Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 321 AM EDT Tue Mar 18 2025 Valid 12Z Tue Mar 18 2025 - 12Z Fri Mar 21 2025 ...The West... Days 1-3... The period begins with an amplifying mid-level trough diving across the eastern Great Basin and into the Four Corners, pushing a cold front eastward into the Rockies. This front will have the potential to produce scattered convective snow showers and snow squalls this morning across UT/CO/WY as its forcing driven by 0-2km fgen moves east into a region of at least modestly unstable air (100-300 J/kg of SBCAPE) and increased low-level RH. While snow accumulations from these squalls will be minimal, brief heavy snow rates and gusty winds could produce dangerous travel through the day. Behind this front, remnant low to mid level fgen and increased moisture will drive a swath of heavy snowfall across much of the CO Rockies and northward into WY and the Black Hills of SD. Large scale ascent through the height falls downstream of the parent trough combined with this fgen will allow for some heavy snow rates, especially into the terrain where upslope flow will additionally produce lift. WPC probabilities D1 for more than 6 inches are high (>70%) across the Wasatch, CO Rockies, and much of the terrain of WY where locally more than 12 inches is possible, especially in the Big Horns. Where the most intense fgen is progged, primarily along the MT/WY border (a bit north from previous model cycles), some lower elevation snowfall is likely which could accumulate 3-6" into the High Plains of southern MT. 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). ...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. ...Interior Northeast... Day 3... Guidance has trended stronger with a developing low pressure Thursday evening across the northern Mid-Atlantic states. While there is still considerable uncertainty at this time range in the strength of this system, most ensemble systems indicate a rapidly developing low pressure between 00Z Friday and 12Z Friday lifting across southern New England. The synoptic setup is favorable for this development. The same trough responsible for the Plains/Midwest blizzard will continue to shift east, and while the primary shortwave will lift into Canada, a secondary southern impulse is progged to deepen over the Central Appalachians. This vorticity lobe will help split the jet stream, leading to an increasingly coupled pattern as the subtropical jet streak pivots over the Carolinas and the polar jet stream lifts towards Greenland. The LFQ of the former and the RRQ of the latter will overlap the greatest height falls, indicating a strong likelihood of low development along a surface cold front/baroclinic gradient. By 12Z Friday, the deterministic global models prog SLP to be sub 990mb, and this rapid intensification within the synoptic setup will likely cause a strong deformation axis to develop NW of the surface low. While the antecedent column is marginal for wintry precipitation, strong ascent in the vicinity of this deformation will result in rapid dynamic cooling, and rain changing to snow, possibly heavy, across Upstate NY and interior New England. Being 6th period, confidence in amounts and placement is still modest, but a band of heavy snow is becoming more likely, with significant accumulations possible, especially in higher terrain. This is reflected by current WPC probabilities that are above 50% for 2 inches and 10-30% for 4 inches, highest in the Adirondacks and Catskills. Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png