Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 333 AM EST Thu Dec 30 2021 Valid 12Z Thu Dec 30 2021 - 12Z Sun Jan 02 2022 ...Western U.S... Days 1-3... Shortwave troughing will slide down the West Coast through today eventually carving out a longwave trough over the Rockies by late Day 2 into Day 3. As this occurs, height falls and embedded shortwaves moving through the flow will drive large scale forcing for ascent. This is expected to produce widespread precipitation initially across the Pacific Northwest during Day 1 that then spreads eastward and southeast toward the central and southern Rockies. By Day 3, upper level ridging in the wake of the passing trough will bring a reprieve to the active winter weather for the west. For D1, the heaviest snow is expected across two distinct regions. The first will be across the Pacific Northwest including the WA Cascades and Blue Mountains of OR where WPC probabilities for 8 inches are high. This is in response to a cold shortwave dropping into the region and pushing a surface low southeast and onshore. The other area is eastward across portions of the Great Basin, Wasatch, southern UT ranges, and the Colorado Rockies. Local amounts greater than 2 feet will be possible. By D2, the axis of the upper trough reaches the southern to central Rockies. Here the anomalously deep heights and cold air work with the available moisture for widespread mountain snow along the Mogollon Rim, the Wasatch, San Juans, and CO Rockies. The latest WPC probabilities for 8 inches are high. Two day totals for the CO Rockies, San Juans, White Mountains of AZ could be greater than 2 feet locally. By D3, residual snow across AZ/NM will taper off quickly as the main upper level energy ejects into the Plains and ridging quickly replaces over the region. Ridging extending through the Rockies and Pacific Northwest will lead to a quieter weather regime for through late this weekend. ...Central Plains to the Great Lakes... Days 2-3... ...Significant winter storm expected for portions of Central Plains to Great Lakes New Year's Day... Shortwave trough energy from both the southern and northern stream will push into the Central Plains and phase Saturday into Sunday. In response, surface cyclogenesis occurs and the low is forecast to track across the central Plains early Saturday toward the lower Ohio Valley by early Sunday. Favorable forcing for ascent will lead to a broad area of winter precipitation on the north to northwest side of the low track. While model guidance continues to feature a large spread in both timing and placement of this low, there is increasing confidence in a significant and impactful winter storm. Well northwest of the surface-850mb low, heavy snow is likely. PW pooled across the Southeast near the Gulf of Mexico is progged to reach nearly +3 standard deviations above the climo mean, and then get advected northward into the system as the trough deepens to the west. This will be accompanied by impressive WAA and moist isentropic ascent at 290-295K which will surge into a very cold column to drive an anomalously deep DGZ with increasing ascent. The SLRs were bumped above the NBM guidance into the 17-19:1 range, particularly early day 3 as the cold surges southward. The WPC probabilities for 6" or more reach at least 30 percent across a large area from northern Kansas, southern Nebraska northeast toward southwest Michigan with the greatest/strongest signal from far northeast KS, northern MO, southern IA, and northern IL where the WPC probability for at least 8 inches is up to 40 percent now. A secondary maximum in probabilities exists near the Lake Michigan SW lake shore where LES enhancement is likely. South of the heaviest snow, a zone of mixed precipitation is likely and could become significant as cold air surges southward. The greatest signal for 0.10" or more of ice accumulation exists from southeast KS through much of central to north-central Missouri where the latest WPC ice probabilities are approaching 40 percent. Weiss/Taylor ~~~Key Messages for the Central Plains to Great Lakes Winter Storm~~~ Accumulating snow and ice are expected New Yearâ€s Day (Saturday) from the Central Plains to the Great Lakes resulting in widespread travel disruptions and impacts. Several inches of snow are likely with potentially heavier accumulations where snowfall rates surpass 1" per hour. Some locations may exceed 6-8†total amounts. Significant freezing rain is possible to the south of the heavier snow, with some ice accumulations greater than 0.10â€. Bitterly cold temperatures will follow on Sunday with highs only in the 10s to low 20s and wind chills near to below zero.