Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 437 PM EDT Wed Mar 29 2023 Valid 00Z Thu Mar 30 2023 - 00Z Sun Apr 02 2023 ...Northeast... Days 1... An amplifying shortwave will continue to sweep east across the Great Lakes and southern Ontario this evening, before taking a negative tilt over the St Lawrence Valley overnight. This will drive a sharp cold front across the Northeast this evening. Sufficient instability and strong ascent is expected to support a convective rain showers changing over to brief but intense snow showers, with embedded snow squalls. The greatest threat for heavy snow and whiteout conditions is expected to extend from northern New York to western Pennsylvania early in the period, before progressing east across northern New England overnight. Please see our Key Messages below for additional details. ...The West/Rockies... Days 1-2... A deep closed low centered along the NorCal coast this afternoon is forecast to gradually fill as it continues to sink south before turning east overnight. Some additional heavy snows are possible along the southern Sierra and SoCal ranges before diminishing tomorrow as the trough axis moves east of the region. Meanwhile, mountain snows are forecast to develop along a well-defined low-to-mid level frontal zone as it moves from Nevada into Utah and Arizona, with locally heavy amounts expected, especially along the favored southwestern-facing slopes. Farther to the north, left-exit region upper jet forcing is forecast to add additional support for heavy snows across the central to southern Idaho and southwestern Montana ranges. As snows spread into the Colorado Rockies, snow showers are expected to continue across the northern Rockies and Intermountain West into late Thu before diminishing early Friday as the upper low/trough moves into the Plains. Overall, widespread heavy accumulations are not expected, however some areas are likely to see a foot or more. This includes the Wasatch, where WPC probabilities for a foot or more are generally above 50 percent. ...Northern Plains and Upper Midwest... Days 1-3... As the trough in the West moves east, mid-level energy and favorable upper jet forcing along the downstream ridge will help support warm advection precipitation developing across the upper Mississippi Valley into the northern Great Lakes Thu night into early Friday. Thermal profiles indicate a wintry mix is likely, with a stripe of accumulating ice expected from eastern South Dakota to northern Wisconsin and the UP of Michigan and some light snow accumulations farther to the north. By early Friday, the trough in the West is expected to move east of the Rockies, with a closed low forecast to develop over South Dakota-Nebraska. Strong upper forcing and low-to-mid level frontogenesis are forecast to support the development of heavy snow on the northwest side of the associated surface low. Model differences remain, however the consensus for heavy accumulations centered across South Dakota continued to grow with the 12Z runs. Strong forcing is expected to support precipitation changing to snow within the comma head as it translates east across the upper Mississippi Valley into the northern Great Lakes Friday night into Saturday. Probabilities for heavy snow continue to increase, with the latest WPC run generally showing probabilities above 50 percent for accumulations of 6 inches or more along this corridor. Along this axis, accumulations of a foot or more can be expected. This will likely include portions of central South Dakota as the system slows and intensifies over the region on Friday. In addition to heavy snow, this system is likely to bring strong winds to the region as intensifies and moves east. Please see our Key Messages below for additional details. ...Pacific Northwest to the Northern Rockies... Day 3... Heavy mountain snows are expected to return to the region as an amplifying shortwave trough originating over the Gulf of Alaska dives southeast and moves across the region on Saturday. Heavy accumulations are likely for the Olympics and the Washington and Oregon Cascades, with WPC probabilities indicating accumulations of a foot or more likely (greater than 70 percent) for portions of the area. Generally, lighter but significant accumulations are expected farther east for the Blue Mountains and parts of the northern Rockies, including the Idaho and northwestern Wyoming ranges, where amounts of 6 inches or more likely. Pereira ...Key Messages for Northeast Snow Squalls... --Snow showers and squalls are expected later this afternoon through the evening and overnight hours along and behind a strong cold front sweeping across the area. --Where snow squalls occur, they will be accompanied by bursts of heavy snow and strong gusty winds. Higher elevation areas more likely to see stronger snow squalls. --Any squalls could produce dangerous travel including whiteout conditions, zero visibility, and quickly snow covered roads. --If driving, remain alert for rapidly changing road conditions and be prepared to react to any Snow Squall Warnings. --Temperatures will rapidly fall below freezing this evening. Any wet roads may quickly turn icy and lead to a flash freeze. ...Key Messages for Mar 31 - Apr 1 Winter Storm... --A strong late winter storm is expected to track across the central High Plains Friday and into the Great Lakes Saturday. --While there remains some uncertainty in the exact placement of the most impactful winter conditions, confidence is increasing that the system will bring periods of heavy snow and strong winds to the region. --The combination of heavy snow and strong winds (gusts up to 50-55 mph possible) could create blizzard conditions, particularly across portions of South Dakota and southwest Minnesota. Difficult to near impossible travel conditions are possible.