Hot A/C

Hello all!

I have a 1994 Passport V6 3.2 with 66K on it. I have noticed that this summer the a/c blows warm air on very hot & humid days and it never really cools. But on cloudy days with temps in the 70's or low 80's it gets cold. I am taking it in next week to get it inspected and I plan on mentioning it to my mechanic who is very honest but I would like to get a heads up of what it could be and its possible cost. Never before have I had this problem.

Thanks in Advance!
GMW72

Probably just needs freon.