MS student in CS meeting advisor for the first time in person (USA). What should I wear? (male)
I've never even visited any universities in that part of the country, but I think your comfortable shorts/T-shirt should be perfect for a cookout in any part of the country.  For the first impression, make sure you choose something comfortable.  I think dress shoes would probably be overkill.  However, a short-sleeved button shirt (i.e. tailored shirt with a collar) would be a nice touch.  Do you have some sort of sports shoes?  That would be a happy medium between dress shoes and flip flops.  If not, I'd go with the dress shoes.

If you ask what food to take to the cookout, I'll chime in on that one too.