Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Mon Dec 24 2018 Valid 12Z Thu Dec 27 2018 - 12Z Mon Dec 31 2018 ...Significant winter storm to move into the Upper Midwest Thursday and Great Lakes on Friday... ...Overview... A powerful and impactful winter storm will track out of the central Plains after early Thursday and proceed through the Great Lakes by Friday, bringing heavy wind-driven snow over some areas in the cold sector and locally heavy convective rains across the South. The trailing front will move off the Northeast/Mid-Atlantic coast Saturday but likely linger over the Southeast U.S. and northern Gulf of Mexico. Another system digging through the West/Southwest in the wake of the Midwest storm will spread precipitation (mostly snow) through the Great Basin/Southwest and central/southern Rockies. Late in the period guidance begins to differ significantly regarding how upstream northern Pacific energy may amplify into the West. ...Guidance Evaluation/Preferences... Early in the period the primary forecast issue is with the storm tracking northeastward from the central Plains. The operational GFS/GEFS runs are somewhat weaker with the dynamics aloft leading to a somewhat farther south surface low track as of early day 3 Thu and then stray faster than consensus by early day 4 Fri. The 00Z FV3 GFS is more similar to the majority solution. A small component of GFS input would not be unreasonable though, in light of earlier guidance that had skewed a little farther south with the system early Thu and some uncertainty over northern stream influence by Fri. By mid-late period guidance becomes increasingly divergent, with respect to Pacific trough energy that reaches 150-160W longitude by Fri and likely separates to some degree before heading toward North America--as well as influence of this evolution on the leading Four Corners/Southwest/northwestern Mexico upper trough. The 00Z GFS and many of its GEFS members offer one extreme with a more phased Pacific trough and much more persistent western Canada/northwestern U.S. ridge, in contrast to the ECMWF/CMC and their ensembles which are quite aggressive with western U.S. trough amplification by days 6-7 Sun-Mon. The combination of EC/CMC guidance tending to provide a better lead on the eastern Pacific/western U.S. pattern recently along with the 06Z GFS trend in their direction would suggest favoring the ECMWF/CMC and their ensembles. However teleconnections relative to D+8 negative height anomalies near 180 longitude and positive anomalies near the British Isles present the risk that the eastern Pacific/North America pattern could end up being shifted somewhat eastward from what the majority cluster is showing. Thus would ultimately favor trending the forecast in the ECMWF/CMC direction but with some 06Z GFS/00Z NAEFS input, both of which provide somewhat more compatibility for blending than either 00Z or 06Z GEFS mean runs. Needless to say confidence is very low for specifics of southwestern U.S. energy that may eject late in the period, with ECMWF/CMC runs differing greatly among each other and run to run in spite of being more similar/consistent with upstream evolution. ...Weather Highlights/Hazards... Precipitation in association with the major Plains/Midwest system will push eastward Thursday with snow (heavy at times with increasing wind and possible blizzard conditions) making travel difficult if not impossible. Several inches of snow are likely over a large area with over a foot quite possible for some areas, especially through parts of the north-central Plains, Minnesota, and northern Wisconsin/western U.P. of Michigan as the surface low tracks toward Lake Michigan. Please consult the latest WPC winter weather outlook probabilities for a daily graphical depiction of the best chance of snow. In the warm sector, rainfall could become locally heavy in the South as the system infuses Gulf moisture northward up and over the warm front. Several inches of rain are possible in some areas with a more widespread 1-2+" likely. The main low should exit through southern Ontario and northern New England where some onset frozen precipitation (perhaps appreciable freezing rain as warm air advances over in situ lower-level cold air) is likely. Next system reaching the West on the heels of the lead storm will bring more dynamically driven snow and some lower elevation rain from the Northwest and Great basin into the Four Corners region. Another cold front will enter the Pacific Northwest with limited moisture east of the Cascades. Stationary front lingering over the Southeast will likely provide a focus for further rainfall over the southeastern quadrant of the Lower 48 and possibly wintry weather on the extreme northern fringe of the moisture shield. Wave details along this front become increasingly uncertain toward late weekend/early next week. Temperatures will be cooler than average in the West and Plains behind the departing Plains/Midwest storm, with coldest anomalies (10-20F below average) likely focused over the High Plains and vicinity). On the other hand the storm will pull much above average warmth (plus 10-30F anomalies) into areas from the Mississippi Valley eastward until the cold front moves through. With reinforcing troughing into the West later in the week, temperatures will stay below average from the Great Basin/Rockies into the Plains, especially over newly snow-covered areas. Cooler air will filter into the East next weekend as the front sinks into the Gulf and northern Florida. Central/southern Florida will stay under the influence of subtropical ridging and above average temperatures. Rausch/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