Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 436 PM EST Sun Jan 1 2023 Valid 00Z Mon Jan 2 2023 - 00Z Thu Jan 5 2023 ...Western U.S.... Days 1-2... The strong low pressure system from the Pacific is moving across the Four-Corners region tonight. The shortwave energy aloft is evolving into a closed low, and the combination of the enhanced lift, orographic effects, and anomalous low-mid level moisture in place will continue to produce moderate to locally heavy snowfall across most of the mountain ranges of northern Arizona, much of Utah, central Wyoming, and central-western Colorado. Probabilities are high for 8-12" for the highest terrain areas, with some locations getting on the order of 1-2 feet through Monday afternoon, with snowfall maxima across the San Juan Mountains of Colorado, the Uinta Mountains of Utah, and across portions of central Wyoming. A weaker storm system from the Pacific is forecast to reach the West Coast by Monday evening, with moderate snow reaching the northern California mountains and then the Sierra by overnight Monday and into Tuesday morning. This low will likely take a more southerly route compared to the ongoing event, with some additional snow for the higher terrain of Arizona and western New Mexico on Tuesday as the system continues to weaken. ...Central Plains and Upper Midwest... Days 1-3... The low crossing the Rockies is forecast to re-develop across southeast Colorado Monday morning after the initial system gradually dissipates over the north-central part of the state. This surface low is progged to track in a general east-northeast direction across northern Kansas, and then across Iowa by Tuesday morning, with central pressures generally in the 990s. It should then reach the central Great Lakes by Wednesday morning. A swath of moderate to heavy snow is expected from northeast Colorado/eastern Wyoming to central Minnesota and far northern Wisconsin, with high probabilities for 4+ inch total accumulations and moderate probabilities for 8-12 inch totals. The snow will primarily be confined to the deformation zone to the northwest of a surface low, where a trowal is also expected to be present. Some mesoscale banding of heavy snow is likely if conditional symmetric instability develops, with snowfall rates possibly reaching 1-2 inches per hour, but future forecasts and model runs will provide better clarity on the placement of these mesoscale features. At this time, it appears the highest totals are likely across south-central South Dakota where the best combination of frontogenetical forcing and lift with the dendritic growth zone will exist, with locally a foot or more of total snow accumulation possible. Overall, there has been a slight increase in total forecast snowfall within the primary band, and there is now better model agreement, with the main axis similar to the previous forecast. The ECENS served as the basis for thermal profiles, along with some contributions from the GEFS/FV3/Nam nest/previous WPC continuity, to derive snowfall totals. Similar to yesterday, the SLR was slightly increased within the expected heavier snow bands. Farther to the south/southeast, a warm nose advecting northward is forecast to bring a transition zone of mixed precipitation with a surface cold layer supporting freezing rain and potentially significant ice accumulations in a corridor from northeast Nebraska, far southeast South Dakota, and much of northern Iowa and far southern Minnesota. There is growing concern for a corridor of 2 to 3 tenths of ice accretion, mainly near the Iowa/Minnesota border. Recent model soundings are depicting a well defined warm nose between 925 to 800 mb across much of this area with a shallow subfreezing layer near the surface with cold air advection from northeast winds. A secondary area of freezing rain and ice accumulation exists for central/north-central WI where one to two tenths of accretion is currently anticipated. Finally, by late Tuesday into Wednesday, the shortwave energy opens up, becoming absorbed into the longwave troughing over the region. Some lingering deformation band snow will likely continue for portions of northern Wisconsin into the U.P. of Michigan. Hamrick/Weiss ...Key Messages for January 1-3 Winter Storm... -An area of low pressure will develop and bring snowfall to the central High Plains beginning later tonight. This low pressure will intensify as it tracks northeast into the Great Lakes by Tuesday, producing a large swath of moderate to heavy snow, sleet, and freezing rain. -Snow rates exceeding 1â€/hr are likely in the heavy snow swath and combined with strong winds, areas of blowing snow will create snow-covered roads and difficult travel. -Localized significant ice accumulations exceeding 0.25†are possible from portions of northeastern Nebraska, northwest Iowa into southern Minnesota. This may lead to dangerous travel and isolated power outages. -There remains some uncertainty in the exact storm track and potential mixed precipitation types and zones. Stay tuned to the latest forecast information.