Just because you don't see fire ants doesn't mean they aren't there. They're just not making visible mounds because of the heat and drought.
With drought, the fire ant colonies move underground where they can obtain water and where temperatures are cooler. They will resume mound building as soon as the area receives significant rainfall. That is why they seem to pop up overnight. Fire ants can become an indoor problem during times of drought, when they come in for food and water. Treating during periods of drought can be very effective because the colonies are weakened anyway.
Individual mound treatments, particularly with contact insecticides, are less effective during hot, dry conditions because the colony is not near the surface. However, because the ants must come out of the ground to find food and water, baits can work quite well. Baits should be applied in the late afternoon or early evening when rain is not predicted, the irrigation system is turned off, and no dew is on the grass. The late application time will keep the bait from spoiling in the sun. Most of the bait will be picked up by ants foraging at night.
Find more information about fire ants in eXtension's Imported Fire Ant Resource Area.