Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 325 AM EST Fri Dec 31 2021 Valid 12Z Fri Dec 31 2021 - 12Z Mon Jan 03 2022 ...Western U.S... Day 1 and Day 3... Digging shortwave energy through British Columbia this morning will help carve out and amplify the longwave trough over much of the western U.S. through the day 1 period while a closed low west of Baja opens up as it gradually phases with the aforementioned northern stream energy. This mid/upper level forcing will work together to provide plentiful forcing for ascent and drive widespread precipitation during the day 1 period. The heaviest snowfall is expected across the Four Corners region for the mountains of southern/central Utah, much of the Colorado Rockies, and the AZ/NM mountains. The latest WPC probabilities for at least 6 inches are at least 60 percent with greater than 80 percent probabilities for the entire Colorado Rockies. After a day long reprieve thanks to pronounced upper ridging in the wake of the departing trough, another potent and anomalous mid/upper level system will approach the Pacific Northwest. This system will have a favorable fetch of Pacific moisture of at least 0.5" PW impinging on the terrain of western OR/WA. Lower heights and colder 850 mb temps will support lower snow levels down to around 1000 ft and heavy mountain snow is increasingly likely. WPC probabilities for at least 6 inches are above 80 percent for the Olympics and WA Cascades and the signal for greater than 18 inches is moderate at 40-60 percent. ...Central Plains through the Great Lakes and into Northern New England... Days 2-3... ...Significant winter storm expected for portions of Central Plains to Great Lakes this Holiday Weekend... Shortwave energy from the Pacific Northwest will phase with a southern stream shortwave originating west of Baja around the Four Corners today before lifting through the central U.S. Saturday into Sunday. At the surface, low pressure is forecast to develop across the mid-MS Valley and deepen as it lifts northeast to the eastern Great Lakes by late Saturday. A large shield of precipitation is expected to break out on the western to northwestern side of the surface low resulting in a swath of heavy snow followed by a corridor of mixed precipitation south/southeast. This forecast cycle continued the trend for a clustering of model solutions and overall the deterministic guidance was in good agreement from a QPF perspective as well as thermally. This iteration showed a slight bump in QPF across northern KS toward the Chicago metro but overall amounts have locked in over the last 24 hours or so. The SLRs were again adjusted above the NBM this cycle to account for the favorable saturation within the DGZ coincident with the forcing, particularly across portions of northern KS, southern NE into southern IA. Ratios approaching 18:1 (perhaps 20:1) will be possible early Saturday morning. Snowfall rates up to 1" per hour will become likely during the height of the event, especially from central KS into IA/MO and perhaps as far northeast as the Chicago metro. The WPC probabilities for 6 inches range from 40-60 percent from northern KS toward the Chicago metro with just a slight signal for 8 inches or more across northern MO and southern IA. South of the heavy snow axis, a swath of mixed precipitation is expected, some of which could be significant (both sleet and freezing rain). Crashing temperatures in the low levels should lead to a quick transition to mixed ptype and the WPC probabilities for 0.10" of ice remain at about 30 percent from eastern KS across much of central Missouri. Taylor ~~~Key Messages for the Central Plains to Great Lakes Winter Storm~~~ Accumulating snow and ice this holiday weekend from the Central Plains to the Great Lakes will lead to widespread travel disruptions. The most significant snow impacts are expected from northern Kansas to the Chicago metro where the combination of snowfall rates up to 1" per hour and strong winds may lead to dangerous travel conditions and severely reduced visibility. Significant mixed precipitation including freezing rain and sleet is likely to the south of the heavier snow, with some ice accumulations greater than 0.10" across eastern Kansas to central Missouri. Bitterly cold temperatures will follow with dangerously cold wind chills Saturday night into Sunday morning.