Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 321 PM EDT Tue Mar 18 2025 Valid 00Z Wed Mar 19 2025 - 00Z Sat Mar 22 2025 ...The West... Days 1-3... Assessment of the latest WV satellite indicates our deep upper trough located just north of the Four Corners with primary ascent pattern located in-of far eastern UT, southern WY, and the western half of CO. Radar trends have materialized as previously forecasted with scattered bouts of snow showers/squalls located within the above areas due to the combination of robust mid-level ascent and the progression of a surface cold front propagating eastward out of the Great Basin through the Inter-Mountain West with the current surface analysis indicating the position over western CO and NM. Elevations above 5500' MSL will be the greatest beneficiaries of heavier snowfall totals with the latest WPC probabilities for at least 6 inches located over the interior of CO with >50% probs located in those higher terrain locales along and west of the Central Rockies. When coupled with the snow that has already fallen, totals exceeding 12 inches will be plausible, especially above 9500' MSL. As this trough moves into the Plains later D1, brief ridging blossoms across much of the West bringing an end to precipitation. However, this respite will be relatively short lived as yet another deepening trough approaches from the Pacific and moves onshore Wednesday night. Height falls, PVA, and LFQ diffluence ahead of an accompanying jet streak will all provide deep layer lift, aided by upslope ascent as the low-to-mid level flow south of the trough becomes more zonally oriented. This overlap of zonal/broadly confluent mid-level flow beneath the upper jet streak will pivot IVT exceeding 250 kg/m/s onshore, leading to an expanding plume of precipitation beginning Wednesday night /late D2, and continuing into D3. Snow levels during this period will be rather low, starting around 2000-2500 ft, and warm only slowly, reaching 3500-4000 ft by the end of the forecast period. This suggests that pass-level impacts are likely, and where the most intense ascent occurs, even the lower valleys could receive some light snow accumulations. WPC probabilities for heavy snow exceeding 4 inches on D2 reach above 70% from the northern CA terrain northward through the Cascades and into the Olympics. On D3, moisture shunts north and east just a bit, causing a wane in precip over the Sierra, but expanding WPC probabilities for more than 4 inches from the Olympics and Cascades east as far as the NW WY ranges and CO Rockies where they exceed 70% in the higher terrain (especially in the Cascades, Tetons, and Park Range of CO). Weiss/Kleebauer ...Central High Plains through the Great Lakes... Days 1-2... ...Significant storm system to bring blizzard conditions to portions of the Central Plains and Upper Midwest. Key Messages are in effect for this event... This system will emerge in response to an amplifying mid-level trough which will be crossing the Great Basin and approaching the Four Corners starts to start the period /12Z Tuesday/. This trough will continue to deepen in response to a sharp shortwave trough and accompanying vorticity lobe swinging through its base, such that by the time it tracks into the Plains Wednesday, 850-700mb heights fall below the minimum within the CFSR climatology according to NAEFS, with 500mb heights dropping below the 2.5 percentile. This is indicative of an exceptionally strong system producing intense synoptic ascent. While height falls and PVA will drive a considerable portion of the deep layer lift, a collocated subtropical jet streak intensifying downstream and gaining rapid latitude (arcing poleward) will place impressive LFQ diffluence atop the region of greatest height falls. This setup will support lee cyclogenesis with rapid intensification, and NAEFS mean MSLP progs suggest that by 12Z Wednesday, this event has a 75-90% chance of reaching an extreme minimum pressure. Intense low pressures in March are not exceedingly rare, but this one will likely result in a wide swath of heavy snow and strong winds, producing blizzard conditions across the Plains and Upper Midwest. While there is some uncertainty still among the various model clusters as to the exact temporal and spatial evolution, confidence is high that after some heavy snow in the Rockies D1, a swath of blizzard conditions will develop farther east. This will result in heavy snow developing first across KS/NE late Tuesday night/early Wednesday morning, and then shifting northeast through Wednesday across IA, SD, MN, WI, and finally the U.P. of MI. During this time, the robust moisture funneling northward on 300K isentropic ascent will rotate into a strong TROWAL, helping to support elevated instability and increased moisture. Where the TROWAL is most impressive and lies atop the strongest deformation from eastern NE through northern WI, cross-sections continue to indicate a risk for CI and thundersnow. This will support snowfall rates that have high chance for exceeding 1"/hr and 2"/hr (90% and 60% chance, respectively, from the HREF), which is supported as well by the WPC prototype snowband tool. Where CI occurs, rates could reach 3"/hr, and this will be more than sufficient to overcome hostile antecedent conditions. While some snow may initially melt, snow accumulation will likely become rapid as SLRs rise due to cooling despite dendritic fracturing on winds as high as 60 kts below the DGZ. This will produce blizzard conditions, and the WSSI-P is forecasting a 90% chance of moderate impacts, and 20% chance of major impacts, even where snow amounts are more modest due to the speed of this system. As far as snowfall amounts, WPC probabilities have increased, and while are modest D1, are high (>70%) D2 for more than 6 inches from far eastern NE through a continuous, but narrow, corridor as far north as the central U.P. of MI. In this corridor, WPC probabilities for 12+ inches have increased to as high as 30-50%, highest from NW IA through the U.P., and WSE plumes indicate a potential for up to 15" in some areas should this band translate temporally over some areas a bit longer. Some higher snow totals are also possible in the Huron Mountains of the U.P. where the deformation axis and some lake enhanced snowfall combine. By D3 this entire system shifts well east as the primary low moves into Ontario and the accompanying cold front surges into the Mid- Atlantic states. While some wrap-around snowfall will likely continue through early Thursday, especially where lake enhancement can occur across the U.P. and northwest L.P. of MI, as well as S/E of Lake Michigan, additional accumulations are expected to be modest, but have a low chance (10-30%) of exceeding 4 additional inches in the U.P. and far NW lower Michigan near Traverse City. Mullinax ...Interior Northeast... Day 3... Guidance has backed off on the potential across the Interior Northeast, in-line with the current ML output with subsequent prob fields degraded compared to previous forecast. The threat for surface cyclogenesis is still prevelant off the coast of Southern New England with most of the deterministic running between 990-994mb as of 12z Friday. Despite deepening within the remainder of the Friday window, the progressive nature of the precip and marginal thermals currently depicted may limit the greatest threat for at least 4" to the highest elevations in Northern New England (Green and White Mountains) with very little chances in the valleys. Threat for at least 2" is still running between 30-50% for the Adirondacks and 50-70% for the Green/White Mountains, so the threat is not muted by any stretch. Considering the potential hovering near the threshold for impactful snowfall across the Interior Northeast, will maintain close assessment of any trends as we move through mid-week. Kleebauer ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png