Ryan/Fracasso WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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