Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 347 PM EDT Mon Mar 13 2023 Valid 00Z Tue Mar 14 2023 - 00Z Fri Mar 17 2023 ...The West... Days 1-3... An active period of weather continues across the Western United States as yet another in this train of atmospheric rivers (AR) moves onshore today and Tuesday. This AR will extend from the Pacific Northwest through central CA, with an elongated surge of PWs lifting onshore D1 into D2, although the highest IVT, progged by CW3E to exceed 500 kg/m-s by increasing GEFS probabilities, and potentially reaching 750 kg/m-s, will lift into CA. This pronounced moisture will be driven by a combination of confluent mid-level flow from the Pacific, subtle shortwaves embedded within this flow moving onto the coast, and persistent, albeit modest, upper level Pacific jet energy arcing northeast into the Pacific Coast and Intermountain West. With the general zonal flow from the Pacific plaguing the region, snow levels will be generally modest at 3000-4000 ft north, and 6000 ft south, but will climb within the most impressive WAA to above 8000 ft in CA late D1 into D2, with these higher snow levels spreading well eastward towards the Central and Northern Rockies. However, the embedded shortwaves will help push a cold front eastward from the Pacific Northwest D1 into the Great Basin D2 and then as far east as the High Plains by D3, with the accompanying cold advection and lowering heights crashing snow levels back down to 1000-2000 ft north, 3000-4000 ft south. However, the most intense precipitation and associated snowfall should occur ahead of this front during the period of higher snow levels. For D1-D2, WPC probabilities for more than 6 inches are high across much o the terrain from the Sierra eastward across the Great Basin and into the Central/Northern Rockies, with local maxima likely in the SIerra, Sawtooth, Wind River/Teton area, and Uintas. Locally 3-5 feet of heavy and wet snow is expected in the Sierra, with 1-3+ ft possible in these other ranges. Late D2 into D3, the best forcing shifts east collocated with the decaying AR shifting into the Four Corners. This will produce heavy snow that is more confined spatially from the Absarokas southward through the Big Horns, much of the CO terrain, and southward through the Wasatch and Sangre de Cristos. WPC probabilities are high for 6+ inches on D3 across these areas, with locally 1-2 ft possible in the higher terrain. ...Central Plains to Upper Midwest... Day 3... Northern and southern stream energy is progged to interact across the Intermountain West Thursday morning and amplify into a full latitude trough as it moves into the Plains by the end of D3. Height falls and modest PVA will work in tandem with a collocated and intensifying coupled jet structure to produce pressure falls in the lee of the Rockies, resulting in a surface low development across eastern Colorado. This pattern seems to support a surface low that will rapidly deepen as it shifts northeast across the Plains on D3, with increasing meridional moisture transport surging into cold air to the north to support a swath of heavy snow. While the models are in pretty good agreement that this will occur, there remains considerable spread among the various ensemble members as to the exact track, strength, and speed of this low, bringing lower than typical confidence for heavy snow on D3. However, there is likely to be at least a stripe of moderate snow across the Central Plains D3, with more impressive snowfall possible into the Upper Midwest as the low wraps up into D4. Current WPC probabilities indicate a large area with a low chance for more than 4 inches stretching from eastern Montana through Nebraska and as far east as the Arrowhead of MN. It is likely these probabilities will rise as the event draws closer, and an impactful winter event is possible for this area later in the week. ...Mid-Atlantic and Northeast... Days 1-2... ...Major nor'easter for the interior Northeast...Continued uncertainty along the immediate coast... The guidance has become much better clustered with the track of a rapidly deepening coastal low which will lift along the Mid-Atlantic coast tonight and become a powerful nor'easter east of New England on Tuesday. Despite better clustering, uncertainty still exists among the details of the evolution due to the low likely becoming captured by a strongly negatively tilted 500mb trough digging south of Long Island, which will result in the surface low either stalling or retrograding back towards Massachusetts late Tuesday and Tuesday night, before pivoting again to the east on Wednesday. The farther west this low tracks the warmer the environment will be across New England, and for some areas this will be the difference between a mix of rain and snow, or heavy snow with exceptional snow rates. However, well inland, especially in elevated areas, will experience the most impactful winter storm of the season. The setup is nearly classic for a major nor'easter across New England and Upstate New York with a retrograding/stalling surface low east of New England. The potent closed low digging southeast from the Great Lakes will deepen to nearly -3 standard deviations below the mean climatological 500mb heights, while a strong lobe of vorticity shedding northeast Tuesday morning. This secondary vorticity impulse will help further negatively tilt the upper trough, which when combined with robust LFQ diffluence of a jet streak arcing over the Southeast, will result in rapid deepening of the surface low, potentially undergoing bombogenesis east of New England. The guidance overnight has backed up the surface track a bit NW, which then brings the low very close to Boston, MA Tuesday evening, and some guidance actually retrogrades the low onshore briefly over eastern MA before it shifts back