Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 344 PM EST Fri Jan 03 2020 Valid 00Z Sat Jan 04 2020 - 00Z Tue Jan 07 2020 Days 1-3... ...Pacific Northwest to the Northern Rockies... A well-defined shortwave trough is expected to momentarily suppress an upper ridge in the West, with strong onshore flow and upper level forcing producing a period of heavy precipitation, including mountain snows across the Northwest Friday night into Saturday. As snow levels drop to below 3000ft, WPC Day 1 guidance (ending 00Z Sun) continues to show high probabilities for accumulations of a foot or more across portions of the Olympics and Washington Cascades. Accumulation rates are expected to diminish as this system moves quickly east of the Rockies by late Saturday. However, onshore flow will continue to support snow showers in western Washington and Oregon before the next system arrives on Sunday. Models show another shortwave trough and associated cold front moving into the Northwest on Sunday, bringing additional locally heavy snows to portions of the Olympics and northern Cascades. Unsettled weather is expected to continue through the end of the period with the passage of a warm front on Tuesday. Along with additional heavy snows for portions of the Olympics, locally heavy accumulations are possible for the northern Idaho ranges as well. For the 72-hour period ending 00Z Tuesday, WPC guidance shows high probabilities for total accumulations exceeding 24-inches across portions of the Olympics and northern Cascades. ...Northeast and Central Appalachians... Southern stream energy will continue to lift northeast from the southern Plains and Gulf Coast region ahead of northern stream trough digging into the Ohio valley early Saturday. Overall, the models have now come into agreement on a less-phased system, with a relatively weak surface wave moving quickly from the Ohio valley to the southern New England coast on Saturday. Higher probabilities for snowfall accumulations of 4-inches or more remain mostly over the Adirondacks and the northern Green and White mountains, while warmer air is expected to limit accumulations farther to the south. Meanwhile, strong northwesterly winds and the passage of the northern stream shortwave will support snows along the central and southern Appalachians and Cumberland Plateau -- with several inches possible Saturday night, especially along the Allegheny Mountains in West Virginia, where WPC guidance continues to show higher probabilities for accumulations of 4-inches or more. Dry conditions across much of the Northeast during the day on Sunday will be followed by the return of wintry weather as the leading shortwave impacting the West over the weekend moves across the Northeast Sunday night and Monday. A period of strong westerly flow across Lake Ontario late Sunday into Monday is expected to support lake enhancement, raising the threat for heavy accumulations across the Tug Hill region. The probability of significant icing (0.25 inch or greater) is less than 10 percent. Pereira