Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 302 PM EST Thu Mar 02 2023 Valid 00Z Fri Mar 03 2023 - 00Z Mon Mar 06 2023 ...Pacific Northwest through Central California and the Intermountain West... Days 1-3... Persistent onshore and confluent flow Friday and Saturday will amplify late Saturday into Sunday in response to a shearing upper low dropping out of Alaska and moving along the Pacific Coast. While the core of this low should remain just offshore by the end of the forecast period, intensifying downstream PVA will surge onshore as lobes of strung-out vorticity shed eastward into the West. At the same time, a pronounced and intensifying Pacific jet streak, progged to reach 130+kts by Sunday, will overlap with the mid-level confluence to surge moisture into much of the West this period. The overlap of height falls, PVA, and LFQ diffluence along the upper jet will provide widespread deep layer synoptic lift to support rounds of moderate to heavy snowfall each day. Snow levels through the period will generally remain low, generally 1000-1500 ft west of the Cascades, and 500 ft or less to the east, but could surge as high as 3000 ft over CA during the period of most intense WAA ahead of the cold front and downstream of the approaching low. While this supports the heaviest snow in the terrain, with impactful snow likely at many of the mountain passes, it should also result in at least moderate accumulations in the coastal ranges, with light snow likely even in the lowlands. For D1, the heaviest snow should remain across the Olympics and WA Cascades, where WPC probabilities for more than 6 inches are above 80% and 1-2 feet of snow is likely in the higher terrain. High probabilities for 6+ inches also extend into the Bitterroots and Tetons. By D2, the heavy snow expands significantly down the Pacific Coast, with high WPC probabilities for more than 6 inches encompassing most of the terrain above 1500 ft from the Olympics and WA Cascades southward through the northern CA ranges and Sierra. The low snow levels will also support accumulations into the coastal ranges and even some of the lowlands near Portland, OR, although confidence is lower here. By D3, the focus shifts to the Shasta/Trinity/Siskiyou region and the Sierra where probabilities for more than 12 inches reach 70-80%, and another major snow event is likely for the Sierra. Additionally on D3, some moisture spillover should result in heavy snow accumulations into the Wasatch Front and Uintas of UT. ...Upper Midwest into the Great Lakes and Northeast... Days 1-3... A fast moving dual-low pressure winter storm will spread heavy snow and some mixed precipitation across portions of the Great Lakes and Northeast through Saturday. The primary driver of this system will be an anomalously strong closed mid-level low moving across TX tonight with height anomalies progged by the NAEFS ensemble tables of -3 to -4 standard deviations below the climo mean. This low will move slowly across TX before beginning to eject more rapidly to the northeast on Friday, reaching the Great Lakes Friday night. This feature will then begin to weaken D2 as it shears out into the westerlies, with a lobe of vorticity shedding beneath it to the east and moving south of New England during the day on Saturday. Aloft, a robust subtropical jet draped across much of the country will intensify to 190kts over New England and split downstream from the primary trough axis, while a secondary jet streak rotates through the trough to produce efficient and impressive coupled jet streaks resulting in strong UVVs collocated with the best mid-level divergence and height falls. This will result in a surface low pressure moving out of TX and then rapidly deepening as it shifts into the OH VLY likely reaching maximum depth Friday evening before beginning to slowly weaken in response to the shearing upper shortwave. Downstream of this low, meridional moisture advection will surge from the Gulf of Mexico, resulting in PW anomalies reaching as high as +3 standard deviations from the OH VLY into the Mid-Atlantic, with the associated strong 285-290K isentropic ascent pushing the theta-e ridge into an impressive TROWAL late Friday into Saturday. While the guidance continues to feature more than typical spread, the trends have been for a slightly more suppressed precipitation shield as the high pressure over Canada trends a bit stronger. This should result in two distinct areas of heavy wintry precipitation with this event. The first will be in a swath from the Upper Midwest through the Great Lakes, on an increasingly intense deformation band that will likely translate along the NW edge of the precipitation. This band will likely have a sharp cutoff on the NW side due to dry air from the Canadian high to the north, but should also produce intense snowfall rates to accumulate rapidly despite its progressive nature from SW to NE. This band is likely to become intense due to its position beneath the TROWAL and favorable overlap of deformation/fgen, especially before the system begins to weaken thanks to the ageostrophic flow into the low helping to producing stronger CAA to sharpen the baroclinicity. Cross sections suggest some -SEPV above the most