Summary

‍An early season winter storm will impact much of southern New England Wednesday night and Thursday. Snow will overspread the region from southwest to northeast Wednesday evening, likely reaching Boston around 10PM and becoming heavy quickly thereafter. The peak of the storm should last from approximately 12AM - 10AM Thursday, gradually winding down through the midday and afternoon hours.

Unlike our last system, where quite a bit of rain got involved and lowered snow totals along the coast, this storm should be all snow for the majority of the region. A bit of sleet or rain may be introduced at the immediate south coast, so I have lowered snowfall totals in that region a bit. Snow should accumulate quite quickly. A broad region of 8-12" amounts is expected, with localized jackpots in the 14-16" range. At this point, the most favored region for the jackpot amounts appears to be along I-90, perhaps in the Metrowest suburbs.

There will be a period of wind with this storm as well -- gusts to 35-45mph may occur along the east and south-facing shorelines at the height of the storm. A period of near-blizzard conditions is possible, particularly across Cape Ann and southeastern Massachusetts.

Discussion

A bit of an unusual setup for a significant winter storm in the northeast featuring some classic elements and others that would point towards a less potent event. All things considered, I think many will see their largest December snowstorm in some time.

Let's start with the basics - cold air and moisture. The antecedent airmass in place across the northeast is quite cold - as of 00Z Wednesday, temperatures were below 0F across southern Quebec and ranged from the single digits above 0F to the mid-20s across New England. A very strong 1035mb high pressure centered over central Quebec and is actually forecast to strengthen into the day tomorrow, locking in the cold airmass in the low levels.

Moist air near and just east of the Carolinas is also present as of 00Z Wednesday. Surface pressure falls should begin in this region overnight tonight, advancing northward through the day on Wendesday, and the moist air will be drawn north accordingly. This process, combined with the previously discussed cold airmass in place in New England, is a classic recipe for significant snowfall.

On to the not-so-classic elements of this situation. The 500mb trough currently over Oklahoma (as a closed low) is going to de-amplify as it advances northeastward through the prevailing flow. As it approaches the east coast, the disturbance will still be strong enough to induce surface cyclone genesis. Though the latitudinally elongated low pressure center is not expected to be particularly deep, 995mb at the deepest, the high ambient pressure of the environment should allow for some decent low level wind fields to develop.

In terms of precipitation, a shield of snow will advance northward from the Tri-State area into New England by Wednesday evening. Timing should be about 7pm in southwestern CT, 8pm in northern CT/western MA, 9pm in RI/central MA, and 10pm in eastern MA/southern NH. Once the snow begins, it will quickly become heavy with the strong mid-level forcing that develops.

Impressive speed convergence will lead to intense frontogenesis near 700mb in a broad west-east band that will advance northward in tandem with the most of the 700mb jet. If this band slows or stalls at any point, rapid snowfall accumulation is possible, as rates under this band will likely be 1-2"+ per hour.

At the surface, a coastal front will develop as the low deepens and organizes that low-level wind field. It appears at this point that the front should spend much of the storm near or just west of I-95, before collapsing to the coast as the low advances east of 40N/70W. On the cold side of the boundary, temperatures will be very cold and support high snow-to-liquid ratios. The warmer side of this boundary will feature temperatures in the high 20s and low 30s, certainly cold enough for snow but the ratios may be held down a bit in this region by the more borderline thermal profile.

As is likely apparent from the above, I am anticipating an all snow event for most of the region. The one exception to this may be along the immediate south coast/Cape/Islands where southerly flow may drag in enough warm air for a flip to sleet/rain for at least part of the storm.