Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 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 exit the Plains Wednesday and push through the Upper Midwest Thursday bringing heavy wind-driven snow and locally heavy warm sector convective rains through the South. The system will move off the Northeast/Mid-Atlantic coast Saturday as the front settles into the Gulf of Mexico. Another storm will dig through the West/Southwest in the wake of the first storm to spread precipitation (mostly snow) through the Great Basin/Southwest and central/southern Rockies. ...Guidance Evaluation/Preferences... A deterministic blend of the 12Z FV3-GFS and ECMWF/UKMET/Canadian served as a good starting point for the major system through the northern tier Thu-Fri as the GFS runs have been a bit too quick but the ECMWF runs a bit too slow. For Saturday through Monday, the flow from the Pacific into/through the West becomes increasingly uncertain with divergence in the ensembles in the evolution of upper troughing. The GFS/GEFS have been more bullish on a closed/cutoff upper low over NW Mexico this weekend while the ECMWF runs have generally favored a weaker positively-tilted trough farther east. Ensembles have not shown much, if any, contraction either over the CONUS or upstream so did not lean on any one/group of deterministic models. A blend between the GEFS mean and ECMWF ensemble mean sufficed for now which was quicker/weaker/farther east than the GFS and slower/stronger/farther west than the ECMWF. Teleconnections still support such an idea that would allow some ridging into California Sunday. In the east, northern stream would take the trough quickly out of the central Rockies between an upper low over Hudson Bay and subtropical ridging over the Bahamas, leaving a wavy front across the Gulf and Southeast (with possible waves along it). ...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 eastern SD/northeastern KS to Minnesota/Wisconsin/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 into 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 in the Southeast could support a surface wave riding along it that may spread some rain/snow through the Southeast/Mid-Atlantic early next week. Temperatures will be cooler than average in the West behind the front but will be well above average over the Mississippi Valley eastward (10-30 deg F anomalies especially overnight) 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. 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