Thunderstorms in New England are not uncommon. Most every part of the USA gets it’s share of convective weather at some point. But supercell thunderstorms in Maine are uncommon. Here’s a recent example.
This is a view of a supercell from the Caribou, ME NWS doppler radar. The BR image on the left shows the classic supercell shape including a hook echo. The SRV image on the right does show a couplet that, while not visually impressive, does show enough rotation to warrant a tornado warning. In fact, this storm did produce a brief tornado close to the time this scan was taken. Considering the past tornadic events that have taken place in the plains and south, I’m sure the people under the warning for this storm took the message from the Caribou NWS very seriously.