Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 424 AM EST Sat Jan 19 2019 Valid 12Z Sat Jan 19 2019 - 12Z Tue Jan 22 2019 ...Ozark region into the lower Ohio valley... While previous model runs showed a good deal of spread and little run-to-run consistency across this region, the overnight runs have moved into better agreement, with the general consensus showing heavier precipitation developing, resulting in higher snow accumulation totals across the region. Several of the overnight models now show moderate precipitation with rain changing to snow within the deformation zone associated with the wave lifting into the lower Ohio valley, resulting in a stripe of significant snow accumulations across the region on Saturday. Latest WPC probabilities show a Slight Risk (greater than 10 percent chance) for snow accumulations of 4-inches or more from northwest Arkansas into southeast Missouri, with a Moderate Risk (greater than 40 percent) farther to east along the Ohio river into southern Illinois, Indiana and northern Kentucky. ...Upper Ohio valley to the Northeast... There remains a strong signal for heavy snow accumulations from the upper Ohio valley northeastward into interior New England as the previously noted wave continues to amplify and track to the northeast late Saturday into Sunday. Strong upper divergence supported by a coupled upper jet, along with low to mid level frontogenesis is expected to produce a swath of heavy snow accumulations from northeast Ohio to central Maine. Highest totals are expected to center from northern Pennsylvania to central Maine, where WPC probabilities show a 70 percent or greater probability for snow accumulations of a foot or more by Monday morning. It is expected that accumulations of 1-2 feet will be common within this region. Further to the south, snow changing to a wintry mix may result in some significant ice accumulations from southern Indiana and Ohio across the Appalachians into southern New England. The general trend of the overnight models was toward a warmer solution, shifting heavier snows and the icy transition zone a little further to the north than their previous runs. Latest consensus indicates that the major cities in the Northeast along the I-95 corridor will see generally light snow/ice accumulations as frozen precipitation at onset changes over to rain in the strong warm advection pattern ahead of the wave. Synoptic scale snows are expected to diminish as the low lifts into Atlantic Canada late Sunday. However strong cold air advection in the wake of low is forecast to support lake effect snow showers, contributing to additional locally heavy totals in the lee of the lakes into early Monday. Significant additional totals are also possible further to the south along the upslope regions of the central and southern Appalachians. ...Western U.S. into the northern Plains and upper Mississippi valley... A frontal band pushing into the northern Rockies is expected to support some locally heavy mountain snows across the northeast Oregon and central Idaho ranges Saturday into early Sunday. Then on Sunday into early Monday, the heavy snowfall threat will become more widespread across the West as a well-defined shortwave trough moves across California into the Great Basin. Locally heavy mountain snows can be expected from the Cascades and Sierra to the northern Rockies, with WPC probabilities indicating a Moderate Risk (greater than 40 percent) for accumulations of 12-inches or more along the northern Sierra. As the trough continues to move east, probabilities for locally heavy amounts increase across the central Rockies, particularly along the Wasatch and the western Colorado and southern Wyoming ranges Monday into early Tuesday. Meanwhile, expect lighter amounts to spread ahead of the northern stream component of the upper trough and along a low level front moving east across the northern Plains and upper Mississippi valley late Monday into early Tuesday. Pereira