Monsoon season is typically in July and August. This brings heavy afternoon thunderstorms, and the chance of flash floods. Flash floods are not a huge threat if you are aware of your surroundings, and keep an eye on the weather. They can also show you a spectacular display of nature's power. If you are not an experienced southwest hiker, just get a professional guide for your trip.
The trailhead at Hualapi Hilltop is 5200 ft in elevation, while the canyon floor sits at 2800 ft. The water maintains a relatively constant 70 degrees throughout the year.
Below are the weather averages for Havasu Falls. Temperatures will drop 5 degrees on average for every 1,000 feet in elevation you climb. Therefore the temperatures at the trailhead (Hualapai Hilltop), are over 10 degrees cooler than the campground.