Arizona monsoon starts around June 15 and ends about September 30. It is caused by an area of high pressure called the subtropical ridge that moves northward during the summer months.

Flash flooding is a serious danger during the monsoon. Dry washes can become rivers in an instant, even when no storms are visible as a storm can cause a flash flood tens of miles away; it is therefore wise to avoid camping in a dry wash during the monsoon. Lightning strikes are also a significant danger. Because it is dangerous to be caught in the open when these storms suddenly appear, many golf courses in Arizona have thunderstorm warning systems.