Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM EST Mon Nov 16 2020 Valid 12Z Thu Nov 19 2020 - 12Z Mon Nov 23 2020 ...Overview... Late this week into the weekend expect flow aloft over much of the lower 48 to be southwesterly, between Gulf of Mexico/Southeast ridging and a positively tilted shortwave (likely consisting of energy from multiple streams) that moves into the West. Meanwhile Canadian surface low pressure will bring a front across the northeastern quadrant of the country, with the front extending into portions of the West. A significant portion of the front should decelerate/stall for a time as it becomes more aligned with the flow aloft. Continued progression of the western shortwave from the weekend into early next week should promote wave development along the surface front along with an area of enhanced precipitation over parts of the central U.S. Meanwhile guidance has increasingly diverged over the past day regarding details of the eastern Pacific/western U.S. pattern by Sun-Mon. The one positive is that even with these differences there is a more common signal for precipitation emphasis over the Pacific Northwest late in the period. ...Guidance Evaluation/Preferences... Based on guidance through the 12Z/18Z cycles, an operational model blend provided a reasonable representation of consensus and continuity for about the first half of the period. As has been the case in recent days, the overall shortwave moving into the West still has at least a moderate amount of uncertainty due to it likely consisting of multiple pieces of energy--favoring a blended approach to the forecast. Continuing with the new 00Z run, the ECMWF has tended to be among the more inconsistent models for handling this energy in recent days. Clustering quickly unravels during the weekend over the eastern Pacific and western U.S. Over the past day GFS/GEFS mean runs have made a pronounced adjustment toward flatter and faster handling of trough energy that moves into the West, with the last couple CMC runs also quite progressive. ECMWF/ECMWF mean runs thus far have been consistently slower and more amplified with the eastern Pacific trough. The 12Z CMC mean advertises a compromise timing while the new 00Z UKMET through the end of its run late Sat sides more with the ECMWF. Most D+8 multi-day mean charts generally agree on a core of positive height anomalies near 40N 170W and associated teleconnections favor eastern Pacific troughing more similar to the ECMWF/ECMWF mean. Latest GFS runs are closer to 160W for that positive anomaly center but telecons still favor more amplified West Coast troughing than what the model is showing. These considerations favor leaning more in the ECMWF/ECMWF mean direction for the latter half of the forecast, with minority incorporation of the 12Z NAEFS mean (the benefit being to include some of the CMC ensembles) to yield a modest compromise. The new 00Z ECMWF trends a bit faster with its East Pacific trough versus the prior run but still keeps the axis offshore as of 12Z Mon and is actually close to where the GFS was 36 hours ago. Farther east there is better agreement in principle with the Sun-Mon Great Lakes/New England wave and trailing cold front though with some differences due to variation in evolution of energy within the supporting upper trough. ...Sensible Weather and Hazards... Shortwave progression from the eastern Pacific into the West late this week will lead to a lighter trend for rain/high elevation snow over the Pacific Northwest/northern Rockies and bring some precipitation into the central Rockies. Then by the weekend the shortwave will interact with the front draped over the central U.S. into Lower Great Lakes, leading to an expansion of precipitation and development of a wave along the front. Currently the best potential for highest totals extends over and a little south of the Midwest as well as into parts of the Great Lakes. Expect mostly rain to the east of the Rockies, with modest potential for a little snow in the northern fringe of the moisture shield. Meanwhile precipitation should increase again over the Pacific Northwest during the weekend/early next week. Orientation of flow aloft is quite uncertain given the model and ensemble spread so specifics of terrain enhancement and timing/coverage at a particular point in time have low confidence. However there is a more common signal for precip to be increasing in this time frame. Initially very warm temperatures from the Southwest/southern Rockies through much of the Plains and into the Great Lakes on Thu will moderate and push southeastward with time, especially from the weekend into next week as the main frontal boundary over the country accelerates. Some plus 15-20F or slightly higher anomalies will be possible Thu and to a lesser extent Fri with scattered daily records also a possibility from Arizona into the southern Rockies and parts of the Plains. By next Mon above normal readings should be confined to the eastern states. The cooler air pushing into the western and central U.S. will likely yield near to modestly below normal anomalies. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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