Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 326 AM EST Wed Jan 25 2023 Valid 12Z Wed Jan 25 2023 - 12Z Sat Jan 28 2023 ...Middle Mississippi River Valley, Lower Great Lakes, & Northeast... Days 1-2... ...Significant winter storm to produce heavy snowfall and icy conditions from the Middle Mississippi River Valley to the Northeast today and through Wednesday night... The winter storm that dropped a swath of heavy snow from New Mexico and the Southern Plains to the Ozarks yesterday is on the move this morning as it heads for the Ohio Valley and then the Lower Great Lakes by this evening. Impressive jet coupling that includes diffluent regions of two jet streaks is facilitating a favorable environment for large scale ascent from the Ohio Valley and Great Lakes this morning to the Northeast later this evening. As a vigorous shortwave trough driving the surface cyclone moves northeast, it will provide a healthy stream of Gulf of Mexico moisture up through the Southeast and Ohio Valley. Some of this moisture will be wrapped around the northwestern flank of the 850mb low, placing a deformation axis of heavy snow over the Middle Mississippi Valley and Lower Great Lakes this morning. Within this band of heavy snow, the 00Z HREF shows a cluster of CAM members producing snowfall rates of 1-2"/hr from southeast MO and southern IL, on northeast into central IN, northwest OH, and southeast MI. The latest WSSI shows a broad swath of Moderate impacts across the regions today, with some areas featuring Major impacts, most notably around the Indianapolis metro area. Major impacts include the potential for considerable disruptions to daily life which include dangerous (even at times impossible) driving conditions. Latest WPC PWPF shows a swath of 50-70% probabilities for snowfall >6" from central IN to southeast MI, including the Detroit metro area. For areas in the upper Ohio Valley, the intrusion of a dry slot at mid-levels and a surging warm nose of >0C temperatures aloft will force a change over to rain, keeping any initial snowfall totals via strong WAA at the onset early Wednesday morning to lighter accumulations generally <3". Farther east, a powerful 70kt LLJ over the Southeast will deliver a highly anomalous plume of moisture to the central Appalachians. Meanwhile, high pressure to the north is helping to keep temperatures in the atmosphere within the interior northern Mid-Atlantic and Northeast sub-freezing. As precipitation arrives, snow will be the initial precip type, but will change over to an icy wintry mix in the central Appalachians while any snow in the western suburbs of I-95 likely make a quick transition to rain. This is due to the same LLJ that is providing both a slug of rich atmospheric moisture, but also a protruding warm nose first within the 850-750mb layer. As the warm front lifts north and southerly surface winds pick up, surface temperatures will rise above freezing as well. The initial frontogentical forcing in the 850-700mb layer will result in a few hours worth of heavy snow across central PA, the Poconos, the southern tier of NY, and the Catskills. The WSSI does depict Moderate impacts along the Allegheny front and Laurel Highlands of PA, with some localized areas in the Poconos and Catskills. As temperatures continue to warm in the low levels, precipitation will transition to sleet and freezing rain in all areas of the northern Mid-Atlantic Wednesday evening. While accumulating snowfall will be shut off by this point, parts of the central and northern Appalachians could see some ice accumulations. WPC PWPF does show 10-30% chances for >0.1" of ice accumulation in these regions. Combined with strong winds, there is the potential for power outages due to strong wind gusts and/or a combination of winds and snow/ice accumulation weighing down tree limbs and power lines. The heaviest snowfall will be found in northern New England where the cold air will hold on the longest. Snowfall rates of 1-2"/hr are expected in these areas throughout much of Wednesday night, before the same warm nose and dry slot aloft arrive by Thursday morning. In addition, a secondary low pressure center forming along the New England coast will introduce easterly, oceanic fetch into New England, causing coastal areas to transition to a wintry mix, and eventually plain rain, sooner than their more mountainous neighbors to the west. WPC PWPF shows 60-80% probabilities for snowfall totals >8" in the White Mountains of New Hampshire and across much of northern Maine. Speaking of northern Maine, they feature the highest probabilities for snowfall totals >12" for this event, reaching as high as 30-50%. Much of northern Maine and portions of New Hampshire are expected to contend with Moderate impacts according to the WSSI, with some portions of northern Maine seeing some Major impacts. Travel conditions will be hazardous in these areas, amd the combination of heavy/wet snow and strong winds may produce scattered power outages Wednesday night into Thursday morning. ...Northern Rockies and Northern High Plains... Days 2-3... The first in a pair of setups resulting in measurable snowfall across the northern Rockies comes in the form of a Pacific warm front resulting in northerly winds upsloping into the higher terrain of northern ID, western MT, northern WY, and into the Black Hills. Northerly upslope flow will also contain some modest 700mb moisture flux, so between the upslope component and sufficient mid-level moisture, periods of heavy snow are possible in these states tallest mountain ranges. Latest WPC PWPF does suggest 30-50% probabilities for >6" of snow in parts of the Absaroka, Big Horns, and Black Hills. The second opportunity for heavy snow will cover more ground across a large portion of the Northern Rockies. A strengthening frontal boundary will reside along the northern slopes of the Rockies, while a strong Arctic high pressure system is dislodged and makes its way south behind an approaching Arctic front. In addition, a northeast Pacific shortwave trough plunging south along the British Columbia coast will impose large scale ascent at upper levels. Between colder temperatures forcing highers SLRs, a better source of synoptic scale lift, and strong upslope flow into the terrain, the footprint of heavy snow will span as far west as the Blue Mountains of northeast OR and the Bitterroots of ID, across western Montana and into the Tetons. Latest WPC PWPF indicates much of the northern Rockies above 6,000' have 60-70% odds of seeing >8" of snowfall on Friday. WPC's WSSI shows some Moderate impacts in the highest elevations of Montana, northern WY's Big Horns, and the Black Hills on Friday. As a wave of low pressure forms along the frontal boundary Friday night, it may lead to periods of heavy snow in the Northern Plains, specifically southern SD and northern NE. WPC PWPF shows some 20-30% probabilities for >6" of snowfall there Friday night. Expect hazardous travel conditions in these areas on Friday from a combination of heavy snowfall rates and gusty winds that may lead to near whiteout conditions. Mullinax Key Messages for Jan 25-26 Winter Storm... --A winter storm will track from the Ohio Valley this morning into the eastern Great Lakes tonight. Bands of heavy snow are expected over the Lower Great Lakes this morning with snowfall rates of 1-2"/hr. --Heavy snow is forecast to spread across the interior Northeast by Wednesday afternoon into Thursday morning as a second area of low pressure develops along the coast on Maine. Some icing is likely from the central Appalachians northeastward into New England. --Peak snowfall rates in the interior Northeast Wednesday afternoon and into the overnight hours are likely to range between 1-2â€/hr. --Snow-covered roads and reduced visibility will make travel difficult across impacted areas. Gusty winds and heavy snow may result in downed trees and power outages.