Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 340 PM EST Sat Jan 04 2020 Valid 00Z Sun Jan 05 2020 - 00Z Wed Jan 08 2020 ...Pacific Northwest through the Northern Rockies... Days 1-3... A pair of shortwaves embedded in fast Pacific flow and jet energy will bring periods of heavy snow to the terrain of the Northwest and northern Rockies through Tuesday. This is apart of an ongoing very active pattern in the northwest CONUS. The next trough arrives into the Pac NW late tonight with a decent plume of Pacific moisture. The following trough axis reaches the PAC NW Tuesday with more moisture and forcing in advance than the previous. Snow levels remain low (1000 to 2000ft) through Sunday night before rising to 4000 to 5000ft Monday/Monday night in the next Pacific moisture plume. Nearly continuous snow with periods of heavy snow is expected through the three day period for the WA Cascades and Olympics with a bit of a Day 1 break for the Northern Rockies before at least two days of heavy snow for the higher elevations. WPC probabilities are moderate to high for 18 or more inches for the WA Cascades/Olympics with 3 day totals likely to exceed 5 ft in the highest terrain. Day 2 probabilities are moderate for 12 or more inches over the highest ID/MT Rockies and moderate for 18 more more inches for the same area on Day 3. ...Northeast...Central Appalachians...and Great Lakes... Days 1-3... Developing low pressure will track northeast from the northern Mid-Atlantic coast and off the New England coast tonight. Snow on the north/back side of this system will shift east across northern New England tonight with Day 1 WPC probabilities for four inches or more in the 20 to 30 percent range for eastern coastal Maine. Meanwhile, strong northwesterly winds on the back side of the low with Great Lakes moisture will support ongoing snows along the central and southern Appalachians with several inches possible tonight, especially along the Allegheny Mountains in West Virginia, where Day 1 WPC probabilities are moderately high for 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 a shortwave currently over the Canadian Prairies shifts across the Great Lakes Sunday and track along the St. Lawrence Valley through Monday night. This trough is accompanied by modest left exit jet streak diffluence. A period of strong westerly flow across Lake Ontario late Sunday into Monday is expected to support lake enhancement, and WPC probabilities for 4 inches are moderately high only for the Tug Hill Plateau. The next wave in a long series of waves from the Pacific Northwest reaches the northeast CONUS by late Tuesday. There are indications of low pressure both near the Great Lakes and along the northeastern Seaboard. Cold enough preconditions from the previous wave could allow warm advection snow Tuesday in the Northeast. The probability of significant icing (0.25 inch or greater) is less than 10 percent. Jackson