Runoff from heavy rain after a storm during monsoon season at the Navajo National Monument in Arizona. Image credit: Al_HikesAZ via Flickr 

We’ve come up with all sorts of ways to describe an abrupt downpour. We say that it’s raining cats and dogs or that it’s a gully-washer when the heaviest storms crank up. Some folks opt to call a drencher a monsoon. “It’s a real monsoon out there” is a popular comment to drop into some hardcore weather small talk. We call heavy rain a monsoon so often that the term has started to lose its meaning. What is a true monsoon?

A monsoon is a change in a region’s weather pattern that marks the beginning of the wet season, or the time of year when that region will see the bulk of its rainfall. The monsoon season begins when shifting winds start to drag moist air—usually from an ocean—into a typically dry region (like a desert), allowing regular showers and thunderstorms to pop up on a daily basis for a couple of months before another seasonal change in weather patterns occurs.

Monsoons are a big deal in many parts of the world, including India, which is home to the world record for most rain ever recorded in one year. The city of Cherrapunji, which sits in the mountains north of Bangladesh, recorded more than 1000 inches of rain between August 1860 and July 1861.

A weather model image showing the amount of moisture in the atmosphere (precipitable water) on August 10, 2016. There was as much moisture over the deserts of Arizona as there was over parts of Virginia and Texas. Image credit: TwisterData

 
Americans are most familiar with monsoons when they come to the Southwest. Heavy rain is commonplace during the late summer months in states like Arizona, Nevada, and New Mexico. If you don't live in the region, it may be hard to envision downpours in the desert, but in reality, only the spring and early summer months are truly desolate with little rain to be found. This is typically the most miserable time of the year in the Southwest, allowing for constant sunshine with virtually no clouds to break the monotonous heat.

A monsoon develops due to a change in large-scale weather patterns; in the case of the Southwest, a broad area of high pressure sets up across the southern United States, generating that endless stretch of brutal summer heat over much of the country. Since wind blows clockwise around a high-pressure center in the Northern Hemisphere, this creates southeasterly flow over the desert southwest, allowing moisture from the Gulf of Mexico to pump into the region. Combine this with additional moist winds blowing off the Pacific Ocean and the Gulf of California and you have a recipe for frequent bouts of torrential rainfall.

A chart showing the average annual precipitation in five southwestern cities between 1981 and 2010. Image credit: Dennis Mersereau

 
When you look at the historical weather data, it’s easy to spot the beginning of monsoon season in the Southwest. A quick look at climate information for five cities in the region (above) shows some precipitation during the first few months of the year, followed by that desolate stretch in spring and early summer. The floodgates open and the deluge begins almost as soon as the calendar flips over to July, bringing a sharp uptick in heavy rain through September.

Even though it’s a yearly phenomenon, the ground in the desert Southwest isn’t capable of handling these heavy thunderstorms. Most cities in the eastern U.S. can handle a quick inch or two of rain as the soil there can absorb the sudden influx of water with relatively few problems. But the arid soil of the desert Southwest is far less permeable than most, so when it rains, most of the water runs off instead of sinking into the ground. 

Flash flooding is a serious threat in the Southwest during monsoon season, even miles away from a thunderstorm where the skies might be clear. Most of the rain that falls in the desert eventually winds up in an arroyo, or wash—a dry riverbed that only fills up after it rains. Arroyos can quickly fill up with millions of gallons of muddy water carrying rocks and trees and other debris, all gushing downstream faster than many pedestrians and motorists can react. It’s flash flooding in the most literal sense of the term: It happens in a flash, and the results can be devastating. 

Several people die every year in the Southwest due to flash flooding. In the past 20 years, flooding has claimed the lives of 68 people in Arizona. In Utah, 20 fatalities occurred in a single day in September 2015 after a flash flood swept down a stream in Hildale, Utah. Several vehicles carrying multiple families were caught in the flood, and most of the occupants couldn’t escape before the water and mud overcame them.