Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 338 AM EST Mon Dec 06 2021 Valid 12Z Mon Dec 06 2021 - 12Z Thu Dec 09 2021 ...Great Lakes... Days 1/2... Deep low pressure over the UP will turn northeast and quickly track across Ontario today. Strong cold air advection in its wake (with winds increasing from the N/NW over Lake Superior and Lake Michigan), intense ascent over the open lakes will result in lake enhanced comma head snow this morning off Lake Superior, transition to strong bands of LES later today. The NW flow of today backs to W tonight and Tuesday. Day 1 WPC probabilities for 6 or more inches are limited to the typical snow belts of the UP, especially from the Keweenaw Peninsula to the Porcupine mountains and the eastern UP Lake Superior shoreline with moderate probabilities for 4 or more inches for the northwest LP, for portions of OH/PA/NY south of Buffalo off Lake Erie and the Tug Hill/western Adirondacks. The westerly flow on Day 2 limits WPC probabilities for four or more inches to the Keweenaw, near Whitefish Pt, and the Tug Hill. ...Pacific Northwest to the Colorado Rockies... Days 1-3... A shortwave trough crosses WA today and reaches the WY Rockies by late tonight. However, now that the western portion of the trough is expected to shear off and move south off the West Coast, the impacts and snow magnitude over the Pac NW and Rockies has lessened vs previous days. Snow levels will be around 1000ft this morning around Seattle and rise from there as high moisture Pacific air spreads in. Mountain snows are expected in the WA Cascades, Bitterroots, and portions of the Absarokas and Tetons and down to the CO/WY border on the Medicine Bow range where Day 1 WPC probabilities are now just moderate for 4 or more inches. Moderate accumulations are likely at Snoqualmie and Stevens Pass today and tonight there is also some moderate probabilities for 4 or more inches in the central Sierra Nevada from the western branch of the trough. The next shortwave trough and accompanying Pacific jet streak shifts into the Pacific Northwest Tuesday night with a reinforcing impulse reaching northern CA by late Wednesday night. The phasing of these features over the Intermountain West makes for a notable system crossing the CO Rockies Thursday into Friday. on Day 3, WPC probabilities are high for 6 or more inches in the WA Cascades and moderate in the OR Cascades, Olympics, Bitterroots, Ranges near Glacier NP, the Tetons and on the northern Sierra Nevada as Pacific moisture spreads inland under the broad trough. ...Mid-Atlantic through Northeast... Day 3... A reinforcing upper trough rounding a low centered over James Bay swings east across the eastern CONUS Wednesday. Surface low development is expected off Cape Hatteras Wednesday with potentially rapid intensification Wednesday night as it tracks northeast. However, uncertainty with the low track and magnitude exist. A stronger and closer to the coast low would bring a heavy snow threat to the Northeastern Seaboard. The 00Z ECMWF is the slowest solution and thus has the least snow for the Mid-Atlantic. Will need to monitor this low track. For Days 1-3, the probability of ice accretion over 0.1" is less than 10 percent. Jackson