Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 434 PM EST Sun Nov 17 2019 Valid 00Z Mon Nov 18 2019 - 00Z Thu Nov 21 2019 Days 1-3... ...Northeast... Surface low pressure positioned off of the Southeast coast Sunday evening will track to the north, approaching southeastern New England by late Monday as it interacts with an upstream upper trough moving across the Ohio valley. As cold air gives way to warmer air aloft, there remains a good signal for a light wintry mix to develop by late Monday across portions of eastern Pennsylvania, northern New Jersey, Upstate New York and New England. Across portions of Upstate New York and New England, expect low-to-mid-level frontogenesis along with upper level forcing to support an increase in precipitation rates Monday night into early Tuesday. Some guidance members now suggest that strong vertical motion will overcome the low level cold air - resulting in higher snow totals and less freezing rain across eastern Upstate New York into central New England. By early Tuesday as the upper trough tracking from the Ohio valley moves across the Northeast, expect most areas across northern New York and interior northern New England to transition to snow before precipitation ends during the afternoon. WPC probabilities still show strong potential for notable ice accumulations - with high probabilities for amounts of 0.10-0.25 inch along the Green Mountains and across northern New Hampshire into northern Maine. Across far northern Maine, where the precipitation type is most likely to remain all snow, probabilities remain high for accumulations of 4-inches or greater. ...Northern Rockies/High Plains... Heavy snow is expected to become likely across portions of western Montana as a upper trough moves east into the region on Tuesday. Strong upper forcing, coinciding with low-to-mid level frontogenesis and upslope flow enhanced by a strengthening wave over the High Plains is expected to support heavy snow developing over the mountains of western Montana. Heaviest amounts are expected to center along the Lewis Range, where WPC probabilities for a foot or more remain high. This system in expected to track fairly quickly to the east, producing lighter accumulations across central into northeastern Montana and North Dakota Tuesday night into Wednesday. ...California/Great Basin/Central Rockies... An upper low currently centered west of Baja California is expected to weaken and eject northeast ahead of an upper low closing off over California on Wednesday. As the ejecting wave moves across the Southwest, snow levels will be high, so widespread heavy snow accumulations are not expected. However, some of the higher elevations across southwestern Utah into the southern Wasatch and the San Juans in Colorado may see some locally heavier amounts. Meanwhile, as the upstream low continues to drop south, lowering snow levels may help support some locally heavy accumulations along the southern Sierra and along the southern coastal ranges of California on Wednesday. Pereira