Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 333 PM EST Tue Dec 10 2019 Valid 00Z Wed Dec 11 2019 - 00Z Sat Dec 14 2019 Day 1... ...Southern Appalachians to New England... A strong surface cold front will continue to move through and off of the East Coast Tuesday evening and overnight. Favorable upper jet forcing and low-to-mid level frontogenesis will generate precipitation well behind the front, while deepening cold air supports rain changing to snow. Overall, the latest guidance is in good agreement -- showing a transition to snow around 06Z that continues into the morning commute from the DC to Boston metros. While an inch or two is expected across most areas, locally heavier totals are expected for portions of the central Appalachians as well as parts of southern New England. Increasing divergence along the right-entrance region of a strengthening upper jet, along with low-to-mid level frontogenesis may support some higher totals from eastern Connecticut to southeastern Massachusetts Wednesday morning. ...Great Lakes... Colder air along with westerly winds in the wake of the previously noted cold front will support lake effect snow showers in the lee of the Lower Great Lakes Tuesday evening. Backing flow ahead of the an approaching shortwave is expected to direct these showers farther to the north overnight before realigning farther south once again on Wednesday. Meanwhile westerly flow and a reinforcing shot of colder air associated with the shortwave moving through the Great Lakes is expected to promote lake effect snow showers and potentially locally heavy amounts across portions of northern Michigan -- particularly the Keweenaw Peninsula and eastern sections of the Upper Peninsula. Day 2... ...Northern Plains to the Upper Great Lakes... Models show a fast-moving shortwave dropping southeast from western Canada into the northern Plains Wednesday evening. This system is expected to produce mainly light snow accumulations as it moves east from the Plains into the upper Mississippi valley Wednesday night. However, low-to-mid level frontogenesis may support banding and a period of heavier snow as it moves across the upper Great Lakes -- resulting in heavier snow accumulations across northern Wisconsin into Upper Michigan on Thursday. Days 2-3... ...Pacific Northwest to the Northern Rockies... Shortwave ridging will give way late Wednesday to fast Pacific flow with several embedded shortwaves and deeper moisture producing periods of organized heavy precipitation beginning Wednesday night and continuing into Friday. Two-day totals of a foot or more are likely across portions of the Olympics and Cascades to the northern Colorado Rockies. Day 3... ...Southern to the Central Appalachians... Precipitation is expected to spread north across the region Thursday night ahead of an amplifying trough over the lower Mississippi valley. Models show a cold air wedge remaining in place long enough to support accumulating ice across the region before a transition to rain on Friday. Pereira