A technical look at Tuesday’s snow

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As many of you have heard by now, a quick band of moderate to heavy snow is expected to affect the region tomorrow, mainly from the mid-morning through the early to mid-afternoon. The fast movement of the storm should prevent significant accumulations, but impressive upper-level dynamics could still allow several inches of snow to fall in a pretty short window of time, causing treacherous conditions.

It’s one thing to just look at a computer model’s precipitation output and just assume that everyone in the area will get that amount of precipitation, but 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.

Let’s take a look at a forecast sounding from the NAM model for JFK, valid for 11:00am tomorrow morning:

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 slight chance that areas near the immediate coast could start as light rain at the onset, but that should quickly changeover 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.

Today’s 12z NAM model valid for 10:00am tomorrow morning shows very fast winds at the 250mb level; between 150 and 200 knots. This supports the storm system moving along quickly.

That being said, the very fast forward motion of the storm is evidenced by the screaming mid and upper level winds. Winds of well over 100 knots in these regions support mid-level features, such as vorticity, moving along very quickly; thus the steadier and heavy snow may only fall in a 3-4 hour period. This also means that the storm may have a bit of a hard time turning the corner to so speak, meaning precipitation will tend to move a bit more west-south-west to-east-north-east (following the strong mid and upper level winds) rather than from north to south. This will also help to hinder the duration of the snowfall event, and may prevent areas north of NYC from picking up more than an inch or two.

Additionally, the very fast mid and upper-level winds and quick movement of any positive vorticity advection means that any heavy snow band will not necessarily be stationary. More support for this is that the jet streak in Northern New England is quite strong and not too narrow, meaning that the enhanced lifting in the right-entrance region will be more broad, as opposed to very localized. Thus, we do not anticipate this being an event where some locations get 1″, and others 8″. Rather, most areas should be able to sit in a heavy band of snow for at least some time, but maybe not for a very long duration, which supports a generally uniform 2-4 or 3-5″ of snowfall potential. Some localized areas that do stay in a band a little longer could pick up as much as 6″. Based on the storm not being able to turn the corner that well, those heavier amounts may be more likely in Central NJ.