Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1054 AM EST Thu Jan 17 2019 Valid 12Z Sun Jan 20 2019 - 12Z Thu Jan 24 2019 ...Major Winter Storm shifts from Great Lakes/Ohio Valley/Appalachians to Mid-Atlantic/Northeast Sunday into Monday... ...Guidance Evaluation/Preferences... The WPC forecast was initially based on a multi-model deterministic blend (including the ECMWF/GFS/CMC) during days 3-5 (Sun-Tue). These solutions were relatively well clustered throughout and the blend maintains reasonably good forecast continuity. Uncertainty increases significantly by days 6-7 (Wed-Thu) as Pacific energy undergoes significant amplification across the central U.S., with model solutions waffling as to the degree of amplitude of the trough as well as potential separation of northern/southern stream flow, with major implications on the development/track/intensity of any potential surface low pressure system across the central/eastern U.S. Given the degree of model variability, the forecast was trended quickly to majority ensemble mean (ECENS/NAEFS) weighting during days 6-7, with exclusive use of ensemble means by day 7. ...Weather Highlights/Hazards... Deep Gulf of Mexico and Atlantic moisture will lift over the eastern U.S. Sunday. Heavy precipitation focuses around a main low/frontal system, overrunning arctic air to the north. Heavy snows shift from the Great Lakes/Ohio Valley and Appalachians through the northern Mid-Atlantic/Northeast, exiting Monday. Some locations could see blizzard conditions in the tight pressure gradient between the surface low and the strong arctic high. Meanwhile, lingering heavy warm-sector rains/strong convection moves through the Southeast/Mid-Atlantic Sunday, with a transition zone between snow and rain to include significant sleet and freezing rain. Meanwhile, Pacific storm systems will reload with new focus as a heavy precipitation threat into the Pacific Northwest this weekend. Wintry weather spreads/moderates inland across the north-central Great Basin/Rockies later in the holiday weekend and across the full U.S. northern tier next week. As mentioned above, models have waffled on the degree of mid-upper level trough amplitude/separation over the south-central U.S., and there is a large degree of uncertainty with the specifics of this flow and associated surface system response. However, most guidance supports increasing Gulf then Atlantic moisture inflow and precipitation pattern next Tue-Thu that will include the threat for a widespread swath of snow/ice on the northern periphery of the large precipitation shield. Upstream, sharp shortwave digging next Wed/Thu over the West to the lee of a building/amplified eastern Pacific upper ridge may provide cooling and risk of a wintry weather swath from the Northwest to the north-central Intermountain West/Rockies/Plains. Ryan/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