Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Sun Dec 23 2018 Valid 12Z Wed Dec 26 2018 - 12Z Sun Dec 30 2018 ...Significant Southern Rockies/Plains/Midwest/Great Lakes Winter Storm Wed-Fri... ...Overview... A powerful and impactful winter storm will lift from the southern Rockies up through the Plains to the Midwest Wed-Thu bringing heavy wind-driven cold sector snows over the n-central U.S. and locally heavy warm sector convective rains into the South. The system will move into the Great Lakes on Friday and then 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 snows) through the Great Basin/Southwest and s-central Rockies. ...Guidance Evaluation/Preferences... The WPC medium range product suite was primarily derived from a composite blend of the 00 UTC GFS/ECMWF and GEFS/NAEFS?ECMWF ensemble means days 4-7. Applied increasingly more weight to ensemble guidance over time amid growing embedded system timing/strength differences. Overall, leaned a bit more weighting toward the less progressive solution envelope given the tendancy in guidance over the last few days to offer a pattern with upstream amplification over the Pacific and Alaska regions. ...Weather Highlights/Hazards... A main weather threat focus will be on the major system in the Plains by Wednesday as it lifts northeastward. Precipitation will expand around and ahead of the deepening low and cold front with the flow out of the Gulf. To the northwest of the surface low, snow will come down heavy at times with increasing wind and possible blizzard conditions (blowing and drifting snow with limited visibility) Thursday. This will make 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 from NW Kansas/NE Colorado northeastward to Minnesota/Wisconsin 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 down into 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 down into to the Four Corners region and s-central Rockies. 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). 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. Schichtel 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