Is this going to affect my chances at Northwestern?

One score isn't going to kill you. You are a very solid student, you shouldn't worry too much as long as you write excellent essays! Good luck! I've seen Northwestern accept much worse students (3.7 out of 4.0 GPA, 2100 SAT, and only 3 APs at a great school) because of essays presumably. When in doubt, interview and you can make up for this one flaw.