Might be too cold for the AC to run, depending on where you live. My suggestion would be to go to a car wash and you sit in the passenger seat and have someone spray the area with water and watch for the leak. The only suggestion I can make is that there is a plastic drip tray that is available from GM that goes underneath the wiper cowl that helps channel water away to the fenders to stay outside the car. Your Impala may or may not have one there. I would buy this piece and install it if it isn't there. Only other thing I can think of is did you have the windshield replaced recently? If so, they could have done a poor job of sealing the windshield properly.