Winter Storm Warnings issued as snowstorm approaches

The National Weather Service issued Winter Storm Warnings and Winter Weather Advisories for the entire area on Monday evening, in advance of a light to moderate snowstorm which is expected to impact the area on Tuesday. The Winter Storm Warnings, in effect through 6pm on Tuesday evening, cover much of Southwestern and Central New Jersey as well as the major cities of Philadelphia and Trenton. Elsewhere, Winter Weather Advisories are in effect for the rest of New Jersey, New York City, Southwest Connecticut and Southeast New York. The advisories currently do not include Eastern Long Island or interior New York State.

The watches and advisories were issued in advance of a forecast snowstorm, which will approach the area on Tuesday. A mid level disturbance sliding eastward, and a frontal boundary providing the focal point for enhanced lift and development of precipitation, will set the stage for the event. A band of moderate to heavy snow is forecast to develop by late Tuesday morning, but forecast models have been struggling with the exact placement of the band. Regardless of that bands eventual position, 1-3″ of snow seems increasingly likely throughout the majority of the region. But the heavy snow band, and enhanced forcing within it, provides extra intrigue for potential higher amounts. We break down Tuesday snowfall event with timing, snowfall totals, and hazards information below.

Storm total snowfall forecast (left) and NWS Watches, Warnings and Advisories (right).

Storm total snowfall forecast (left) and NWS Watches, Warnings and Advisories (right).

What’s the deal with this storm, and what is causing it?

Tuesday’s storm system is being driven by a mid level disturbance which will slide eastward through the Northeast United States, originating from the Great Lakes and Ohio Valley region. The mid level disturbance will kick off a weak low pressure system, which will develop along a frontal boundary which will be located just off the coast. The juxtaposition of those two features will be the focal point for the development of precipitation on Tuesday morning, with enhanced winds aloft and frontogenesis at the surface increasing the likelihood of the development of a band of moderate to heavy snow which will move eastward throughout the area.

The system will shift northeastward fairly rapidly on Tuesday, admist a very fast flow aloft (quick jet stream). This will limit the snowfall potential and preclude any major amounts. However, the positioning of the moderate to heavy band of snow is imperative to the forecast — and remains highly uncertain. Areas that sit underneath the band of heavy snow could receive amounts in excess of 4″ and possibly approach 6″ in some locations.

What is the timing with this storm system?

Precipitation looks to begin during the morning hours on Tuesday — essentially during the later half of the morning commute. Snow will begin from southwest to northeast, first over Southwestern New Jersey and Southeast Pennsylvania. As it streams north and east, the intensity will increase in a relatively narrow band of stronger forcing and lift, and resulting heavier precipitation. This band will shift eastward during the next several hours, pushing through New Jersey and potentially New York City and Long Island during the late morning to early afternoon hours.

Light to moderate snow will continue before and after this heavier band of snow, but the band itself will contain the lowest visibilities and best snow rates. During the early to mid afternoon, snow will lighten considerably. This is a fast system — it likely will be winding down by the evening commute. But the accumulated snow will likely create quite a mess.

What can the forecast models tell us right now?

In situations such as this one, it is very easy to “read and rip” forecast models, or in other words, take them very literally. It is important to remember that all forecast models, both global and higher resolution, will struggle with the placement of the heaviest banding. Forecast models are intended to be used for guidance — not the do all, end all to a forecast. That being said, we can take away some important information from them in situations like this as well. in order to really understand what a model is showing and to get a good understanding of what is going on with a storm, one must look at a forecast sounding. Soundings give a detailed look at the temperature, dewpoint, and wind throughout the entire atmosphere, which can help give us a better understanding as to how much snow will fall.

Forecast model sounding for JFK valid for 11:00am tomorrow morning shows favorable conditions for heavy snow.

Forecast model sounding for JFK valid for 11:00am tomorrow morning shows favorable conditions for heavy snow.

For people who are newer to soundings, the red line indicates the temperature, the green line indicates the dewpoint, the slanted, dotted blue lines indicate the temperature that they are connected to at the bottom (in Celsius), the numbers on the left are feet above the ground in thousands, and the colored lines on the right indicate wind direction and speed.

Several important features to note include:

1) It’s cold! Cold air advection behind yesterday’s storm will allow relatively mild temperatures now to quickly fall to below freezing at all levels of the atmosphere. There is a chance that warmer boundary layer temperatures near the coast could bring a period of rain, but even those areas should eventually flip over to snow as temperatures throughout the atmospheric column are quite cold — very suitable for a snowflake to survive.

2) It’s saturated everywhere. There are some NW winds near the surface, which may try to create a bit of a downsloping effect. That being said, there are no dry layers in the atmosphere, as evidenced by the dewpoint and temperature lines nearly overlapping in the entire sounding.

3) The snow growth looks excellent. The white line on the left represents the amount of lifting in the atmosphere, and the yellow line on the temperature line represents the snow growth region. The NAM model is showing impressive values of lifting that coincide with the same height as the snow growth region. The unit of lifting here is microbars per second, and once we see values in the -15 to -20 range (like shown above), that’s when eyebrows get raised a bit. When there is sufficient lift in the snow growth region, snowflakes are formed very easily, allowing for the potential for heavy snow.

4) Snow to liquid ratios might be higher than 10:1. This is sort of a continuation from point #3, but it dives in a bit further. There is significant lifting in the snow growth region, and the snow growth region is saturated. Additionally, temperatures in the snow growth region are between -12 and -18C, which favors the formation of snowflakes called dendrites. Dendrites tend to be a bit fluffier and can accumulate very efficiently, especially if the entire atmosphere below it is still quite cold. And this is what we have here.

Thus, the likelihood of areas receiving heavy banding of snow that will accumulate quickly is quite high.

Wrapup, Hazards, and Travel Information

Wrapping up the situation this evening — it appears a light to moderate snowfall is likely throughout the area on Tuesday. This won’t be a widespread significant snowfall for the entire area, so hold off on gassing up the snowblowers for now. That being said, heavy snow is likely underneath the band of heaviest precipitation which will shift quickly east throughout the area on Tuesday morning and afternoon — and areas that stay under the heavier banding for a longer period of time could see snowfall totals in excess of 4″.

Travel delays look likely throughout the day on Tuesday, and the PM Commute may be especially treacherous with new snow on the ground and light snow potentially continuing. Other hazards include obvious low visibilities during heavy snow — and we urge caution if you are traveling at all during the day on Tuesday.

For further updates, stay tuned here and to our social networks. We will post a live blog thread — likely around midnight tonight — so you can follow along with our meteorologists during the event to see the latest updates as they are posted.