Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 345 AM EST Thu Dec 12 2019 Valid 12Z Thu Dec 12 2019 - 12Z Sun Dec 15 2019 ...Eastern Appalachians/Piedmont... Days 1,2... The combination of a cold air damming surface ridge from a high departing the Mid-Atlantic today and increasing southerly flow ahead of an amplifying mid-upper level trough over the Mid-South states will support precipitation spreading north up the Appalachians tonight through Friday night. Subfreezing surface temperatures in the cold air wedge will support freezing rain across much of the southern and central Appalachians and western Piedmont of NC/VA/MD/into PA tonight into Friday. Cold air damming is often slow to erode, slower than guidance suggests, so in this case a surface low passage Friday night from south to north may mark the end of freezing precipitation. QPF looks to be generally half to one inch ahead of the surface low, so pockets of thick icing are possible given the cold preconditions, but as of now the highest WPC probabilities for a quarter inch are Day 1.5 west of the Blue Ridge Mtns in western NC and SW VA. Cold air damming also tends to linger in the northern Potomac Highlands which has elevated sheltered valleys, so despite model guidance not suggesting the risk for a quarter inch of ice, freezing QPF lingering through Friday night ahead of the surface low would raise a risk for thicker ice there. ...Northern Great Plains and Upper Great Lakes... Days 1-2... Low-to-mid level frontogenesis with a shortwave trough, along with right-entrance region upper jet forcing supports a period of enhanced snowfall over the UP of MI where there are low to moderate Day 1 WPC probs for 8 or more inches. A second, weaker shortwave trough crosses ND tonight and MN/WI Friday. This system is expected to track farther north than the first, with less potential for heavier amounts. WPC probs for Day 2 are moderate for 4 or more inches over the Arrowhead of MN. Sprawling low pressure lifts up the interior Northeast Saturday with lake effect snow, particularly off Lake Superior on northwesterly flow expected Saturday night. Western U.S.... Days 1-3... ...Pacific Northwest to the Northern Rockies... Fast Pacific flow continues to overspread the western US through tonight before drifting south through Saturday night ahead of the next trough shifting in from the Gulf of Alaska. Embedded shortwave energy and deep moisture will continue to support periods of particularly heavy mountain snow from the Cascades and to the central and northern Rockies into tonight. Day 1 WPC probs are high for 8 or more inches across the higher elevations of WA/OR/ID/MT/WY/and northern UT/CO. The breadth of incoming moisture narrows a bit as it shifts south, making for lower Day 2 prob coverage for 8 or more inches, generally limited to southern OR/northern CA and east to central CO. The same could be said for Day 3 with a focus from the northern Sierra Nevada, the Wasatch, and most of CO having moderate to high probabilities for 8 or more inches. Jackson