So what "squishes" the low in the ECMWF's depiction?

A shortwave trough moving through New England Sunday PM.

The northerly flow behind this feature suppresses the burgeoning storm on Monday, letting it slip SE on Tuesday.

The GEM has this feature *much* weaker and farther NE. pic.twitter.com/b7vXWn3EBA

— Jack Sillin πŸ‡ΊπŸ‡¦ (@JackSillin) January 29, 2021