Now that temperatures have plummeted below -15 and stayed there, I feel it necessary to bring this up, because it's driving me batty:
The wind chill has no effect on whether or not to plug in your vehicle.
Every night on the news they say, "It's -15 outside, feels like -27 with the wind chill. Bundle up and plug in your vehicles!"
Your engine will only cool off to the ambient air temperature and then stay there. The wind chill will not make your engine "feel like -27". It's easy to remember if you just realize that engines don't feel.
Further reading here if you don't believe me.
The more you know.
Oh... but it's ok if you bundle up. It's effing cold up in this bitch!