Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 413 AM EST Fri Mar 03 2023 Valid 12Z Fri Mar 03 2023 - 12Z Mon Mar 06 2023 ...Pacific Northwest through Central California and the Intermountain West... Days 1-3... A trough over the Gulf of Alaska amplifies as it tracks southeast, closing into a sprawling low late Saturday centered off the WA coast where it stalls through Sunday. While the core of this low will remain just offshore into or through Monday, intensifying downstream PVA will surge onshore as lobes of vorticity shed eastward into the West. At the same time, a pronounced and intensifying Pacific jet streak, progged to reach 150+kt 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 left jet exit 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 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. On Day 1, WPC snow probabilities for >6" are above 80% in the WA Cascades and Olympics. The heavy snow focus shifts south Saturday as the leading trough axis comes ashore over far northern CA with Day 2 probabilities for >8" high over the Klamath/CA Cascades and Sierra Nevada. 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 given the lower precip focus under the upper low. Similar areas have high probabilities for >8" again on Day 3 with 1 to 3 (locally 4+) feet over 48hrs in the higher terrain. ...Upper Midwest into the Great Lakes and Northeast down into the Mid-Atlantic... Days 1-2... 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. A deep and rapidly developing low pressure system over North Texas will lift northeast today and cross the Midwest this evening before weakening as it develops a coastal low over Long Island tonight. Ahead of this low a surge of Gulf moisture, 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 this afternoon into Saturday. 00Z guidance has come into much better consensus on the snowband potential with heavy snow now expected from northeast IL (generally south from Chicago), northern IN, and over much of southern/southeastern MI. This Midwest/Great Lakes snow swath will develop on an increasingly intense deformation band translating along the NW edge of the precipitation shield. 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 (and this being the afternoon in March). 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/thundersnow. 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 this afternoon into tonight across this region. Day 1 snow probabilities are now moderately high for >6" over northern IN and high over southeast MI. There is a 20 to 30% risk for a max of a foot of snow over southeast MI into the Thumb which will occur this evening with nocturnal trends aiding accumulation. 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 p-type changes from snow to wintry mix to rain across PA and 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/The Adirondacks 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 Day 1.5 WPC snow probabilities indicate a greater than 90% chance for >8" focused across the Adirondacks, and north-central New England including the Greens and Whites to the Merrimack Valley and the southern Maine coast, with some areas likely receiving around 1 foot of snow. South of this heavy snow, a region of moderate sleet and freezing rain is expected, especially in some of the terrain from the Potomac and Laurel Highlands through the Catskills where pockets of freezing rain could become significant, particularly in the typical cold air damming max north from around Frostburg, MD through the Laurels where is a 20% risk for >0.25" ice glaze. By Saturday evening the coastal low will have pulled far enough into the Atlantic with rapid dry advection occurring in its wake to bring an end to precipitation over New England. ...Northern Plains to Upper Midwest... Day 3... A lobe of vorticity ejecting from the Pacific coast will be aided by the left exit regions of the Wly Pacific jet streak over the Intermountain West to increase lift across the Northern Plains north of a lee-side low on Sunday. Moist advection ahead of this low lifts up the MS VLY and into the 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. 00Z guidance has come into better consensus on the snow swath being located near or north of the SD/ND border, stretching east over north-central MN into northern WI. Forecast soundings suggest an environment favorable for above-climo SLR to produce efficient accumulations with Day 3 WPC snow probabilities indicate a 40-70% chance of >4" for much of ND and north-central MN with enhancement off Superior along the North Shore in MN. Jackson ...Key Messages for Midwest through Northeast Winter Storm... --A potent winter storm will produce swaths of heavy snow over the Midwest and northern New York and New England today into Saturday. Significant sleet and freezing rain is possible south of the heaviest snow. --For areas over the Midwest, heavy snow rates of 1-3"/hr are possible, which when combined with gusty winds will produce dangerous to potentially impossible travel. The heavy wet snow could also result in scattered power outages. --Across the Northeast the combination of heavy snow rates exceeding 1"/hr and gusty winds will likely lead to blowing snow which will limit visibility and produce difficult travel. --Freezing rain and sleet are likely for portions of the Mid-Atlantic states and into southern New England as well. This could produce slippery travel and power outages.