Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 327 AM EST Wed Jan 23 2019 Valid 12Z Wed Jan 23 2019 - 12Z Sat Jan 26 2019 ***Great Lakes Region to Northern New England*** The rapidly developing surface low across the Midwest will continue tracking northeastward across Michigan on Wednesday before becoming occluded Wednesday night over southern Ontario, which is being sustained by a potent shortwave trough and right entrance upper level jet dynamics. This will lead to greater divergence aloft, and low to mid-level frontogenesis will further aid with ascent and result in heavier mesoscale banding within the developing deformation zone northwest of the surface low. This is most likely to manifest itself from central Wisconsin to the northern lower peninsula of Michigan. With the passage of an arctic front on Thursday, intense cold air advection over the lakes will bring a return of lake-effect snow and higher snow-to-liquid ratios for the end of the week. There will likely be some snow as well for northern New England Wednesday evening prior to changing over to mixed precipitation and even rain before ending owing to strong warm air advection ahead of the low. The corridor of heaviest snowfall with this event should remain north of the border across southern Quebec given the northward extent of the surface low track. ***Central Appalachians to Upstate New York*** Strong warm air advection is expected across the entire East Coast region on Wednesday ahead of a cold front approaching from the Ohio Valley. The lower troposphere should be warm enough at the start of the event to support mainly rain south of the Adirondacks and lakes Erie and Ontario. One of the forecast challenges will be determining the amount of freezing rain/drizzle that falls on the leading edge of the precipitation. The recent hard freeze across much of the northern Mid-Atlantic region and Northeast U.S. has caused ground temperatures to be well below freezing, including road surfaces. Although temperatures are expected to rise above freezing Wednesday afternoon, the ground may remain cold enough to result in more freezing rain and icy conditions that what the models are currently indicating. This especially holds true for interior valleys from West Virginia to Upstate New York, and could lead to icy roads and hazardous travel conditions. By later Wednesday night, temperatures should warm up enough for just a cold rain for most locations. Colder air rushing in behind the front across the central and southern Appalachians will likely result in a brief changeover from rain to snow early Thursday, even though the heaviest QPF will have exited the region by that time. This would mainly affect areas from eastern Tennessee to western Pennsylvania. ***Northern and Central Rockies*** A surface low tracking inland across the Pacific Northwest Wednesday morning, in conjunction with a mid-level shortwave trough, will dive southeastward across the northern Rockies and then Colorado Wednesday night and into early Thursday. Most of the heavy snow should be winding down across the Cascades by 18Z Wednesday once the trough axis passes by, with most of the snow falling above 3000-4000 feet elevation. Snow intensity and coverage will be increasing across the mountains of western Montana, north-central Idaho, and western Wyoming by the afternoon with an upper level jet max overhead and appreciable upward vertical velocities within the dendritic growth zone. A cold front sinking southward from the Canadian prairies should also provide additional forcing for ascent Wednesday night across Montana, with some enhanced snowfall across northeast facing terrain. D. Hamrick