to the east on Wednesday. During this evolution, moisture will become quite anomalous noted by PW anomalies reaching +1 standard deviation above the climo mean, fueled by moist 290-295K isentropic ascent with mixing ratios of 3-4 g/kg. Additionally, an 850mb jet north of the low is progged to reach -4 standard deviations with respect to U-wind (east), driving intense Atlantic moisture advection onshore. This indicates that there will be plentiful moisture for which this system to ingest, and both QPF and snowfall will be impressive. The biggest challenge for this event will be forecasting the low-level thermal structure. Ahead of the low, there is generally a lack of arctic air, with surface temps a few degrees either side of freezing. Additionally, by mid-March the sun angle and warmer ground temperatures will at least somewhat inhibit accumulations, especially during the daylight hours. Of course, this can be overcome by heavy snow rates, which look likely in this event. The same theta-e ridge being driven northward by the isentropic upglide will lift into a robust TROWAL overhead New England late Tuesday and Tuesday night, and this will provide enhanced instability aloft. Within this environment, the setup appears contextually to support a pivoting deformation band, potentially multiple bands, with one over eastern Upstate NY early in the event, and a secondary band developing later across southern/central New England also pivoting eastward with time. There may also be a third band along and just northwest of the coastal front lying along the ME/NH/MA coast. However, model trends to the N today and a more intense dry slot rotating around the low and onshore New England will act to somewhat inhibit snow amounts both on the SW edge of the swath, as well as the NE. Identifying the exact placement of these bands is challenging at best at this time range, but enhanced mesoscale ascent through fgen/deformation beneath -SEPV noted in cross sections supports a high likelihood of CSI and even upright convection for thundersnow within banded structures. Where these bands occur, snowfall rates will likely exceed 2"/hr, possibly reaching 3-4"/hr in thunder, as noted by the WPC prototype snow band tool, and this is despite SLRs that will generally be below to well below climo in the saturated but marginally thermal environment. The combination of these intense snow rates with strong winds will likely result in an extremely impactful storm due to snow rate and snow load, which is reflected by high WSSI/pWSSI. While the exact details are still uncertain, especially outside of terrain and closer to the coast, this storm will likely produce more than 12 inches of snow in many areas as reflected by WPC probabilities that are above 80% across a large portion of upstate NY, the Berkshires, Greens, and Monadnock region of NH. While the heaviest snow will be in the higher terrain, where locally more than 2 feet is possible, the aforementioned bands should allow for heavy snow accumulations even in the valleys, and WPC probabilities for more than 4 inches encompass nearly all of New England except the immediate south coast. Additionally, as these bands pivot S/E with time, and as the ageostrophic component of the wind allows thicknesses to crash back into the low Tuesday aftn, rain will turn to heavy snow even down towards the coast, although uncertainty in when this occurs will have significant impacts on how much snow can fall. As noted above, the guidance today has trended a bit warmer near the coast limiting the snowfall potential for I-95, but there is still some threat for impactful snow there, especially between Portland and Boston. As the low pulls away Wednesday, there is some potential for enhanced OES for Cape Cod, but WPC probabilities are less than 10% for 4 inches east of the Canal before the system exits and allows clean up to begin late on Wednesday. Additionally, impressive upslope flow behind the departing system will result in heavy snow accumulations in the favored upslope region of the central Appalachians, where WPC probabilities are 30-50% for more than 4 inches on D1-D1.5. Weiss ...Key Messages for March 10-15 (Upper Midwest to Northeast) Winter Storm... --A coastal low will strengthen rapidly into a major nor'easter that significantly impacts the Northeast through Wednesday. The greatest impacts are expected late tonight through Tuesday night. --Heavy snow rates (2-3"+/hr) and strong winds will produce dangerous to impossible travel. The heavy-wet nature of the snow, combined with max wind gusts of 55 mph, will result in scattered to widespread power outages and tree damage. --Snowfall totals >12" are forecast over much of New England and Upstate New York. Localized max totals of 24-30" are possible, particularly in terrain. --Widespread minor and localized moderate coastal flooding and beach erosion are forecast tonight through Wednesday as the low pressure stalls and deepens off the coast of New England. ...Key Messages for Mid-March (Western U.S.) Winter Storms... --Heavy Rain and Snow Tonight Into Wednesday Considerable flooding impacts below 5000 feet elevation are expected to shift south across much of the California coast, Central Valley, and Sierra Nevada foothills. --Heavy Rain and Snowmelt Expected Heavy rain, combined with snowmelt in terrain below about 5000 feet to result in more widespread flooding from Tuesday into Wednesday, particularly in low elevations and areas with shallow and warming snowpack. Creeks and streams will continue to be vulnerable to flooding from additional rain and snowmelt. --Sierra Nevada Snow Load Impacts Worsen Heavy rain absorbed into the particularly deep snowpack in the Sierra Nevada along with heavy snow, measuring in feet above about 7500 feet, will further compound ongoing snow load impacts and issues.