Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Tue Jan 15 2019 Valid 12Z Fri Jan 18 2019 - 12Z Tue Jan 22 2019 ...Another major central to eastern U.S. snowstorm is forecast, with Arctic cold air involved... ...Overview... A trough moving into the Western U.S. on day 3 (Fri) will aid in surface cyclogenesis in the lee of the Rockies. This surface low will track across the central and eastern U.S., creating a variety of hazardous weather. Another trough is forecast to approach and move through the West by day 6-7. ...Guidance Evaluation/Preferences... Model guidance is in generally good agreement in the large scale, and has been for the last day or two. Thus was able to maintain continuity with previous WPC forecasts by using a blend of the operational EC and 00 and 06Z GFS, as well as the EC and GEFS ensemble means. The model differences that exist include the degree of northern and southern stream flow separation in both the first trough on day 4 and the trough on day 6-7, which will affect the surface lows. The EC has shown a closed upper low in the Southern Plains on day 4 for a couple of runs, along with the UKMET. The operational GFS has trended that direction too so elected to include a closed low at 500 mb for day 4 (Sat). For the second trough, models have not been trending any particular direction, so will have to keep watching to see if a trend emerges. ...Weather Highlights/Hazards... Amplified southern stream mid-upper level trough energy/height falls digs through the Southwest Friday with pooled moisture and precipitation working through the Intermountain West/Great Basin/Rockies with windy conditions and valley rain/mountain snow. Snow levels will fall as heights lower which will turn some lower elevations over to snow as precipitation diminishes Saturday. Pacific storm systems reload farther to the north later for the holiday weekend to then focus an associated heavy precipitation threat into/across the Northwest. Downstream surges of frigid arctic air into the central U.S. Friday through the weekend occur as the northern stream amplifies around a Hudson Bay upper vortex. A lead system will spread some wintery weather over the northeast Friday. Upstream, a more robust system ejecting from the West organizes over the south-central Plains with increased lead Gulf of Mexico moisture and lift up through the east-central U.S. Saturday as an arctic front to the north pushes through the Northeast. As precipitation overruns the arctic airmass to the north, heavy snow will spread out of the Central Plains through the Midwest/Ohio Valley then Mid-Atlantic/Northeast through the holiday weekend along with some threat of blizzard conditions in the tight pressure gradient around a main low that lifts through the eastern U.S. and offshore. Meanwhile, heavy warm-sector rains/convection move through the South/Southeast and a transition zone between snow and rain will include significant sleet and freezing rain. Please consult WPC Day 4-7 QPF and Winter Weather Outlook for a probabilistic assessment of the significant winter weather threat. Tate/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