Is Pennsylvania in the Midwest?

Number-cruncher extraordinaire Nate Silver Tweeted Tuesday morning that he thinks Pennsylvania is a Midwestern state, and he's surprised that so few people agree with him.

Uh, Midwestern?

Philadelphia, which is an hour or so from the Atlantic Ocean, is in the Midwest?

An hour later, Silver amended his earlier statement, saying that "the Midwest begins at the Phillies-Pirates dividing line."

That sounds a little more sensible.

What do you think? Is Pennsylvania a Midwestern state?