Having hunted Val Verde County and environs many times during and immediately after a hard rain I can tell you that snakes will NOT be moving much, if at all. You will probably see lots of anurans (B. valliceps, B. punctatus, B. speciosus, and S. couchi, R. pipiens), but little else in the way of herps. If things have a chance to dry out significantly, then herps will move and road cruising can be very good. Personally, I would not visit Val Verde County until Sunday or Monday, and that's providing there is no more rain after last night.
As an aside, this has been a lackluster year for snakes in the Trans-Pecos. A very few herpers have reported moderate success in spotting snakes while road cruising or walking the cuts, but the average trip almost anywhere in the region has been a bust. My log this year is so far showing far fewer entries than it did for the same time last year for the same amount of trips. It will be interesting to see how the rain affects things in the short term.