Yes, the monsoon can even affect Southern California

Most summertime weather in Southern California is fairly benign. But clouds, rain and thunderstorms can occasionally come from another source: The same North American Monsoon that mainly affects the desert southwest.

Powerful monsoon storms moved through Arizona, Utah and Colorado on Aug. 21-23, 2024, with intense downpours, hail, heavy winds, flooding and lightning. In comparison, most of the summertime in Southern California is usually fairly benign.

Much of Southern California has a Mediterranean climate with generally mild, wet winters and dry, warm summers. While many of the world's major cities, from New York and Chicago to Tokyo and Sydney, deal with afternoon and evening thunderstorms amid summertime heat and humidity, cities such as Los Angeles and San Diego bask in sunshine all summer—assuming the May Gray and June Gloom aren't too dastardly.

Besides the characteristic early morning coastal clouds and fog that can mar an otherwise beautiful day, clouds, rain and thunderstorms can occasionally come from another source: the same North American Monsoon that affects mainly the Desert Southwest.

On the weekend of Aug. 10-11, the National Weather Service (NWS) reported a total of 8,953 lightning discharges across San Diego County, including 713 that were cloud-to-ground strikes. Most of the impact was felt inland, including the high-elevation town of Ranchita, which received 1.43 inches of rain.

But what is the North American monsoon, and why can it sometimes spill into Southern California?

A monsoon primer

First, here's a quick rundown on monsoons: The most well-known monsoons happen halfway across the world. The Asian monsoon and Indian Ocean monsoon are known for bringing intense rainfall after an extended dry period. Goa, a state in southwestern India, is generally rainless from January through March. However, between June and August, Goa receives nearly 94 inches of rain, more than most places receive in an entire year. Yet while many associate monsoons with rain, the phenomenon actually refers to a seasonal shift in the wind direction, AccuWeather Meteorologist Dave Houk said. That shift can either bring either more persistent rain or predominantly dry weather for several months.

The word "monsoon" comes from the Arabic word meaning "season." This helps clarify that the monsoon isn't just about rain. Think of it as two sides of the same coin. "When the flow for a season switches from land to ocean in some monsoonal affected regimes, that flow delivers dry air from the continent leading to a 'dry' monsoon," Houk said. "However, when the land is warmer than the surrounding ocean, the flow [comes] from the ocean to the land helping to deliver increased moisture to fuel the seasonal rains or 'wet' monsoon."

In the North American monsoon, breezes bring moist air northward from Mexico that originated from Gulf of California, the tropical Pacific Ocean and even the Gulf of Mexico. This can lead to intense summer rain and thunderstorms in cities such as Phoenix, Las Vegas and Albuquerque.

"The North American monsoon typically starts in northwestern Mexico in June and expands northward into the southwestern U.S. in July before settling down in September," Houk said.

Southern California monsoons

Often, California is sheltered from the worst impacts of the North American monsoon due to the cool Pacific Ocean, which stabilizes the atmosphere close to the coast. The topography of Southern California, including its Peninsular Ranges and Transverse Ranges, also keeps some of that moisture at bay. There may be a monsoonal incursion to these mountains but not beyond. It's not unusual to see cumulus and cumulonimbus clouds building up over the ranges in inland Southern California during the summer, but rain often eludes the state that time of year.

But what causes monsoonal moisture to flow beyond that point? The North American monsoon is marked by a broad area of high pressure, usually parked in the Four Corners region, but the exact position of that high can influence exactly where (and how much) moisture flows.

"The deepest moisture concentration can vary day to day and week to week depending on the position of what is commonly referred to as the 'Four Corners' high and how that upper-level feature directs the steering flow for the moisture and resulting showers and thunderstorms," Houk said.

Another factor is tropical systems. Southern Californians won't soon forget former Category 4 storm Hilary, which brought 13.5 inches to the mountains of San Bernardino County and more than 3 inches to bone-dry Palm Springs in August 2023. It even transformed Death Valley into a massive lake. However, sometimes the moisture from these tropical systems from the south and west can connect with moisture from the monsoon coming out of the east, leading to impacts for Southern California.

In July 2021, monsoonal showers brought rain as far west as Hermosa Beach, California, with Downtown Los Angeles recording 0.18 of an inch of rain in a 24-hour period. But before everyone starts thumbing their noses at melodramatic Californians, there have been much bigger monsoonal events. Strong monsoonal flow hit in July 1956 for several days, with daily thunderstorms, even west of the mountains. On July 24, 1956, an evening thunderstorm hit Barstow with about 1 inch of rain in a mere 30 minutes for an arid area that gets 4 inches of rain annually. More recently, in July 2015, moisture from Hurricane Dolores combined with monsoon moisture to create showers and storms throughout Southern California. Rainfall ranged from 0.5-4 inches, including a record 1.71 inches at San Diego on July 18.

It was the San Diego Padres' first rainout in July ever.