First snowflakes of the season tomorrow morning?

Although the big snowstorm that some models had for Wednesday night and Thursday is definitely not going to happen, the NYC Metro area may still see its first snowflakes of the season tomorrow morning during the rush-hour commute. Yes, this does include coastal areas.

The main culprit for this is the Arctic cold front that will move through the area. There is not a whole lot of moisture with this front, as the main story will be the falling temperatures, as temperatures will struggle to get out of the 30s tomorrow and Wednesday. However, there is some moisture; particularly behind the front, where temperatures will be colder. This is called an anafront, as opposed to the more typical katafront, where most of the moisture is out ahead of the front in the warmer, more unstable airmass.

Current surface observations in the Great Lakes show the frontal boundary quite well, with many observations of snow on the cold side of the boundary. This front will cross our area during the overnight hours and should give most of the area its first snowflakes of the season.

Normally, it is much easier to generate precipitation ahead of a front rather than behind it, due to the airmass being warmer and having more moisture ahead of the front. But sometimes, due to jet stream dynamics among other things, the best source of lifting actually occurs behind the front. Of course, the colder airmass and lack of a strong storm system will somewhat mitigate the amount of precipitation that does fall; but the temperature gradient, lift, and moisture aloft do support some precipitation.

This morning’s high-resolution NAM model shows a band of light to moderate snow moving into the area around 6:00 a.m.

Precipitation should start as light rain around 2:00am in further northwest areas, and closer to 4:00am in coastal locations. As temperatures cool, the rain should change to snow about an hour or two after the initial onset of precipitation, fall lightly to at times moderately, and then end between 9:00am and 11:00am from west to east. This means that the immediate NYC Metro area could see a period of steady snow between 6:00am and 7:00am. Accumulations will be very light and limited to only the cold surfaces. Perhaps a dusting to a coating in coastal sections, and a coating to as much as an inch further north and west. The main roads should primarily be just wet, but it may be a good idea to save a bit of extra time tomorrow morning, as some colder roads could be a bit white and slick. It may appear quite wintry at times, due to the strong winds blowing around any snow that does fall.

Where will the lift be coming from? Associated with the powerful front is a strong jet streak. Upward vertical motion (lift) is favored when one is either in the left exit, or right entrance region of a jet streak.

This morning’s NAM model valid for 7:00am Wednesday morning shows a potent jet streak in New England, placing us in the right entrance region at the same time that temperatures will be plummeting.

The above image shows a nice jet streak moving through New England, which places southern New England and the northern Mid Atlantic in the right entrance region of that jet streak. Also important to note is that this jet streak will be located further west when the front is further west — meaning when the area is still on the warm side of the front, the lift will not be supportive of precipitation. The fact that this jet streak swings through the area after the cold front has passed is crucial to the development of any kind of snow, especially considering that the moisture is somewhat limited to begin with.

Also note the wind direction aloft. They are in a WSW direction as opposed to a more WNW direction — this tends to support more moisture aloft, given that a southerly source has more moisture than a northerly source. Thus, there will be cold air advection at the surface when the front passes, but a decent amount of moisture and lifting aloft as well. These details can further be elaborated on when we check out the forecast model soundings.

Attached below is this morning’s 12z NAM forecast sounding generated from BUFKIT valid for 7:00am tomorrow morning for LaGuardia Airport.

This morning’s 12z NAM forecast sounding valid for 7:00am tomorrow morning for LaGuardia Airport supports a brief burst of snow. The white line on the left represents the amount of negative omega (lift), and the yellow line on the temperature profile represents the snow growth region.

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.

A few key things to note about this sounding are the northwest winds near the surface, but the strong, southwest winds aloft. For one thing, this helps to advect cold air at the surface; the temperature at the ground is only ever so slightly above freezing, and it is well below freezing once you go above the surface — suitable for a snowflake to survive. If this warm layer were thicker, the snowflake would melt, but since it is such a shallow layer of warmth surrounded by abundant cold above it, snowflakes will survive to the ground; they just may not accumulate all that efficiently. Additionally, the strong southwest winds aloft help to keep the entire atmospheric profile saturated in this cold airmass.

Looking a bit higher in the atmosphere, we can denote two important features. One, the amount of lifting, which is indicated by the white line, and the snow growth region, indicated by the yellow line. The further to the left the white line is, the more lift there is in the atmosphere. The strongest area of lift is coincides with the snowgrowth region, which helps to support the development of snowflakes. Although I would not call these values of lift all that impressive, they are still relatively strong.

We can also see that the snowgrowth region is saturated or close to it, given how close the dewpoint is to the temperature. Additionally, the snowgrowth region has temperatures between -12C and -18C, which supports the formation of dendrites; the type of snowflake that tends to survive, accumulate most efficiently, and of course for the snow-lovers, look prettiest.

Caveats include the fact that there does not appear to be a whole lot of lifting at the surface, and the fact that temperatures will initially be warm. However, the potential formation of dendrites and lifting aloft does support a period of snow. Either way, it looks like most of the region will see its first snowflakes tomorrow morning.