Winter storm expected in the Northeast tonight, moderate accumulations likely

A winter storm will approach the Northeast states this evening, beginning first in the Mid-Atlantic states and gradually approaching from the southwest towards New England. The storm system is developing on the heels of a frontal boundary that crossed the Northeast states on Friday evening and Saturday morning (you may have noticed it’s a bit colder outside) and that frontal zone will serve as a highway for the development of low pressure.

In the atmosphere’s mid levels, the system remains somewhat progressive – in other words, this is not a huge, powerful Nor’Easter. But there is plenty of moisture, aided by a strong low level jet stream, and that will act to enhance precipitation rates as the storm moves by. There are still a few uncertainties remaining with the system:

Precipitation Type

Precipitation type remains a significant concern and uncertainty with this storm system, as it does with many of our winter storms. This will be especially true south and east of the I-95 Cities of Philadelphia, New York and Boston. Here, warmer air will remain entrenched at the surface – and the lack of a true cold air source or high pressure will make it difficult for snow to fall for a long period of time.

Here’s an important thing to remember: In order to get snow of significance in the coastal Northeast states, a cold surface high pressure to the north is critical. With the warmer ocean waters lingering to the east and south, keeping cold air in place is a huge part of the forecast. This storm, synoptically, is not “Classic” for snow along the coastal plain by any means, as winds turn southeasterly during the storm and begin to draw in some of that warmer, maritime air. As a result, we anticipate that precipitation will flip to rain along the coast before the more steady snow begins and starts accumulating. The big cities and areas to the northwest, however, will fall along the gradient, which may actually enhance snowfall rates for a period of time.

HRRR model showing heavy snow in parts of the Northeast tonight.

Atmospheric Dynamics

Heavily involved in any forecast for snow is the presence and evolution of atmospheric dynamics. This is the straw that stirs the drink, so to speak; what is going on in the atmosphere that is going to cause snow? Where will it be heaviest and why? In this particular storm system, there are a few conflicting signals to pay attention to. Firstly, the synoptic setup in the mid levels of the atmosphere is not overly favorable for snow in the Northeast States, especially the Northern Mid Atlantic, as heights remain high and the flow remains progressive. The best vorticity in the atmosphere’s mid levels tracks to the north and west and the surface low is spawned by energy coming in from the Ohio Valley.

With that being said, forecast models depict a very favorable and strengthening 250hPa (upper level) jet streak over New England and Southeast Canada. This is a major factor in allowing for precipitation to expand and properly vent – while opening up room for bands of heavier snow. A strengthening low level jet is also expected to aid in the development of increased moisture (as mentioned above), which will allow the atmosphere to produce better low level dynamics and, as a result, heavier snow as well.

It is not surprising, then, to see forecast models producing bands of heavy snow as the storm system evolves.

Accumulations

As a result of everything discussed above, we feel comfortable forecasting 3-6″ of snow in a band where the best dynamics and cold air remain juxtaposed. In some regions, snowfall amounts over 6″ are possible – but we did not have enough confidence in these amounts being widespread to include a 6-9″ area. The gradient in snowfall totals is also expected to be stout, especially near the coastal plain, especially in Southern Long Island and along the Southeast NJ Shore. Some very slight tweaks to the forecast are possible moving forward, but we feel comfortable with this general idea as the storm approaches tonight.

Snow is expected to wrap up on Sunday morning area-wide.