You’ve probably heard something about a storm coming toward New England on Saturday. For me, this one has been full of red flags and problems. So why is that? Let’s start out with the Euro depiction of the storm Saturday afternoon.
What’s missing from this picture? A high pressure! A cold Canadian high is a prerequisite for almost all major I-95 snow events. There are a couple reasons for this. One is obvious and the other may not be so obvious.
The obvious reason you want a cold high pressure banked to the north of New England is that it provides a steady stream of cold and dry air into the storm. This ensures that the precipitation will be in the form of snow even in the coastal plain.
The less obvious reason is that the cold high to the north and warm Gulf Stream off of the east coast produces a sharp temperature gradient (large difference in temperature over a short distance). The stronger the temperature gradient (aka baroclinicity) the stronger the storm may get. Also, as warm moist air from the over the ocean rides up and over the cold, dry, and dense air near the surface you get a blossoming precipitation shield well north of the low. This is known as isentropic upglide.
The stronger the temperature gradient the stronger the isentropic upglide can be. This schematic from NWS in Louisville shows the process nicely.
So, where does that leave us? This storm will struggle for many areas. Marginal cold air will result is borderline temperature profiles between rain, snow, and sleet. Additionally, the lack of strong isentropic ascent north of the low will cause the precipitation shield to be quite compact.
As opposed to seeing a regionwide snow event we’ll likely see a narrow band of very heavy, wet snow. Whoever can get into the narrow band of heavy snow will get hit pretty good by this storm. Others seeing drips of rain and mangled snowflakes will wonder what all the fuss is about.