Thump of Snow Incoming to New England: Impacts, Timing

A deepening low pressure will likely be pulling out of the Mid-Atlantic and pushing northeast toward New England early next week. The overall setup and position of this system will favor cyclogenesis, especially with the colder air setting up over land and a still relatively warm ocean here at the beginning of the season. The storm's center will likely pass New England just inside the benchmark. This will yank some milder air in from the ocean for the coastal plain.

ALERTS (as of 8am)

The following watches will be upgraded to warnings or downgraded to advisories by this afternoon.

TIMING

The storm itself will be an open wave for most of the time its affecting New England. This wave will be positively tilted, so precipitation will arrive well before the center of the storm makes its closest pass. Snow will likely break out across western New England after sunrise and steadily spread eastward through the morning hours. Snow will generally be light to moderate during this time. Areas near the coast of southern New England may see a quick burst of snow at the start, but daytime heating will change this over to rain rather quickly.

Below: HRRR showing potential weather Tuesday morning around 8am (1st image) and around midday Tuesday (2nd image):

The bulk of the storm will likely occur for most on Tuesday afternoon and evening. Snowfall rates will become steady during this time, with the potential for snowfall rates to reach up to one inch an hour. This is also when the rain/snow line will make its furthest push inland. It will likely be steadily raining at this time across the lower elevations of southern New England and near the immediate coast of northern New England. The storm will steadily wind down from west to east through Tuesday night.

Below: HRRR showing potential weather around mid-afternoon Tuesday (first image) and Tuesday evening (2nd image):

SNOWFALL

For the past few days, this storm has looked like a classic widespread 4-8 inch snowstorm. The 4-8 inch jackpot zone will likely be rather large, ranging from central and western Massachusetts through the southern half of Vermont and New Hampshire and interior Maine. Amounts will gradually taper off moving into the north country as moisture gradually tapers off. There may be a sharp gradient in snowfall across southern and central New England when moving toward the coast due to the rainfall.

There are a few factors that point to this storm being a higher elevation snowstorm and lower elevation rainstorm for southern New England despite likely being a "benchmark" storm. For starters, this will be an open wave-type storm rather than a closed low with a tight circulation, which is really needed to produce a large and all-inclusive snowstorm. This will also be a progressive storm, moving through the region within 12 hours as there's no downstream blocking to keep it around for too long. There's also no high pressure to the north of New England to lock cold air at the surface.

All of the above factors point to less snowfall across lower elevations of southern New England. There has also been a subtle downward trend in snow-liquid ratio (SLR). This means a generally heavier and wetter snow is more likely rather than very light and fluffy snow. This, along with the other factors mentioned above, have led to a subtle downward trend in total snowfall. This trend hasn't been enough for a major re-thinking of the storm, but trends will be watched through the day today. Currently, probabilities of at least 8 inches of snow tops out at 60% for central New Hampshire, and gradually decreases moving out from there.

Below: Current probability of at least 8 inches of snow:

There's still the potential for a rather narrow mesoscale band to set up somewhere across central New England. Areas directly under this band could see heavier snowfall rates and the highest overall totals. Snow amounts may drop sharply both south and east of this band (due to mixing and rainfall) as well as to the north and west of it (due to lighter snowfall and the fact that the band could rob moisture from areas to the north). Where this band sets up will come down to the exact track as it may be as narrow as 15-25 miles.

As stated before, this storm isn't set up to rapidly intensify and create excessively heavy snowfall rates across the board. Central New England (northern MA, southern VT & NH) are With that said, moderately heavy rates are possible for a time, with a 40-50% chance for at least 1 inch an hour snowfall rates. Probabilities drop to 10-20% for up to 2 inches an hour. An inch an hour rates are most likely across interior New Hampshire.

One of the biggest questions for snowfall will be across the Massachusetts North Shore, New Hampshire Seacoast and southwest Maine. These areas have the greatest uncertainty on just how far the rain/snow line pushes inland. Should these areas remain all snow, they will be included within the 5-8 inch zone. Should mixing be able to push inland, snow amounts will drop pretty greatly in these areas. Right now, we generally have these areas in the 3-5 inch zone, but they may need to be upgraded to the 5-8 inch zone depending on today's trends.

Below: Current probability for at least 6 inches of snow. You can see probabilities drop from 70% near Fitchburg, Manchester and Rochester to 20-30% near the coastline:

RAINFALL

QPF totals will likely be in the half inch to inch range for southern and central New England during this storm. Most of this will fall as rain across the southern New England coastal plain. There will be an excess of moisture available across eastern Massachusetts and Rhode Island, which will likely lead to over an inch of rain falling across southeast Massachusetts and Rhode Island. As colder air crashes in behind the system, these areas may change to snow briefly before the storm wraps up entirely.

BEYOND THIS STORM

An Arctic cold front will crash through New England during the daytime Thursday. This will bring very cold temperatures for Thursday night and Friday, with a full-on mid-winter chill expected. This front may be favorable for snow squall development Thursday afternoon, which will be the main thing to watch in the immediate aftermath of this incoming storm. After that, another system is likely to approach next weekend.

Below: Current weather map for Saturday (December 6):