A strong inversion has been firmly in place for the past several days. It's pretty easy to see on the ARVTP. Of course its not working right now.
The persistance of the inversion seems rather unique for the region. Typically in New England an inversion will form on clear, calm nights, but this inversion isn't mixing out during the day. It reminds me a mid-winter inversion in the Salt Lake Valley. Also, its windy in the valleys and there is no snow. Two additional factors that typically help mix inversions.
I guess the driving factor is the very warm air aloft. It has trapped a thick dome of cold air below. And the low sun angle just cant heat the valleys enough to break the cap.