What it means
When a Scottsdale local says the monsoon, they mean the stretch of summer, roughly July into September, when the sky stops being empty. The dry, clear heat of June gives way to humidity, afternoon clouds stacking up over the McDowells and the Superstitions, and evening storms that arrive as a haboob first, then wind, then rain so heavy the washes run. Strictly, a monsoon is a seasonal wind shift, Arizona’s being the northern edge of the North American monsoon. Loosely, it is anything the sky does between the Fourth of July and Labor Day. You will hear “we had a monsoon last night” from people who know better.
The National Weather Service runs it on a calendar: June 15 to September 30, every year, since 2008. Locals still mark the real beginning by the first storm, the one that drops twenty degrees in half an hour, floods the dip at the intersection and makes the city smell like wet creosote. The practical rules, for driving, hiking and patios, are in the monsoon field manual.
Where you’ll meet it
Every forecast from mid-June on, the freeway signs, the summer hiking rules that get stricter once the humidity arrives, resort rate cards in the summer playbook, and the morning after, when the Greenbelt is doing its job as a floodway.
Where the word came from
The word is Arabic by way of Portuguese sailors: mawsim, a season, the name Arab traders gave the wind pattern that governed sailing across the Indian Ocean. Meteorologists borrowed it for any seasonal reversal of winds, and in the Southwest it names the summer pattern in which the prevailing westerlies give way to southerly flow that pulls moisture north from the Gulf of California (in what forecasters call gulf surges), the eastern Pacific and the Gulf of Mexico. The desert heats all day, moist air rises over the mountains, and by late afternoon thunderheads build and drift into the Valley after dark, if they come at all. Language Log, the linguists’ blog, credits Phoenix forecaster Robert Ingram, co-author of the 1972 paper that gave Arizona the word haboob, with putting both Arabic words into local weather talk.
For decades the start of the season was a measurement, not a date. Phoenix declared the monsoon under way after three consecutive days with an average dew point of 55 degrees or more; Tucson used 54. That produced a start that wandered from late June to late July and a civic ritual of arguing about whether it had begun. In 2008 Tony Haffer, the meteorologist in charge of the Weather Service’s Phoenix office, led the change to a fixed statewide Arizona Monsoon Season, June 15 through September 30. The reasons, as he laid them out: damaging storms often erupted weeks before the dew point tripped the threshold; Tucson and Phoenix used different numbers; better instruments showed humidity varying across the region; and the old rule had no end date at all. “The real issue is raising awareness of the violence of this weather,” he said. The fixed window captures about 99 percent of the region’s most violent summer storms.
What it is not
A monsoon is not a storm. A single evening of wind and rain is a monsoon thunderstorm, and “the monsoons” is a plural the Weather Service and the ASU climatologists would like you to drop. It is also not the rain itself: a dry monsoon season, with the humidity and the dust but little rain in the Valley, is a normal and disappointing outcome.
The fine print
The old onset rule still gets computed each year, and it shows how arbitrary the calendar is. In 2018 the dew point criterion put the start at July 8 in both Phoenix and Tucson, which the University of Arizona’s monsoon tracker called slightly later than average; early July is the typical first week of real storms, three weeks after the official opening. Nothing stops a storm on June 20 or a dust wall on October 2. The dates exist so safety messaging starts before the weather does.
The Phoenix office lists seven hazards: dust storms, downbursts, flash floods, excessive heat, lightning, wildfire and the occasional tornado. The numbers are worth memorizing. Thunderstorm gusts here almost always exceed 40 mph and the worst exceed 100. Lightning can strike up to 60 miles from the nearest rain, which is why the rule is thirty minutes of quiet after the last thunder before going back outside. Six inches of moving water knocks an adult down; one to two feet floats most vehicles, SUVs included. Arizona’s stupid motorist law exists because people test that last one every July.
Heat is the quiet part of the monsoon. June is hotter on the thermometer, but July’s humidity makes the same temperature harder on the body, and the Weather Service calls heat the Southwest’s number one weather killer. That is why the hiking rules tighten when the moisture arrives, and why the smart local reads the dew point every morning until the first cool night in September.