National Weather Service says:

[dsrtao]

Tuesday Night: Mostly cloudy, then gradually becoming clear, with a low around 8. Wind chill values as low as -50.

[LATER... IN THE BATCAVE:]

Clicking around the NWS map produces interesting results. Somerville gets -50, Weston gets -1. Belmont gets -1, Arlington gets -50.

I don't believe all these.