Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 343 AM EST Tue Jan 12 2021 Valid 12Z Tue Jan 12 2021 - 12Z Fri Jan 15 2021 ...Pacific Northwest and Northern Rockies... Days 1/2... The strong Atmospheric River will persist into Wednesday before shifting farther inland. The strongest integrated water vapor transport will continue through today with a focus on the OR coastal range/Cascades before beginning a slow ease tonight. This will produce exceptional precipitation near the coast and remain fairly heavy for the northern Rockies of ID/MT. However, snow levels will be 7000 to 8000ft into this evening before crashing to around 3000ft as a shortwave trough passes overnight Day 1 snow probabilities are high for 8 or more inches in the WA Cascades, the Blue Mtns of OR over to the Sawtooth, Salmon River Mtns of ID as well as northern Rockies ranges of northern ID/MT. This expands east to the Tetons of WY for Day 1.5. Precipitation rates really drop off later Wednesday in the wake of the shortwave trough passage. Snowfall at pass levels in the WA Cascades look to be an additional 4-10". ...Eastern Great Lakes... Day 1... An upper level shortwave trough currently over Lake Superior will cross southern Ontario this afternoon and will add some instability to westerly flow over Lake Ontario and southwesterly flow over Lake Erie. Single LES banding is expected with briefly heavy snow rates possible, especially in the Tug Hill Plateau east of Lake Ontario. However, at least modest directional wind shear will allow some transience of the band and a lack of moisture/lift in the rather elevated DGZ should preclude significant snow amounts. Day 1 snow probabilities are 20 to 30 percent for 4 or more inches in the Tug Hill Plateau. ...Upper Midwest... Days 2/3... The low trailing the Atmospheric River flowing in the Pacific Northwest will the WA/BC coast tonight, cross the Canadian Rockies and push onto the Canadian Prairie on Wednesday then dive southeast into MN Thursday morning. From there this low is expected to slow/stall with uncertainty how far east it will make it before doing so. Model preference for this system is with the less progressive 00Z ECMWF/UKMET solutions which get the low to around the MN/WI border Thursday night. A deep low in this area would feature rain/mixed precipitation in the warm sector to the east, lake enhanced precipitation (likely all snow) in northern MN off Lake Superior, and a broad comma head/wrap around of snow. However, even subtle changes in low position make a large difference in the wintry precipitation accumulations. Day 3 snow probabilities are moderately high for 4 or more inches across northern MN/WI with low probabilities for 8 or more inches in portions of these areas. As of now Days 2/3 ice probabilities are moderate to high for at least some ice accretion in eastern MN/western WI and northeast IA, but zero percent for a tenth inch or more. As this system has the potential to persist over the Upper Midwest into the weekend, there remains a good deal of uncertainty at this time. For Days 1-3, the probability of significant icing (0.25-inch or greater) is less than 10 percent. Jackson