The real reason is the jetstream. It's basically dipping way down into the south, then shooting up again straight into the midwest/canada.
See for yourself:
http://www.intellicast.com/National/Wind/JetStream.aspx .
The way it's sitting right now, about 750 miles north into canada, it's pushing all that warm air up here.
Nothing to do with carbon emissions or global warming. Just a very odd jetstream pattern.