strongly sloped fgen, especially across lower MI, which could result in CSI and even upright convection in isolated locations. The marginal thermals will likely result in some areas changing from rain to heavy snow, but the rapid transition during this band combined with dynamical cooling should still result in efficient accumulations despite modest SLR in the moist environment. The combination of intense snow rates, which could reach 2-3"/hr or more at times as shown by the WPC snowband tool, with strong winds, will produce major impacts late Friday into Friday night across this region. The axis of heaviest snow will likely shift a bit more as the event draws closer, and there is still concern about how far north the rain/snow line will get and how much rain, especially in the lower Great Lakes, can cut down on snow accumulations but WPC probabilities for more than 4 inches are above 40% near Chicago where some enhanced lake moisture transport could produce higher accumulations, and 70+% in lower Michigan where, depending on the band placement, double-digit snowfall is possible in some areas as noted in still wide spread in the WSE plumes. Farther to the east, the aforementioned strong WAA driving the meridional moisture transport will be most intense and lead to an expanding precipitation shield from the Mid-Atlantic through Upstate New York and much of New England. There is high confidence that the associated warm nose will cause a p-type change from snow to rain across PA, as well as parts of Upstate NY and southern New England, but exactly how that evolves remains uncertain. The challenge to this part of the forecast will be the rate at which that secondary vort lobe shears eastward to help drive coastal low development south of New England late Friday night into Saturday morning. The faster this transfer occurs, the more cold air can get locked into the Northeast, with again the ageostrophic drainage out of the Canadian high pressure helping to cool the column, but if this occurs more slowly the warm nose may pivot much farther north. Additionally, where this energy transfer leaves at least a modest inverted trough beneath the maximum theta-e ridge, a prolonged period of heavier snow is likely. At this time, the best chance for significant accumulations continues to focus across eastern Upstate NY into central and parts of northern New England where the leading WAA snow will transition to a colder snow with time, but never changeover, and without any break in falling precipitation. Higher elevations should also do well in this event, especially north and east of the Catskills, and current WPC probabilities indicate a greater than 90% chance for 6+ inches of snow focused across central New England and into the Adirondacks, with some areas likely receiving around 1 foot of snow. On the southern edge of this heavy snow, a region of moderate sleet and freezing rain is likely, especially in some of the terrain from the Laurel Highlands through the Catskills but changeover and heavy rates should overall limit ice accretions. Still, WPC probabilities for more than 0.1" reach as high as 30% in some of this area. By Saturday night the system should pull away into the Atlantic with rapid dry advection occurring in its wake to bring an end to precipitation in most areas. ...Northern Plains... Day 3... Strung out vorticity advecting from the Pacific coast will interact with the LFQ of a Pacific jet streak angling onshore to produce increasing lift across the Northern Plains on Sunday. This setup will also support some lee cyclogenesis across CO the latter half of D3, with downstream moist advection lifting across the MS VLY and into the Central and Northern Plains. At the same time, a cold front will be digging out of Canada in advance of a cold high pressure centered over the Northwest Territories, with increasing fgen beneath the upper jet supporting a stripe of moderate to heavy snowfall developing from west to east. There is quite a bit of latitudinal spread as to where this band of snow will setup, primarily due to timing differences of the cold front and how it will interact with the northward surging moisture. However, confidence is high that this swath of snow will develop somewhere across the area, and forecast soundings suggest an environment favorable for above-climo SLR to produce efficient accumulations. Overall the forcing looks to be modest and transient, but current WPC probabilities indicate a 40-70% chance of at least 4 inches for much of southern ND. Weiss ...Key Messages for Plains to Northeast Winter Storm... --A potent winter storm will produce possibly narrow bands of heavy snow and mixed precipitation, including sleet and freezing rain to the south of the snow, from the north-central Mississippi Valley through the Great Lakes and central New England Friday through Saturday. --The combination of heavy snow rates exceeding 1"/hr and strong winds would result in hazardous travel as well as impacts to infrastructure. --Uncertainty remains with the location and strength of the snow swaths over the Midwest/Great Lakes as well as the areas of greatest impact over the Northeast. Keep up to date with the latest forecasts as this storm evolves.