Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 349 PM EDT Thu Mar 21 2024 Valid 00Z Fri Mar 22 2024 - 00Z Mon Mar 25 2024 ...Upper Midwest/Great Lakes... Day 1... Broad upper troughing over central/eastern Canada will favor a continued WNW flow in the mid/upper levels astride the U.S./Canadian border. Vorticity on the SW side of the broad circulation will swing through the northern Plains into the Upper Midwest tomorrow morning beneath the RRQ of a ~100kt jet across Lake Superior. Snow will quickly expand out of ND into MN and WI this evening in a somewhat focused WNW-ESE band, driven by ~700mb FGEN well to the north of a surface low along a boundary over NE. Southerly flow in the lower levels will drive PW values to around 0.50" to along the rain/snow line (roughly northern IA eastward along the WI/IL line). Models have had a tough time with placement and mode of heavier snowfall, driven by FGEN in the 650-850mb layer, and whether or not there would be two areas of heavier snowfall (one just north of the rain/snow line and one farther north into the deeper cold air). Forecast has continued to focus on the southern axis, where WPC probabilities for at least 4 inches are high (>70%)... centered over southern WI and continuing eastward into central Lower Michigan. By tomorrow morning, snow will continue streaming eastward with some embedded heavier rates >1"/hr (HREF probs ~40-70%). Snowfall overnight will be easier to accumulate (MN/WI) but have trended ratios down from NBM guidance due to the time of year, milder ground temps, and time of day (farther east). Nevertheless, still expecting modest totals across Lower Michigan as the system keeps a steady pace into southwestern Ontario tomorrow afternoon. ...New England... Days 2-3... System exiting Lower Michigan D1 will continue eastward into D2, spreading snow across northern NYS into northern New England as mid-level troughing digs into western NY/Lake Ontario and a persistent band of FGEN translates eastward into the region. At the same time, a system in the Southeast will start to lift northeastward along the coast, bringing a surge of moisture northward on strong 50-60kt winds at 850mb across Long Island. Aloft, jet will start to buckle a bit and increase quite smartly to over 150kts across Quebec, with much of northern New England in the right entrance region of the jet. Surface high pressure will be split to the north and east of the region, but a low pressure track along and off the New England coast should maintain some northeasterly cold air drainage into interior sections. This could set up a narrow band of sleet and/or freezing rain in between snow over interior locations and rain along the coast as mild SE flow overrides the marginally sub-freezing interior locations. Snowfall could be quite heavy at times, with HREF probs >1"/hr already ~60-80% early Saturday over northern NY/VT/NH, with snow continuing eastward thereafter into Maine (beyond the 12Z HREF end time). Snow ratios may be quite variable as the system evolves and may also be dependent on time of day as well (esp with lower rates outside the favored areas). Regardless, significant accumulations are possible from the Adirondacks into the northern Green/White Mountains and into northern Maine. There, WPC probabilities for at least 12 inches of snow Sat-Sun are high (>70%). Lower amounts around 4 inches are possible as far south as about I-90 across NY and then turning northeastward across south central NH to coastal Maine, especially west of I-95 in southern Maine. ...Pacific Mountain Ranges... Days 2-3... An upper low off the Pac NW coast on Friday will direct a moisture plume into NorCal and the OR Coast ahead of a cold front that will slowly but steadily sink southeastward this weekend beneath a very vigorous upper jet (200-250mb winds >175kts which is outside the CFSR climo database for this time of year). Upper low is forecast to weaken and come inland into OR late Sat/early Sun with a lead southern vort max slipping into SoCal/Southwest, helping to expand precipitation into the Great Basin/Southwest. The brunt of the snowfall will be over the Sierra owing to higher moisture, favored SW/upslope flow, stronger height falls, and robust upper divergence beneath the strong jet. There, WPC probabilities for at least 18 inches of snow are highest (>60%) generally above about 6500ft or so over the 48-hr period. Lower snowfall amounts exist farther north, away from the best IVT flux, across the Cascades and into the Blue Mountains. Into D3, pre-frontal-forced snowfall will continue into the Great Basin while snowfall associated with the upper trough continues, but wanes, over the Sierra. SW flow aloft will generally favor some of the Utah ranges but much moreso into the San Juans and CO Rockies owing to more orthogonal flow. Snow levels will be fairly high ahead of the front (7000-8000ft) limiting snowfall over the Mogollon Rim, but snow levels will lower post-FROPA to about 5000-6000ft. WPC probabilities for at least 6 inches of snow are >50% mostly over the CO Rockies on Sunday and above 8000ft or so. ...Northern Rockies, Northern Plains, Upper Midwest... Days 2-3... Impressive shortwave energy embedded within the broad longwave trough moving across the Western U.S. this weekend will lead to a strengthening surface low over eastern Idaho than then moves into Wyoming by Saturday evening. This low pressure, combined with building Canadian high pressure to the northeast, will lead to fairly anomalous easterly winds and drive an impressive upslope snow event and all the ingredients are coming together for a potential significant winter storm from the northern Rockies to the Northern Plains this weekend. Confidence continues to increase in this scenario and trends in the latest ensemble guidance, deterministic grids, and probabilistic data are upward. The latest WPC snow probabilities for Day 1 (00Z Fri-00Z Sat) show low chances for 6" across the far Northern Rockies in northwest MT but by Saturday / Day 2 those 6" snow probabilities grow substantially to above 50 percent across a large portion of the Northern Rockies for elevations generally above 6000 ft and eastward across much of central/eastern Montana. By Day 3, another embedded shortwave trough swings through Colorado and induces another area of low pressure in the lee of the Rockies. This low will deepen rapidly as it lifts northeast toward southern Minnesota by Monday. Impressive and broad forcing for ascent and higher moisture flux into the region will produce a longer duration snowfall event across the Northern Plains into portions of the UPper Midwest through Sunday evening (and beyond into Day 4). For Day 3 (00Z Sun-00Z Mon), there is a large swath of moderate to high (40-70 percent) probabilities for 6 inches across eastern MT through the Dakotas into southern/central Minnesota. Fracasso/Taylor ***Key Messages for Northern Plains Winter Storm*** -- Impactful winter storm likely Confidence continues to increase that a large storm system will produce an area of heavy snow over the Northern Plains into the Upper Midwest along with some areas of mixed precipitation this weekend into early next week. -- Widespread heavy snow possible Heavy snow is expected to overspread eastern Montana late Saturday, then expand into the Northern Plains and Upper Midwest by Sunday night. There is a high chance (>70%) of at least six inches of snow from the North Dakota/South Dakota border eastward into Minnesota and northern Wisconsin. --Significant impacts due to snow and wind A combination of heavy snow and gusty winds will likely produce areas of blowing and drifting snow along with low visibilty. Travel may become hazardous late Saturday into Monday due to falling snow, with continued blowing snow into Tuesday. --Forecast changes anticipated Uncertainty remains with the timing and location of the storm track and precipitation type which will affect where the most significant impacts will occur. Keep up-to-date with the latest forecasts as the storm evolves.