The monsoon season in Arizona officially starts on June 15 and runs through September 30 each year. This period marks a shift in prevailing winds from dry westerly to moist southerly flow, promoting thunderstorm activity mainly over mountainous regions. While the official start is mid-June, actual monsoon rains often begin closer to July and August, which are typically the hottest and rainiest months in Arizona. The monsoon provides significant ecological benefits and relief from extreme heat but can also bring hazards like flash flooding and dust storms.