The second winter storm in 48 hours time will impact the area beginning Tuesday Night and continuing into Wednesday. Despite occurring less than two days after a significant snowstorm which blanketed the area with 8-10″ of snow from Sunday into Monday, this system will feature an entirely different set of threats — and will also feature a completely different evolution. A low pressure system is forecast to drive from Tennessee Valley into the Ohio Valley and Western Pennsylvania, while a secondary surface low redevelops off the coast of New Jersey. The result will be heavy precipitation overspreading the area Tuesday Night, and mid level warm air advection rapidly surging northward.
At the surface, cold air looks to be stubborn — especially away from the coast. The result will be a storm featuring snow which will quickly transition to sleet, freezing rain and rain throughout the area. Coastal locations and areas farther south will be warmer initially and likely will changeover to plain rain after just light accumulations. But farther north and across interior New Jersey, Southeast NY and parts of CT — significant low level cold will be slower to scour out. And the result will be the potential for several inches of snow followed by icing, which could create a high impact storm system with travel hazards and power outages into Wednesday. The National Weather Service issued Winter Storm Warnings in advance of the system, which are in effect for Tuesday Night into Wednesday.
How does an icing/freezing rain threat develop?
These forecasts are finicky, tough to predict, and feature an amazing atmospheric process. First, it is important understand that precipitation will develop into the area tonight as a result of “warm air advection”. In essence, the low pressure is still well back to our south and west, but southerly winds and increased lift with warm air advecting northward in the mid levels will allow moderate to heavy precipitation to surge northward. This means that temperatures will increase in a “warm layer” in the atmosphere, some thousands of feet above our head.
At the surface, northerly winds will begin to fight this warmer layer aloft. So what develops is a layer of warm air above a layer of very cold air at the surface. This cold surface layer is weaker at the coast for a variety of reasons (wind direction, ocean, etc.) — and so often the precipitation will turn to plain liquid near the coast much faster. Inland, the precipitation will fall as snow, sleet or freezing rain — and if it is freezing rain, the cold surface temperatures will cause the rain to freeze. And thus, a significant icing threat develops.
The above forecast sounding from the NAM model for Morristown, NJ shows the threat. Think of the temperature lines as a track for snowflakes or rain drops. You can see that they form in freezing air, then fall through a very warm layer, and then reach a freezing surface. This is a strong signal for rain falling and freezing at the surface when it does fall.
Where will the icing threat be most severe?
The icing threat will certainly be most noticeable west of the NYC Metro in interior portions of New Jersey. This is where the warm layer is most pronounced in the mid levels, and also where the surface cold is sustainable enough to support a threat for freezing rain. Across Central NJ, much of Long Island, and most of NYC there is also the potential for some ice. But warming surface temperatures should eventually alleviate the threat and allow for precipitation to change to plain liquid rain. That said, there will certainly be a period of very hazardous travel (icy, slick roads) even in the NYC Metro Area.
Farther north over SE NY and interior CT, precipitation is expected to be more snowy and sleet — with a less pronounced warm layer. So the freezing rain threat over that area will be somewhat less pronounced, although a period of freezing rain is also anticipated as precipitation ends.
What is the expected timing of the system?
Snow and winter precipitation will begin spreading into the area from southwest to northeast during the late evening on Tuesday and early morning hours on Wednesday. Precipitation which begins as snow or sleet over Southern NJ is expected to quickly transition to rain. Farther north, a period of snow and sleet is expected into the early morning hours on Wednesday. Precipitation will slowly begin to transition from sleet to freezing rain from south to north during the morning commute on Wednesday — and may be freezing rain during rush hour even in the city.
Near the coast and in the city, a transition to rain is then expected. Across the interior, precipitation will continue to fall as sleet and freezing rain, with snow confined to the well northern and western suburbs by the mid morning hours. By late morning, the transition to plain rain and drizzle is expected to push north of NYC as precipitation will then begin to wrap up after 2pm throughout the area. When all is said and done, our expectation is that most areas near NYC will receive between 2-4″ of snow and sleet, followed by a period of freezing rain and then rain. Higher amounts of wintry precipitation are expected inland, with little to no snow accumulation south of the NYC Metro Area.
Clearing is expected by late Wednesday afternoon into Wednesday evening as the storm rapidly pulls away. Colder temperatures will quickly funnel back into the area.