Plain-language summary:
A major and unusually slow and unpredictable Nor'easter will produce very heavy snow totals over the higher terrain of central New England and the eastern Adirondacks, with a sizable snowstorm elsewhere south of the U.S./Canada border except for northern Maine. Snow will be wet especially in lower elevations. Another storm will move faster and track farther north Friday into Friday night, leading to snow in Ontario and Quebec and rain farther south. The weather pattern beyond that still appears favorable for snow, albeit not particularly cold with the inevitable more marginal surface temperatures in late March.
Meteorological discussion:
A major but somewhat unusual Nor'easter is forming and will rapidly intensify tonight into tomorrow. Due to most of the intensification occurring offshore, convection over the Gulf Stream, and two surface low-pressures interacting with each other, models have had an incredibly difficult time trying to simulate what will happen. Several days ago, it looked like all of northern New England would get a big snowstorm with a more traditional Nor'easter, but then it trended southwestward due to more ridging and blocking developing to the north with an earlier maturation and occlusion. Today, it has trended back northeast at the last minute, almost to where it was expected several days ago. I think we finally have a good idea of where the heaviest precipitation will occur, which will be in eastern New York to central and southern New England. Significant precipitation will also spread to near the U.S./Canada border from the northern Adirondacks to northern New Hampshire and then into central Maine. Snowfall will still be tricky and will come down to nowcasting due to marginal surface temperatures and resultant dependence on precipitation rate and time of day (due to the strong March sun during the day).
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| Source: TropicalTidbits |
Moderate rain at lower elevations and moderate snow at higher elevations is beginning in southern New England and will spread to central and most of northern New England later in the night. This is due to an elongated inverted surface trough on the east side of a negatively tilted upper-level trough that is about to cut-off, pulling Atlantic moisture into New England. North of the Vermont/Massachusetts border, it might rain initially at lower elevations, but dynamic cooling will cause a quick changeover to snow for everyone, albeit a wet snow that mostly sticks to non-paved surfaces in lower elevations. With the absence of the sun, snow will have little trouble accumulating on non-paved surfaces. The heaviest snow will occur close to the inverted trough, from eastern New York into southern and central Vermont, especially in higher terrain where it will be colder and in eastern facing slopes that will receive upslope enhancement. However, by tomorrow morning, a new surface low will strengthen south of New England and cause the inverted trough and its associated snow to weaken. Instead, a heavy curved snow band will develop in eastern New England and rotate northwestward into New Hampshire and southern/central Maine by tomorrow afternoon as the storm first gets thrust northwest to just east of Boston by the upper-level low. Outside this band, late morning and afternoon snow accumulations will be minimal due to marginal surface temperatures and the March sun angle. Then, the band will weaken over Vermont and stall out tomorrow night as the storm vertically stacks, stalls, and occludes just east of Boston. The northerly flow on the storm's back side, combined with the stalled band, could lead to some enhanced snow in the eastern Champlain Valley and western Green Mountains. It will also cool down tomorrow night as somewhat colder air gets entrained from the north and northwest, leading to a somewhat drier snow. Eventually, the whole vertically stacked storm will slowly weaken while moving slowly eastward through late on Wednesday, when any lingering snow showers will dissipate.
The higher terrain of central and southern Vermont and southwestern New Hampshire look to get the jackpot, receiving heavy snow from both rounds, with 24-36" (60-90 cm) expected. 12-24" (30-60 cm) is expected for higher elevations of the eastern Adirondacks through central New Hampshire, with 8-12" (20-30 cm) for lower elevations. Much less snow will fall in northern Maine and north of the U.S./Canada border farther west, due to the ridging to the north causing the cut-off low to slide too far south, taking the precipitation with it. However, some light snow has fallen in eastern Ontario from the weak parent Great Lakes storm, and light snow from both rounds will penetrate into at least far southern Quebec. The snow will generally be wet (lower than 10:1 snow-to-liquid ratio), except in the highest elevations and also with any snow that falls tomorrow night, when temperatures will be slightly colder.
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| Source: PivotalWeather |
Despite the slow-moving nature of the storm, the upstream pattern, with one trough digging into Hudson Bay and the other one into the central Rockies, will cause systems to move much more quickly by Thursday. The Hudson Bay trough will be accompanied by a surface low pressure passing well to the north, perhaps leading to light rain or snow north of the U.S./Canada border. The Rockies trough will lead to a strengthening storm ejecting from the Rockies into the Plains and then into the Great Lakes. This more familiar track we saw most of the winter will lead to southerly winds and mostly or all rain south of the U.S./Canada border, but with snow, possibly moderate to heavy at times, from Ottawa to Montreal and northward into the Laurentians. This will occur Friday into Friday night. However, the storm will be weakening and occluding as it moves east from the Great Lakes, leading to lessening precipitation amounts in Maine. The occlusion will also prevent the truly warm air mass from reaching northern New England. A strong upper-level trough follows just behind, leading to colder air and widespread snow showers, especially in higher elevations and western-facing slopes.
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| Source: TropicalTidbits |
The overall weather pattern continues to not look particularly cold with little true Arctic air penetrating into New England, but the southward shift in the storm track that started in late February and continued modest blocking likely mitigating storm tracks into Ontario and Quebec will lead to more opportunities for snow. It will be late March however, so snow accumulations will be precipitation rate and elevation dependent due to marginal surface temperatures, unless there is a strong Arctic air mass in place.
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| Source: TropicalTidbits |
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