Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 332 PM EDT Fri Mar 22 2024 Valid 00Z Sat Mar 23 2024 - 00Z Tue Mar 26 2024 ...Northeast... Day 1... Shortwave energy moving through the base of a broad northern stream trough will support a sharpening baroclinic zone that will slide east from the eastern Great Lakes overnight. Amplifying low level flow will support increasing moisture advection into an area of large-scale ascent affording in part my right-entrance region upper jet forcing. Precipitation will blossom across much of the Northeast by daybreak, with heavy wet snow is likely to develop initially across portions of the Adirondacks into the Green and White Mountains Friday night into Saturday morning. Meanwhile, an icy transition zone is likely form to the south along New York's Southern Tier through the mid Hudson Valley. Heavy snow will continue to spread further east across during the day, stretching across interior Maine during the late morning hours. As the northern and southern stream begin to interact, a coastal low will track along the Mid Atlantic coast -- directing warm air further north that will support rain across southern New England and an any icy transition zone that will shift north from central into northern New England. Some areas, especially across interior northern New England may return to snow before precipitation ends as the low tracks into Atlantic Canada and the front slides offshore late Saturday. The latest WPC PWPF shows high probabilities (greater than 70 percent) for snow accumulations of 8 inches or more extending from the Adirondacks through most of interior Maine. The WSSI indicates moderate to major impacts extending across much of the same area. Moderate probabilities (40 percent or greater) for ice accumulations of 0.10 inch or more stretch from the Catskills to along portions of coastal Maine. ...Pacific Mountain Ranges... Days 1-3... A strong, large closed upper level low positioned just off the West Coast this afternoon will push a plume of higher moisture into the Pacific Northwest tonight/Saturday, bringing heavy mountain snow to the region. IVT values reach 400 kg/m/s through tonight and this should produce heavy snow for the northern CA ranges initially then into the Sierra Nevada late tonight/Saturday. Snow levels will initially start out between 4,000-5,000ft tonight but drop to as low as 3,000ft by Saturday as the upper low moves into the Pacific Northwest. However, snow totals that likely surpass 12" will be at elevations >5,000ft Saturday and through Sunday morning. Farther north, while the moisture transport is not as pronounced, the Pacific Northwest lies directly ahead of the approaching upper low, which still supports prolonged synoptically- forced ascent aloft to go along with falling snow levels. Snow levels will be a near carbon-copy of the scenario farther south in the California Ranges with the Cascade Range and Olympics >5,000ft having the better odds of seeing cumulative snowfall totals through Sunday topping 12". As the upper low moves ashore and over the Northwest on Saturday, the axis of Pacific moisture will be oriented farther inland and lead to periods of heavy mountain snow for elevations >6,000ft in the Great Basin, the Blue Mountains of eastern Oregon, the Bitterroots, Tetons, Absaroka, and as far inland as the Wasatch and Colorado Rockies by Saturday night. WPC snow probabilities are high (>70%) for at least 12 inches of snowfall for the event across the Sierra Nevada and the Salmon/Siskiyou/Trinity mountains. For the higher elevations, generally above 7500-8000 ft, there are moderate/high probabilities for at least 18 inches. Once the snowfall threshold dips to >6", the Olympics and Cascades, as well as the Absaroka, Tetons, and portions of the western Colorado Rockies sport moderate-to-high chances (50-70%) for those snowfall totals. ...Northern Rockies & High Plains, Central Rockies, Upper Midwest... Days 1-3... **Major Winter Storm Set to Impact Northern/Central Rockies to Upper Midwest Beginning This Weekend** The combination of lower pressure over the West/Intermountain West and sprawling high pressure over the Canadian Prairies will bring a long duration easterly flow regime to the Intermountain West/Northern Rockies through tomorrow. Aloft, a jet streak positioned over the Dakotas will put the region in the favored right entrance region of the 110 kt jet while heights will begin to lower with the approaching shortwave energy. This is a recipe for periods of snow through tomorrow for the western Montana Rockies as well as central Montana and further south into the Absaroka and Big Horns. Accumulations should be fairly minor with only the Big Horns seeing any substantial probabilities for greater than 4 inches. For Day 2 (Saturday evening/Sunday), a stronger shortwave trough rounds the base of the larger trough over the Four Corners/Rockies regions and this will kick off a strengthening low pressure in the lee of the Rockies. Moisture anomalies increase significantly, reaching well north into the Northern Plains, wrapping around the northern flank of the strengthening surface low. All around, favorable forcing for ascent provided by the falling heights, deepening low pressure, and left exit region dynamics from a strong jet streak over the S. Plains. This will bring a shield of heavy precipitation from the Four Corners region including the Colorado Rockies, northward into the Northern Rockies then along the WAA wing into Minnesota and western Wisconsin. For Day 2, the WPC snow probabilities are high for 6" across western CO Rockies and are in the moderate to high range (40-70%) across much of central/eastern MT through central MN and far west-central WI. For Day 3 (Sunday evening/Monday), the low pressure deepens more and lifts to the northeast, reaching the MN/IA border by 00Z Tue. A classic setup for long duration snowfall north/northwest of the 700 mb low sets up with a favorable TROWAL signature. Moisture anomalies are very impressive, with a strong tap into the Gulf of Mexico moisture. QPF probabilities for 1"+ in the 24-hr period are moderate/high from north-central NE through southeast SD and southwest to central MN. A tricky thermal/ptype forecast as a sharp rain/snow (maybe embedded mix transition zone) and trends in guidance have lifted the zone of greatest snow to the north some. Some waffling in the track/thermals expected over the next couple of days. 8 inch snow probabilities are high (>70%) for a large area from northern NE through MN for Day 3 and are solidly in the moderate range (40-60%) for 12 inches. 3-day totals may reach 18-24 inches across portions of eastern SD through central MN. This will bring a significant/major winter storm to the region where the combination of snow amounts, snow load, and blowing snow/winds will create disruptive and significant travel impacts. The WSSI-P highlights a near 100% probability for Moderate Impacts across portions of the region and for Major Impacts, the probabilities reach 30-40%. Taylor/Peireia/Mullinax ***Key Messages for Northern Plains Winter Storm*** --Significant winter storm likely There is high confidence that an extensive storm system will produce widespread heavy snow and gusty winds over the Northern Plains into the Upper Midwest. --Widespread heavy snow expected Heavy snow will likely spread across much of central and eastern Montana by Saturday night, then expand into the Northern Plains and Upper Midwest on Sunday. There is a high chance (>70%) of at least eight inches of snow extending from portions of the Dakotas and north-central Nebraska northeastward through Minnesota into northern Wisconsin. --Hazardous impacts due to snow, snow load, and wind A combination of heavy snow and gusty winds will likely produce areas of blowing and drifting snow, as well as possible blizzard conditions Sunday into Monday. Hazardous travel, along with some disruptions and closures are expected late Saturday into Monday. The combination of strong winds and heavy, wet snow on trees and power lines may result in tree damage and power outages. --Additional forecast changes anticipated Uncertainty remains with precipitation type for southern-most areas and a possible sharp northern snowfall gradient in northern- most areas. Closely monitor the latest forecasts as the storm evolved over the next 24-48 hours.