Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Dec 27 2020 Valid 12Z Wed Dec 30 2020 - 12Z Sun Jan 03 2021 ...Snow/ice threat across the upper Midwest to the Great Lakes Wednesday will be followed by a second storm bringing heavy rain threat across the South to the Northeast, with snow and wind threat possible across the Midwest later this week... ...Overview and Guidance/Predictability Assessment... For the past day or so, global models have been showing a rather decisive adjustment with the overall synoptic pattern evolution across the eastern two-thirds of the country during the medium range period. Instead of merging the northern and southern stream energies over the central Plains midweek, an abrupt trend of separating the two streams was established. The consequence of this flow separation now leads to the formation of a low pressure wave near the western Gulf Coast on New Year's Eve, which could develop into a deep cyclone as it tracks generally up the Mississippi Valley towards the Great Lakes and into the Northeast for the remainder of the week. Meanwhile, the low pressure system associated with the northern stream energy is now weaker and more progressive across the Great Lakes later on Wednesday. The GFS began to indicate this scenario yesterday before the 00Z ECMWF adjusted toward it. Models this morning continue to indicate a trend toward a deeper and slower-moving cyclone tracking up the Mississippi Valley from Thursday to Friday. The 12Z ECMWF did adjust toward this scenario as well. Despite this abrupt change in the forecast pattern, model agreement was actually rather decent this morning although additional adjustments of the model consensus is still possible. The WPC medium range product suite was derived from a composite of the 00Z ECMWF/EC mean, the 06Z GFS/GEFS, together with some 00Z CMC, and 00Z NAEFS for Days 6 and 7. This necessitated a rather significant change in WPC continuity by introducing a deeper and slower-moving cyclone up the Mississippi Valley later this week. ...Weather/Threats Highlights... A swath of snow and some ice can be expected to spread across the upper Midwest to the Great Lakes on Wednesday as a lead low will appear to move away into eastern Canada on New Year's Eve. Farther south, precipitation/convection will expand and intensify from the Plains eastward from midweek as the system intensifies and begins to draw deeper Gulf moisture and instability. As a second low develops near the western Gulf Coast on Thursday, the best potential for significant wrap-back snowfall, ice and high winds will once again develop from the central Plains to the Midwest/Great Lakes. Widespread areas along and well to the southeast of this axis will see heavy rainfall/strong to severe convection and some runoff issues in a favorable right entrance region of a potent upper jet from the south-central Plains to the mid-lower MS/OH Valleys, the South, then up the Appalachians/East Coast to finally close out 2020 and thankfully begin 2021. Wintry precip is possible from the mid-upper OH Valley/Appalachians to interior Northeast. Meanwhile, areas from the Pacific Northwest to northern CA will continue to see a return of rain/mountain snow this week in periodically enhanced fetch with approach of an uncertain but protracted series of Pacific systems. Highest totals will focus over favored coastal terrain and into the Cascades and Sierra. Additional activity will work over the Northwest/northern Rockies. There is an emerging signal to increase moisture feed into next weekend. Kong/Schichtel 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