Thanks for the replies. I don't have a sunroof or moonroof. I also rechecked the AC drain and it is still clear. It is definitely related to rain, as the first time it happened I had left the car outside overnight and we had a flash-flood kind of rain. When I got in the car the next day, the pass side was sooo full of water it actually overflowed from the front floor into the back! Literally about two inches of standing water inside the car. Checked all the windows - all up tight. The second time it was caught in the rain, the same thing happened.

While it was still raining, I pulled the lower trim from under the dash and I saw water dripping from between the AC box and the firewall. As soon as I get a chance on a dry day, I'm going to run the hose on the windshield, let the water drip into the cowl vents and see if any comes in. I'll keep you posted.