Yesterday I sat here in the weather office looking at the weather maps for Tuesday and thought, “that’s probably going to come west – at least on 1 or 2 model runs” and that’s exactly what has happened. These systems always seem to want to sneak a bit closer to the coast and this one definitely had that look.
The actual low pressure that forms is forecast to be well off the coast – far southeast of Nantucket and the Cape but there’s a fairly large swath of precipitation that extends far northwest of the low center. Why is that?
The GFS closes off an area of low pressure just east of Atlantic City about 5000 feet above the ocean and you can see a band of convergence up through southern New England. In fact, as the large scale trough sharpens there’s a fairly notable zone of frontogenesis that develops from 650mb-850mb across southern New England.
Frontogenesis in the atmosphere forces a thermally direct circulation to develop in order to restore thermal wind balance. That circulation results in upward motion on the warm side of the temperature gradient – and that is what will force precipitation well west of the surface low.
The ensemble mean off the 18z GFS brings >0.25″ of liquid as far west as New Haven, Bridgeport, Middletown, and Willimantic. Additionally, the atmosphere will be quite chilly in the low-mid troposphere. The 18z GFS shows good snow growth with a deep layer of high RH through the dendritic growth zone (-12c to -18c). The one downside to the GFS BUFKIT sounding below is that the best lift (omega) is above the DGZ here in HVN though it is better in GON where the best lift is a bit lower and deeper in the troposphere.
So what’s going to happen? That’s what we all care about of course! At this point I’d say some accumulating snow is likely especially in southeastern Connecticut. The amount of snow is still up in the air and we won’t have a better idea until later tonight. We should be able to throw out some numbers tomorrow morning but from a probabilistic sense here’s what I’m thinking.