Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Dec 19 2020 Valid 12Z Tue Dec 22 2020 - 12Z Sat Dec 26 2020 ...Heavy precipitation threat over parts of the eastern half of the country mid-late week... ...Overview and Model Guidance Evaluation/Preferences... Guidance continues to show an evolution toward an increasingly amplified pattern over the lower 48, with a western ridge/eastern trough aloft likely to be their strongest late in the week. Shortwave energy pushing in from the Pacific by Fri (Christmas Day) and Sat should start eroding the western ridge followed by some height rises over at least southern parts of the East. As the pattern amplifies expect a northern tier storm system and its trailing front to pull increasing amounts of moisture into the eastern half of the country after midweek, with one or more frontal waves possibly helping to enhance warm sector rainfall--and even cold sector snow given the sharp temperature contrast defined by the front. While the amplifying evolution is a consistent and agreeable theme in the guidance, the details are still fairly uncertain. If anything the new 00Z guidance is lowering confidence somewhat given a return to more spread for exactly how Eastern Pacific shortwave energy reaching the West evolves in the short range/early medium range time frame, with the 00Z UKMET/CMC not comparing particularly well with recently established consensus. To some degree these issues feed through the rest of the period. Interaction from Canadian flow provides added complexity. Among the 12Z/18Z solutions the GFS/ECMWF/CMC clustered fairly well in principle, suggesting steady progression of the northern tier system and then one or more waves (possibly with fairly strong development) embedded along the trailing front that crosses the eastern U.S. Another surface low could form in the vicinity of the Great Lakes (or elsewhere nearby) based on these solutions closing off an upper low. Ensembles show greater spread including hinting at some potential for slower shortwave energy such as what the new UKMET/CMC suggest, while keeping the overall upper trough open. For the purposes of a single deterministic forecast, preference sided with an operational 12Z/18Z model blend (more emphasis on the GFS/ECMWF) for about the first half of the period and then some inclusion of the 18Z GEFS/12Z ECMWF means to tone down the GFS/ECMWF/CMC specifics given the detail uncertainties farther out in time. Meanwhile near the West Coast, relative to typical day 6-7 spread there was decent agreement for the shortwave energy reaching the region near the end of the week. The aforementioned model/ensemble blend provided a good intermediate solution for this aspect of the forecast. Finally a southern stream upper low (possibly composed of some trough energy that may separate in the short range) may track near the western U.S.-Mexico border toward the end of the week. Scale/complexity of this feature lead to fairly low confidence in specifics. ...Weather/Threats Summary... Early in the period the primary effects from the northern tier system will be snow over the northern Rockies and then extending eastward into the Great Lakes. The associated band of meaningful snow should like near or just north of the Canadian border, while cold sector flow around the initial low and possibly a residual trailing low may serve to enhance snowfall over the Upper Great Lakes by mid-late week. Some of these areas may see brisk to strong winds as well. Meanwhile the flow of Gulf moisture will increase Wed-Thu ahead of the trailing cold front as it pushes into the Mississippi Valley and beyond. One or more frontal waves whose development could range anywhere from modest to significant may provide added enhancement. Expect areas of moderate to heavy rainfall in the warm sector while snow of varying intensity may be possible behind the front/embedded wave(s)--possibly yielding a white Christmas for some areas that currently have no snow cover. Depending on wave/front orientation, the system could tap Atlantic moisture near the East Coast. Consult the days 4-7 Winter Weather Outlook for a probabilistic representation of areas most likely to see significant snowfall. Areas in the Northeast will need to monitor this event closely given the potential for a period of moderate/heavy rainfall on top of currently deep snow cover. From around Fri/Christmas into the weekend expect moisture to return to the Northwest. Highest totals of rain and mountain snow will likely be over favored terrain of the Pacific Northwest and northern California with lighter activity extending into the northern Rockies. The lower-confidence southern stream upper low/trough tracking near the western U.S.-Mexico border late in the week may begin to generate some rain near the western Gulf Coast early in the weekend. Well above normal temperatures over the Plains on Tue (plus 10-25F anomalies) should modify a bit as system/front progression pushes the warmth into the East after Tue. Colder trend behind the front should briefly bring readings down to 5-15F below normal over parts of the western/central U.S. Wed-Thu and then into the East late week/early weekend. At the same time temperatures will rebound over the Plains and Southwest late in the period with the northern-central Plains likely to see highs at least 10F above normal. Rausch Additional 3-7 Day Hazard 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