The winter is ending with a grand finale. Two separate systems, one over the Midwest U.S. and another one in the U.S. Deep South, will partially phase and form a somewhat stronger albeit elongated storm by tomorrow. As this happens, the northern system will siphon moisture from the much more moisture-rich southern system, leading to a much bigger area of moderate to heavy precipitation. With arctic air ahead of the northern system, precipitation will fall as snow across almost our entire area, except for southern Vermont to southern Maine, where the southerly flow will be strong enough to cause snow to change to ice pellets, freezing rain, or even rain, substantially cutting accumulations. The heaviest snow accumulation will occur in Maine, where more phasing of the two systems will have occurred by then.
Amazingly, there is still considerable disagreement on the amount of precipitation and therefore snowfall will occur. The Canadian models (GDPS, RDPS, and HRDPS) has persistently showed less snow with moisture not getting as far north, whereas the HRRR and NAM 3-km shows substantially more. This shows that the evolution is very
sensitive to small details in what happens today with both systems and
how they interact starting tonight. Where is the fork in the road? One clue might be the differences already present by late this morning with the northern system: the HRRR has a band of snow that is stronger, a bit farther north, and oriented more southwest to northeast than the HRDPS, indicating a stronger northern system that can back the flow to more southerly near the U.S. East Coast and siphon more moisture from the southern system. The radar shows that the reality is probably in between the two models, not surprisingly.
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| Source: Aviation Weather Center |
There is also the uncertainty of snow to liquid ratios. With no strong consolidated low pressure system, very heavy frontogenetic bands with extreme snowfall rates are not expected. Instead, high snowfall totals will come from subtropical moisture being forced to overrun over the arctic air mass in place, with the dendritic growth zone being quite high in the atmosphere, perhaps above the zone of greatest ascent (at least where the precipitation is the heaviest), and there being winds aloft of 60-70 kt. Such a setup isn't conducive to high snow to liquid ratios, despite subfreezing temperatures throughout the column, including right at the surface, leading to no melting. I would guess a snow to liquid ratio of 10-12:1 in most places, but lower to the south near the mixing zone and higher to the north where there will be less moisture and colder temperatures. The WPC 50th percentile snowfall forecast is shown below and is probably a reasonable guess, though I suspect it is a little too high in general, especially on the northern edge. Still, many places along the axis of heaviest snow from eastern Vermont to central and northern Maine will likely receive 12-18" (30-45 cm) of snow, with less to the southwest over northwestern New York due to the phasing occurring a little too late for that area. Southern Quebec will get grazed on the northern edge of the storm, with little or nothing northwest of the St. Lawrence Valley.
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| Source: Weather Prediction Center |
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| Source: TropicalTidbits |
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