After a dry winter, monsoon season has finally arrived in Northern Arizona. If you’ve hiked the Grand Canyon in late summer, you might have experienced these storms firsthand. Beautiful, clear mornings give way to torrential afternoon rains that can disappear just as quickly as they blew in. But what actually causes monsoons, and why would they appear over the desert Southwest?
The North American Monsoon System owes its existence to the interaction between two different air masses. One air mass is a low pressure zone caused by intense summer heating over Northern Mexico and the American Southwest. The other is a belt of high pressure air, called a “subtropical ridge.” The subtropical ridge is caused by convection currents in the earth’s atmosphere. Between late spring and early fall, the subtropical ridge migrates away from the equator. As it does so, it encroaches on the low pressure zone.
The subtropical ridge brings moisture from the Pacific Ocean and the Gulf of California. As a general rule, the warmer the surface temperature over the Gulf of California, the more precipitation we can expect to see over Arizona. Even though the southern air mass is in contact with warm water in the Gulf of California, it is still relatively cool compared to the hot, inland deserts.
Elevation is another basic ingredient in monsoon systems. Air masses compress as they move over high-elevation features on the landscape, causing precipitation. The South Rim, with elevations greater than 7,000 feet above sea level, sustains juniper-pinyon and ponderosa pine forests. At 4,500 feet above sea level, the nearby Painted Desert is sparsely vegetated by comparison.
The entire Colorado Plateau is a large, high-elevation feature. Hence the monsoons become especially intense in the area surrounding Grand Canyon. The exact location of a monsoon storm depends on hard-to-predict, localized variations in atmospheric instability. Tourists at Mather Point might be treated to a thorough soaking while other visitors enjoy a pleasant sunset at Hermits Rest.
Although the general causes of monsoons are well-established, Arizona hikers might be surprised to learn that scientists have struggled to gain a more precise understanding of the North American Monsoon System. In 2004, the National Oceanic and Atmospheric Administration (NOAA) described it as “perhaps the least-understood of all of large-scale circulations patterns that affect the United States.” So, what does it mean for hikers now that monsoon season has finally arrived? It means that there are two big hazards to be aware of: lightning and flash floods.
According to the National Park Service, Grand Canyon is struck by lighting about 26,000 times per year. The safest course of action is to avoid being in an exposed area in the first place. Hit the trail early, and plan your hikes with an expectation of early afternoon thunderstorms. Stay away from the rim if a storm is approaching. Front-country campers are safest inside their vehicles so long as they don’t touch any metal components. NPS provides lightning safety advice specifically for Grand Canyon hikers (PDF), including what you should do if (in spite of your best preparations) you are caught out in the open.
Hikers seldom venture into a deadly flash flood on purpose. They are more likely to be caught off guard. In canyon country, steep rock walls can hide distant precipitation that eventually appears as a flash flood in or along your route. Again, plan to hit the trail early, and identify escape routes that will take you out of harm’s way. Never camp in dry washes. Grand Canyon National Park has more safety information on their website.
The monsoon season officially began on June 15th and will end on September 30th. In the past, monsoon season was defined by average daily dew-point temperature. However, in 2008 the National Weather Service switched to a fixed time period in order to better communicate the risks and hazards of monsoons.