Expanded Summary:

The first significant winter weather event of the year will impact much of New England tomorrow, though some of the details still need to be ironed out. A relatively broad region of 6-12″ snowfall totals is expected to extend from the northern Worcester Hills across much of New Hampshire and into portions of western Maine. Region-wide, snow will be very wet and heavy, increasing the risk of power outages where snowfall totals exceed 6″.

‍Along the coast – particularly in Massachusetts – there is considerable uncertainty about how much snow will fall due to questions surrounding the eastward progression of the rain/snow line and the precipitation rate post-transition. As it stands, I would forecast an 80% chance of snowfall totals between 3″ and 5″ in much of metro Boston, with a 10% chance that totals fall below that range, and a 10% chance that totals exceed that range.

Additionally, the snowfall gradient on the western fringe is also not clear. I am currently forecasting a gradual decrease in amounts west of I-91. In reality, dynamics storms can feature sharp cutoffs, and this one should be no exception, with a sharp gradient of rapidly decreasing snow totals winding up somewhere between the I-87 and I-91 corridors.

Analysis: I am very confident that a potent storm system will impact pretty much all of New England Saturday and Saturday night. As we will walk through, confidence breaks down quite a bit on the details in a few key regions due to the marginal thermal regime and to-be-expected uncertainty regarding banding structures late in the storm.

Currently, the disturbance that will kick off our storm is apparent on water vapor imagery over Tennessee/Kentucky, and a secondary shortwave in the northern stream is digging south out of Wisconsin. As these two pieces of energy merge, they will eject eastward/northeastward off of the mid-Atlantic coast and generate a rapidly deepening surface cyclone that propagates toward the Gulf of Maine during the day on Saturday.

The antecedent airmass is quite warm – high temperatures in the Boston metro approached 60F today. Despite this, it appears the rapidly deepening cyclone should induce sufficient dynamic cooling processes to cool the atmosphere enough for snow to take over as the favored mode of precipitation. I am highly confident that this changeover will occur in the interior, where ensembles indicate sub-0C 850mb temperatures through much of the event with persistent-to-strong northwesterly flow. Though cold enough for snow, even in the interior thermal profiles will be marginal (likely 30-32F 2m temperatures at a minimum), resulting in snow with a very heavy/wet consistency. With >1″ of liquid equivalent precipitation expected over this region, storm total snowfall could be in the 9-12″ range, with locally higher amounts in the hills where snow:liquid ratios are more favorable.

At the coast, the low-level flow will initially be more northeasterly, advecting in warmer marine air off of the ~50F Atlantic waters. As the low pressure center advances, winds will become northerly and eventually northwesterly by mid-afternoon. Once this wind shift occurs, colder air should begin to drain southeastward, dragging the rain/snow line along with it. There is some question about the exact timing of this process – snowfall totals at the coast will depend on two factors: 1) how quickly the rain/snow line advances east and 2) the intensity and duration of precipitation after this changeover.

This system will be quite dynamic. Deterministic guidance indicates impressive vertical velocities, including in the dendritic growth zone – likely a signal that snowfall accumulation could be efficient where it occurs despite the marginal thermal profiles. Given the modeled lift profiles, heavy precipitation should overspread southern New England by 15Z, gradually expanding northward with time. Initially, this precipitation will be in the form of heavy rain at the coast – some location in the coastal plain could approach 3″ of storm total liquid equivalent precipitation. Inland, heavy snow may begin to develop as early as 14-16Z as the column cools, with the first flakes probably flying in the higher terrain of the Worcester Hills/Monadnock region. Heavy precipitation should persist in southern New England through 21Z, with more banded snow/rain areas persisting until perhaps 04-05Z.

One wildcard for snowfall totals in eastern MA is the potential for a comma head signature to form as the surface low deepens rapidly. A signature along these lines, certainly not a rarity in dynamic systems, appears to be implied in the H700 Vertical Velocity plots from the NAM. Global models do not key in on the signature as clearly, but it is worth keeping an eye on as more persistent snowfall in eastern MA could mean storm totals will need to be increased in the highly populated Boston-Providence corridor.

One other element to note with this system – wind. A 50kt+ northerly low level jet appears likely to evolve on the west side of the H85 low during the afternoon hours on Saturday as it deepens. At least some of this wind should mix down given the heavy precipitation that will likely be ongoing near the coast. Expect gusts 40-50mph across coastal Essex, Middlesex, Norfolk, Suffolk and Plymouth counties. Gusts a bit higher – perhaps to 65mph – are possible on Cape Cod/the Islands as the surface low pulls away.

The most impactful period in the I-95 corridor will likely be 21-03Z as moderate to heavy snow overspreads the region and visibilities are reduced amidst blustery northerly winds.

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