Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 316 PM EDT Wed Apr 3 2024 Valid 12Z Wed Apr 03 2024 - 12Z Sat Apr 06 2024 ...Michigan U.P.... Day 1... An occluded surface low placed directly beneath a vertically- stacked upper low will continue to sustain cyclonic flow over Lake Superior. This will keep rounds of heavy snow in the forecast over parts of the Michigan U.P., particularly over the Huron Mountains. WPC's Snowband Probability Tracker (SPT) using the HREF shows snowfall rates of 1-2"/hr are likely to stick around this afternoon and linger until roughly 06Z. While the heavier rates will back off by Thursday morning, the upper low will still be in the Ohio Valley, maintaining steady (albeit weaker) cyclonic flow over the Upper Great Lakes. This still favors occasional rounds of lake effect snow bands over the Michigan U.P. with light-to-moderate snow over the tip of Michigan's Mitt through midday. Also, given less dynamic support aloft and a modifying air-mass, low level lapse rates will not be as impressive and boundary layer temperatures more questionable. Overall, the period of accumulating snow is likely to conclude by mid-morning Thursday as snow will be increasingly more difficult to accumulate due to the strong early April sun angle. Latest WPC PWPF shows moderate-to-high chances (50-70%) for additional snowfall totals >6" in the Hurons. There are also low-to-moderate chances (20-50%) for additional snowfall >4" in the Porcupines a little farther to the west of the Hurons. ...Northeast... Days 1-2... As the vertically-stacked and occluding storm system in the Great Lakes slowly track east this afternoon, farther east, an impressive IVT over the Southeast is directing some of its associated moisture into the Northeast. An icy wintry mix has been ongoing in parts of the Northeast thanks to a Canadian air-mass anchored by high pressure over Quebec that is just sufficiently cold enough within the boundary layer to support wintry precipitation. This air-mass supporting the impending onslaught of disruptive wintry precipitation is not expected to exit the region any time soon due to an omega block over eastern Canada and the North Atlantic that is locking the dome of high pressure in place. With cold air locked in over New England, as the occluded front works north through the Mid-Atlantic this afternoon, intense PVA and robust 850mb FGEN from southern Pennsylvania to the southern New England coast will strengthen over the Delmarva Peninsula this evening and head for the southern New England coast by early Thursday morning. Much of the interior Northeast will initially start out as a wintry mix (lone exception the White Mountains based on latest CAMs) due to the >0C warm nose protruding aloft within the 800-700mb layer. However, as the 850mb low forms near Long Island, winds shift more out of the east and a cold conveyor belt (CCB) will aid in the transition to all snow from the Catskills and Adirondacks on east through the Green Mountains, and the Berkshires by early Thursday morning. As Thursday unfolds, the deformation axis of heavy snow will inch north over New York's North Country, Vermont's Champlain Valley and Northeast Kingdom, the Great Woods of northern New Hampshire, and through central Maine. Hourly snowfall rates will generally range between 1-2"/hr in these areas Thursday morning and through Thursday afternoon, but it is in the Adirondacks, Green Mountains, White Mountains, and central Maine that could occasionally top 2"/hr. By late Thursday afternoon, snow rates will diminish over northern New York while rates increase across northern Maine. By thursday night, the storm system will find itself near Portland, Maine and a dry slot will end the snow potential in central Maine and lead to a mix of snow/sleet/freezing rain in central Maine and even parts of the White Mountains. The storm will remain in the Gulf of Maine on Friday with cyclonic flow kicking up additional periods of snow across the Northeast. WPC PWPF depicted high chances (>70%) for snowfall totals >12" in the Adirondacks and both the Green and White Mountains. Meanwhile, the Blue and Boundary Mountains of western and central Maine sport moderate-to-high chances (50-70%) for >24" for the event. This is triggering the WSSI to highlight Major Impacts in all these listed mountain ranges, which suggest considerable impacts to daily life that include dangerous to impossible travel and widespread closures. In addition to the snow, gusty winds up to 40 mph combined with a heavy, wet snow in areas closer to the coast will result in some tree damage and power outages. Elsewhere, Moderate Impacts are forecast in parts of the Catskills, the Berkshires, the Worcester Hills, and along the New