To keep up with the weather at Cedar Hill State Park, go here:
http://www.srh.noaa.gov/ifps/MapClick.php?CityName=Cedar+Hill&state=TX&site=FWD
Late May in North Texas is generally warm and sunny with days in the mid to upper 80's and nights in the mid to upper 60's. There's always a chance of scattered thunderstorms in May and the humidity ranges in the 25 to 30 percent range.
If there is threatening weather in the area (you'll know it when you see it) head for shelter! Do not be caught out in the open!
I recommend wearing short pants, short sleeve shirts, bug spray, sunscreen, and a hat or a bandana. Pack a rain poncho and carry plenty of water if you're going out for a hike or bike.
Historical weather data:
http://www.srh.noaa.gov/fwd/CLIMO/dfw/normals/dfw05nrm.html