Weather Watch

It’s about to get hot, humid — and rainy? — in SLO. What causes the California monsoon?

Thunderstorms hit SLO County as monsoon weather sweeps the Central Coast

San Luis Obispo County gets rain, lightning and thunder on Wednesday, August 2, 2017, when it's usually dry on the Central Coast.
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San Luis Obispo County gets rain, lightning and thunder on Wednesday, August 2, 2017, when it's usually dry on the Central Coast.

The long-range numerical models, the oracles of weather forecasting, have been advertising the return of monsoonal moisture to the Central Coast by the first week of August.

This phenomenon is part of a seasonal pattern called the North American monsoon. When this occurs, we often experience higher relative humidity levels and plenty of virga — visible streaks of rain that fall from a cloud but evaporate before reaching the ground.

In other words, it changes our dry and comfortable climate to a humid, subtropical, and sticky one reminiscent of Florida.

This condition can also bring rain and thunderstorms to a typically dry time of the year. We live in a Mediterranean climate, which generally means dry summers and wet winters. San Luis Obispo and Santa Barbara counties only receive about a third of an inch of rainfall during the July, August and September time frame — regardless where your location may be along the Central Coast.

The North American monsoon causes most of the summertime precipitation. Sure, it may not be as glamorous as its Asian counterpart. Nevertheless, it can produce large amounts of rain in northern Mexico, much of Texas, New Mexico and southern Arizona. On rare occurrences, it can produce heavy rain in Southern and Central California, also.

So, what atmospheric condition causes this subtropical moisture to stream northward into California? Typically, when the desert southwest heats up in summer, it creates a thermal low. This low-pressure zone can change the direction of the jet stream, which steers subtropical moisture northward toward the Central Coast.

If the subtropical moisture source is plentiful — especially when remnants of hurricanes and tropical storms from the eastern Pacific Ocean and the Gulf of Mexico move over our area — periods of rain and thunderstorms can interrupt our Mediterranean climate.

This condition occurred on July 18-20, 2015, when abundant subtropical moisture from the remnants of Hurricane Dolores produced record amounts of warm July rainfall. Previous records for July were 0.59 of an inch in Paso Robles and 0.46 of an inch in San Luis Obispo; both records were set back in 1950. A weather station in Paso Robles reported 3.55 inches, while numerous weather stations around San Luis Obispo reported around an inch on July 20, 2015. All this rain nearly delayed the start of the Mid-State Fair due to localized flooding.

The Santa Maria Airport reported 0.14 of an inch of rain, while Vandenberg Air Force Base recorded 0.28 of an inch. The highest amount of rain recorded in Santa Maria in July occurred in 1950 when 0.62 of an inch of precipitation reached the ground. Accompanying the rain was approximately 35,000 lightning strikes, according to the www.SLOweather.com.

August 1976, the wettest August on record, saw two subtropical weather systems that produced nearly 1.5 inches of rain in San Luis Obispo, while Santa Maria saw nearly 1 inch of rain between Aug. 15-20. Later in September, Hurricane Kathleen developed in the eastern Pacific and took an unusual path northward through Baja California. It crossed the U.S.-Mexico border near El Centro, east of San Diego, as a tropical depression. Kathleen produced gale-force southerly winds and widespread flooding in many parts of the west, especially in California’s Imperial Valley.

The day before this storm reached San Luis Obispo County, temperatures reached 99 degrees in SLO and 101 degrees in Paso Robles. Subtropical moisture from Kathleen produced 1.74 inches in Santa Maria and 1.72 inches of rain in San Luis Obispo on Sept. 10 and 11.

As the summer progresses, the chance of receiving this type of rainfall becomes greater until it peaks during September.

As the atmosphere and oceans continue to warm, long-range climate models are predicting the North American monsoon could become more prevalent in our state in late summer. Recent studies have already identified a shift in seasonality of the monsoon to September/early October with increased amounts of rain late in the monsoon season.

Of course, the increased amount of rain is welcome; however, lightning accompanying these events can spark wildfires. To learn more about fire safety, please visit www.pge.com.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.
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