Hampshire and Maine coasts. Those coastlines in particular are largely driven by the Blowing Snow component in the WSSI algorithm tonight and through Thursday. ...Central Appalachians... Days 1-3... As the primary low swings through the Ohio Valley and towards the central Appalachians Thursday morning, snow levels will fall and result in a burst of heavy snow. As the 850mb low tracks east across Pennsylvania on Thursday, WNW upslope flow will increase and begin a prolonged period of moderate-to-heavy upslope snow Thursday afternoon and lasting through the day on Friday. Rates will not be overly impressive (generally <1") but due to the coastal low's position off the Northeast coast, a nearly 48 hour period of cyclonic flow over the Great Lakes and oriented south to the Central Appalachians will sustain a persistent onslaught of snow through Friday night. Snow should diminish sometime on Saturday as cyclonic flow gradually weakens. WPC PWPF sports moderate-to-high chances (50-70%) for snowfall totals >12" in the Potomac Highlands >3,000ft. Minor impacts (localized Moderate Impacts possible) are most likely at elevations >2,000ft in eastern West Virginia through Saturday morning. ...Western U.S.... Days 1-3... An anomalous upper low barreling into the West Coast tonight is responsible for the barrage of Pacific moisture enveloping the West Coast tonight and through the day on Thursday. Strong upslope flow is the culprit for heavy snow in the central and southern Sierra Nevada (same in the Oregon Cascades but upslope enhancement is not as strong), while precipitation flowing along a frontal wave in the Northern Rockies fosters mountains snow in the Blue, Sawtooth/Boise, Bitterroots, and Lewis Range. Snow will pick up in intensity along the southern Sierra Nevada and over the central Great Basin Thursday afternoon and through Thursday night, with portions of the Blue, Sawtooth/Boise, and Owyhee Mountains also contending with heavy snow through Friday morning. The upper low will move east and farther inland throughout the day on Friday, pushing a cold front associated with low pressure in the Northern Rockies and its tongue of Pacific moisture into the Rockies. By Friday afternoon and into Friday night, mountain ranges such as the Wasatch, the Tetons, Absaroka, Big Horns, and Colorado Rockies will take their turn seeing heavy snow. The snow in the Big Horns, the cluster of mountains in central Montana, and as far south as the San Juans in southern Colorado will see snowfall become more associated with a developing area of low pressure in western Nebraska. There remains some differences in the track of the storm in the central High Plains, but there is a growing consensus that mountain ranges in Montana, Wyoming, and Colorado are likely to witness periods of moderate to heavy snow the first half of the weekend. WPC PWPF between now and Saturday afternoon show high chances (>70%) for snowfall totals >12" in the central and southern Sierra Nevada, portions of northern Nevada (elevations >6,000ft), the Blue Mountains, the Boise/Sawtooth Montana, and the northern most portions of the Lewis Range. The WSSI shows Major Impacts possible in parts of the central and southern Sierra Nevada, as well as in parts of the Transverse and Peninsular regions also show Moderate to even Major Impacts above 5,000ft due to the potential for 8-12" of snowfall there. Across the remainder of the Intermountain West ranges, from the Blues and the Great Basin to as far east as the Colorado Rockies, most impacts from the expected snowfall will be Minor level impacts, but some Moderate level impacts could be felt in the higher peaks. Mullinax ***Key Messages for Early April Nor'easter*** ---Northeast snow and wind through Friday Heavy, wet snow and some significant sleet will develop this evening over north-central New England and northeast New York this evening and then spread north through Maine on Thursday. Rates then decrease, but snow continues into Friday over northern New England. Snowfall accumulations of 1-2 feet are likely for much of Maine, New Hampshire, Vermont, and the northeast portions of the Adirondacks. ---Significant impacts from heavy snow and wind Heavy snow and gusty winds will result in dangerous travel, with whiteout conditions and snow-covered roads. The combination of wet snow, a high snow load, and strong wind gusts could also cause tree damage and power outages. ---Moderate coastal flooding in the Northeast Prolonged onshore flow through Thursday will result in moderate coastal flooding for the northern Mid-Atlantic and southern New England coasts. Impacts include widespread roadway flooding, coastal and bayside flooding, impassable roads, and some damage to vulnerable structures.