Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1056 AM EST Sun Nov 18 2018 Valid 12Z Wed Nov 21 2018 - 12Z Sun Nov 25 2018 ...Record cold temperatures on Thanksgiving for much of the Northeast... ...Overview and Guidance/Uncertainty Assessment... Upper troughing in the Northeast Wed/Thu will give way to weak ridging as lead and then reinforcing troughing push through the West. This yields a very cold start to the period in the Northeast and a decidedly wet several days (and white at higher elevations) west of the Divide. Farther south, southern stream shortwave energy may attempt to spin up a low pressure system along a linger frontal boundary in the Gulf of Mexico by next weekend. Model consensus was good during days 3-4 (Wed-Thu), and the forecast was based on a multi-model blend including the ECMWF/GFS/FV3. Day 5 (Fri) is when we begin to see some differences emerge in the guidance that grow through time. In general, models/ensembles have shown a trend toward greater amplification of features by late in the week, with the ECMWF seeming to lead the way. This trend holds true for shortwave energy crossing the south central U.S. on Fri,and perhaps amplifying further as it reaches the lower Mississippi Valley or Tennessee Valley by Fri night/Sat. Looking at the past several runs, the ECENS seemed to show the first signs of this trend and has been the most consistent, while the GFS/GEFS seems to be playing catch up. The 00Z deterministic ECMWF became significantly more amplified with this system by Sat, with a closed upper low developing across the lower/mid-Mississippi Valley by Sat. It is possible the ECMWF may have gone too far with this, but given the trends it certainly cannot be ruled out. These trends favor the development of a more significant wave of low pressure than previously thought along a lingering frontal boundary in the Gulf of Mexico by Fri night/Sat, with perhaps a track near/along the Eastern Seaboard. The same holds true for the next shortwave which reaches the Northwest late Fri and then amplifies as it crosses the Rockies and reaches the Plains Sat-Sun. Each consecutive run of both the ECMWF and GFS has become more amplified with this feature, with the ECMWF/ECENS again leading the way. Hemispheric patterns provide some support for the increasingly amplified pattern, with D+8 anomalies from the ECENS and GEFS showing likely development of blocking/strongly negative NAO/AO in the North Atlantic with perhaps some degree of blocking setting up across the north central Pacific in the form of a broad and persistent mean trough south of the Aleutians. The combination of these would support a trend toward flow amplification across the CONUS, and teleconnections associated with these anomaly centers favor rebuilding of the western ridge and amplified troughing across the East. Given these considerations, and increasing uncertainty, weight was shifted toward ensemble means during days 5-7, but with particularly heavy weight placed toward the ECENS relative to the GEFS, and some limited influence from the deterministic ECMWF continued through the end of the period. ...Weather Highlights/Threats... Thanksgiving summary (Thu 11/22) - 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. 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. The entire west coast and much of the interior will see widespread rain/elevation snow to start the period Wednesday. Precipitation focus still appears to be around northern California and the northern Sierras into southwestern Oregon as the upper trough splits N-S. 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 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. Confidence in this aspect of the forecast, however, is low at this time, but the trends (at the moment) seem to be in that direction. Much below average temperatures in the Northeast will be slow to ease but will trend back toward typical late November values by next weekend. 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. The Plains and Upper Midwest will see milder temperatures Thursday-Saturday, generally above freezing for afternoon highs. Colder air will probably work back south across the border by next Sunday. 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