Late summer rain events are rare in the Bay Area. Weather disturbances that deliver clouds, showers or thundershowers to California during August and September are usually the result of a weather anomaly called the North American Monsoon (NAM).
The term monsoon is often used to describe heavy rain events, but it actually refers to a seasonal wind shift. In fact, the word comes from the Arabic mausim, meaning season.
This wind pattern causes the weather to switch from hot and dry to humid and stormy. Monsoon winds form over warm gulf and ocean waters. Moist, low-level air is pushed northward toward land masses that are typically hot and dry in late summer.
The Asian Monsoon is the most well-known as it encompasses an area that extends from India to Southeast Asia. A large percentage of annual precipitation in countries affected by this monsoon falls during this season.
The monsoon is a key factor in the economy of the area, because agricultural harvests depend on ample monsoon season rains. However, extremely strong monsoons are responsible for devastating floods that can ravage the region. Weak monsoons can trigger food shortages.
The NAM is a much smaller circulation than its Asian cousin. It is similar in scale to monsoons found along the Brazilian coast and southern Africa.
In the case of our local monsoon, the main moisture sources are the gulfs of Mexico and California, which record their warmest water temperatures in late summer. During a NAM event, the subtropical jet stream buckles, allowing southerly winds to push moist air northward into western North America.
The storms brought on by the NAM typically produce up to 70 percent of a year’s rain from west Texas to Southern California. During an average monsoon season, cities across the American Southwest receive up to 10 inches of rain.
The northward extent of NAM rains depends on the strength and orientation of the jet stream over the warm tropical waters. The mountains of Southern California typically form the northern boundary of monsoon-related rainfall in the state.
Several times per year, however, a strong and persistent NAM jet stream will steer moisture into the Sierra Nevada and northern mountains of California.
It takes a “perfect storm” of jet stream conditions to deliver monsoon moisture to the Bay Area. When that does happen, we can experience some fantastic lightning displays and beautiful sunsets from our backyards in the Concord/Clayton area.
Woody Whitlatch is a meteorologist retired from PG&E. Email your questions or comments to