Premium: Snow Squalls Possible on Sunday

Although we have generally been basked in a sea of warmth, we are getting to the time of year where snow becomes a more realistic possibility. A large storm system traversed the central part of the US, bringing blizzard conditions to the upper Plains and upper Midwest. While the pattern across most of North America has been pretty warm thanks to a raging Pacific Jet, this storm is powerful enough to tap into some cold air up north — while that air is not as cold as it “should” be this time of year, that air is still cold enough that if tapped into, could yield snow, if there is precipitation, of course.

Ahead of this storm, more warmth has moved into the Northeast, with high temperatures in the upper 60s today. A little bit more in the way of onshore flow is expected on Saturday, which may keep temperatures a tad cooler, but they still will be overall quite warm for this time of year — into the low to mid 60s. But part of this warmth stems from being ahead of a cold front, and once this cold front crosses, cold air advection will be taking place, but it will also be aided by a deepening secondary low pressure system, funneling in cold air from Canada and tapping into the aforementioned cold air.

Usually it’s pretty rare to have secondary low pressure development on the East Coast when the parent low goes into Wisconsin. The storm ran into some resistance via some higher heights and ridging in Canada, so it stopped its northward progress. Additionally, the pattern is very progressive out west, which forces a more west to east trajectory — which is crucial for pushing secondary development further east. A large lobe of strong vorticity will develop at the base of the trough with this storm, and as the synoptics get pushed east, so will this area of vorticity at the base of the trough. The trough will then take on a negative tilt, which fully helps to tap into the temperature gradient with the cold air from the northwest, and the warmer air to the southeast — a lot of baroclinicity. This will help a new area of cyclogenesis take place at the base of this trough, with a lot of lift for precipitation. This will be the main impetus for rain on Saturday night and Sunday morning, before perhaps changing over to lightly accumulating snow well into the interior, and then some leftover scattered snow showers for the whole region on Sunday afternoon and evening.

Taking a more local look at things, we can see that a strong cold front associated with this storm system will be approaching the region late Saturday night into Sunday morning. Lift, precipitation, and winds will be enhanced by the secondary area of low pressure. It turns out that most of the moisture with this front is post-frontal, rather than pre-frontal. In other words, it’s behind the front rather than out ahead of it, because the best forcing, lift, and baroclinicty are behind the front since all of the “action” is flowing cyclonically from the secondary low pressure. This is also because of the fact that the roaring Pacific Jet adds a bit more Pacific moisture to the atmosphere in general. The fact that a lot of the moisture is post-frontal leads more credence to the idea of some snow from this system.

Of course, even behind the front, it will initially still be way too warm for snow in most locations, since the pre-frontal airmass is quite cold and it will take a lot of time for the airmass to cool off to levels that support snow. In the beneath image, there is plenty of positive vorticty advection in the atmosphere, which means you are adding a lot of spin to the atmosphere which often helps develop storms and generate precipitation. But at that time, it is still way too warm for snow for most of the area. There are enough dynamics involved here with cold mid and upper levels and frontal forcing that there could even perhaps be some embedded heavy rain and a rumble of thunder with these showers. But notice how some of the positive vorticity advection “bends back” towards the colder air to the northwest. This will help to generate lift and moisture in areas that are colder, which could support some wet snowflakes mixing in with these showers in elevated areas of NW NJ, NE PA, and SE NY State. Winds are also quite high in the entire atmospheric profile due to the healthy dynamics involved, so wind gusts easily in excess of 40mph may mix down to the ground in these showers.

As the overnight goes on, the secondary low pressure will move further northeast, and the cold front will cross the area. Thus, the initial frontal forcing will wane and we will lose some of the more widespread lifting mechanisms with the storm. But remember, there is still plenty of resistance for the storm to gain latitude because of the strong ridging and NW Atlantic Vortex, so the storm will cut off in the mid-levels. This will prevent it from completely moving out of the way, giving it more time to tap into the cold air to the northwest, as well as drastically lower our mid-level temperatures on Sunday. This greatly increases atmospheric instability and will allow for individual lobes of positive vorticity advection to rotate around the back side of this system. It will essentially turn into a summer-time thunderstorm event, but with snow showers instead of rain showers and thunderstorms.

Notice how on Sunday night, the positive vorticity advection is much more spotty. This means that there won’t be one area of widespread, organized snow accumulating in our region, but rather hit-or-miss snow showers. And considering the strong dynamics with this system, the atmospheric instability, the moisture in the mid-levels, the degree of positive vorticity advection even in the patchy form, and the strong winds — it is possible that these snow showers will be snow squalls. Another scenario that’s similar and will probably be occurring in conjunction with these snow squalls is that there will be a lot of Lake Effect snow behind this system as well — and the very strong west and northwesterly wind component combined with still some moisture and instability in our atmosphere may allow some of these Lake Effect bands to move towards our area as well. They will of course weaken being far removed from the Lakes, but some remnants may survive — or what we call a Lake Effect streamer.

To summarize, we are expecting a line of showers with perhaps some embedded convection to move through late on Saturday night into Sunday morning. These may mix with some snow flakes well north and northwest of NYC. After a lull in the activity for several hours, more mid-level moisture will wrap around the system and will lead to scattered snow showers and snow squalls on Sunday — centered on the afternoon and night. Winds could gust up to 50mph at times on Sunday. It is even possible that these snow showers could make it all the way to the coast, given how cold the temperatures aloft are. If the precipitation is not heavy enough, it could still fall in the form of rain instead of snow. It is important to remember that once the frontal forcing moves through and we don’t have any true cold-conveyor belt precipitation, that the coverage of precipitation will be very hit-or-miss. It’s possible that one location could get a quick dusting of snow and a wintry feel, while another location five miles away sees nothing.