Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 340 PM EST Mon Jan 21 2019 Valid 00Z Tue Jan 22 2019 - 00Z Fri Jan 25 2019 ...Central Rockies... Days 1 and 3... A positively tilted trough will shift southeast across the central and southern Rockies tonight with surface low pressure developing in the lee of the CO Rockies. Gulf sourced moisture wraps around this low tonight with heavy snow from the Black Hills/southeast WY/northern CO to the Uintas where WPC Day 1 probabilities indicate moderate to high probabilities for six inches. Another Arctic surface high descends the northern Great Plains Wednesday night through Thursday. Upslope flow and upper level divergence brings moderate WPC Day 3 probabilities for six inches to northern CO. ...Northern and Central Plains to the Upper Midwest... Days 1/2... Snow develops on the north side of low pressure shifting east from the CO Rockies tonight and spreads across the northern Great Plains through Tuesday and the Upper Midwest Tuesday night into Wednesday. Strong deformation aloft will support moderate snows north of the low track, with moderate to high WPC Day 1 probabilities for four inches across the northern half of IA. Day 2 probabilities for four inches are moderate from eastern WI across northern MI as the low continues across the LP of MI Tuesday night. Additionally, a northern stream shortwave crosses MN Tuesday with bands of snow over northern MN/the Arrowhead with moderate WPC Day 1 probabilities for six inches of snow. ...Northeast to the Mid-Atlantic...Ohio Valley and Central and Southern Appalachians... Day 2/3... Low pressure over the upper Great Lakes Tuesday morning is expected to track farther to the northeast into eastern Ontario and southern Quebec. Precipitation ahead of the low is forecast to begin as snow, resulting in some light accumulations before changing over to rain across many locations across Upstate New York and northern New England on Wednesday. A trailing cold front will push east from the Ohio valley into the Mid-Atlantic Wednesday night into Thursday morning. A well-defined surface wave lifting north along the front is expected to send moisture onto backside the front, supporting a change from rain to a wintry mix, resulting in some light snow and ice accumulations before precipitation over the Mid-Atlantic comes to an end Wednesday night into Thursday morning. Upslope snow over the west side of the Appalachians can be expected with moderate Day 3 WPC probabilities for 4 inches across the Allegheny Highlands of WV down to eastern KY. The absence of a surface low along the front over the Ohio Valley precludes any snow mention over an inch despite low Days 2/3 WPC probabilities for an inch across the lower OH Valley and down the MS Valley. Precip onset on the east side of the southern Appalachians may start as ice with low WPC Day 2 probabilities for a tenth inch of ice. ...Pacific Northwest to the Northern Rockies... Days 1-3... A strong upper ridge over the Pac NW breaks down Tuesday with warm advection precipitation ahead of shortwave trough moving across the top of the ridge will spread across the region with models the wave dropping into WA Wednesday. This will support some heavy mountain snows, especially across portions of the northern Cascades and northern Idaho ranges by Wednesday morning. Models show the system continuing to drop southeast Wednesday into early Thursday. This wave along with a frontal band dropping south through the northern Rockies will promote some light to moderate snows across the northern Rockies Wednesday into Thursday. Jackson