This is considered the monsoon season at the Grand Canyon. Heavy rains lead to flash flooding which can lead to all sorts of problems with your vacation if you arrive on a “monsoon” day. If you are hiking into the inner canyon you will want to check with the Back Country Office before beginning your hike. (You should do this regardless of the weather.) If you are taking a tour, it may be cancelled due to roads being washed out or hazardous weather conditions existing in all or part of the Canyon. You always want to check the weather at Grand Canyon before you come.
Recently Jock Whitworth, superintendent at Zion National Park, announced the closure of one of the trails there because of a rock slide. The rock slide was triggered by rainy weather in Zion National Park. Eleven people were trapped for about 3 hours but were rescued without injury. One of the campgrounds in the area was covered in about 6 inches of mud! Fortunately, no lives were lost but vacations were disrupted.
If you are at the Grand Canyon when monsoon rains come, your tour may be cancelled so you should have a back-up plan. You may want to spend that time watching the IMAX movie or visiting one or more of the museums in the Grand Canyon Village. If nothing else, go to your hotel room or sit in your car and plan what you will do when the rain stops and you are able to enjoy the views of the Canyon once again.