Weather Watch

August can be monsoon season along the Central Coast

Subtropical moisture can produce beautiful sunset photos.
Subtropical moisture can produce beautiful sunset photos.

August is dry, averaging only about 0.05 inches of rain in San Luis Obispo.

And as we’ve seen most years, it can be quite overcast and foggy along our coastline.

Some coastal residents describe the month as Fogust.

In the past, I’ve seen this cycle of low coastal clouds broken when subtropical moisture from the south migrates across our area. This moisture often mixes out the marine layer and can produce rain and thunderstorms. In just a few hours, our Mediterranean climate can change to a humid, subtropical one reminiscent of Florida.

These storms are part of a seasonal pattern called the North American monsoon.

The monsoon system can form when the desert southwest heats up during the summer months, creating a thermal low. This low-pressure zone can change the direction of the jet stream, steering subtropical moisture toward the Central Coast.

When this occurs, we often experience higher relative humidity levels and dew-point temperatures. The dew point is the temperature at which air must be lowered for it to become saturated — in other words, the temperature when dew forms.

Relative humidity, on the other hand, is the amount of water vapor the air could hold at a particular temperature. The actual humidity level changes as the temperature goes up or down.

Dew-point temperature tells how much moisture is in the air, regardless of the temperature. In my book, dew point is a far more straightforward indicator of how sticky it feels outside.

Last year on July 30, the city of Bandar Mahshahr at the far northern part of the Persian Gulf in Iran reached an unimaginable dew-point temperature of 90 degrees. At the same time, the air temperature was 115 degrees, which produced a heat index level of 165 degrees.

I’ll take Death Valley any day over that type of heat.

If the subtropical moisture source is plentiful, the remnants of hurricanes and tropical storms from the eastern Pacific Ocean and the Gulf of Mexico can cause periods of rain and thunderstorms to interrupt our Mediterranean climate.

That happened last year, right before the start of the California Mid-State Fair, when abundant subtropical moisture from the remnants of Hurricane Dolores produced 3.55 inches of rain in Paso Robles. Weather stations around San Luis Obispo reported about an inch of rain. Accompanying the rain were approximately 35,000 lightning strikes, according to sloweather.com.

In August 1976, the wettest August on record, two subtropical weather systems produced about 1.5 inches of rain in San Luis Obispo between Aug. 15 and Aug. 20.

Later in September, Hurricane Kathleen developed in the eastern Pacific and took an unusual path north through Baja California. It crossed the U.S.-Mexico border near El Centro, east of San Diego, as a tropical depression. The day before rain from this storm reached San Luis Obispo, temperatures reached 99 degrees in San Luis Obispo and 101 degrees in Paso Robles.

Kathleen produced gale-force winds and widespread flooding in many parts of the West, especially in California’s Imperial Valley. Subtropical moisture from Kathleen produced 1.72 inches of rain in San Luis Obispo on Sept. 10 and 11.

San Luis Obispo is located on the fringe of the North American monsoon. Rains associated with this weather phenomenon are seldom destructive. However, as the summer progresses, the chance of receiving this type of rainfall becomes greater until it peaks during the month of September. However, as the climate continues to warm, greater amounts of subtropical moisture are expected to stream further northward in the future.

With that being said, monsoon rains can be a blessing or a curse. Some of the long-range numerical models are indicating that subtropical moisture with rain showers could move over our area by mid-August. This type of rain can help Cal Fire to extinguish wildfires, like the Soberanes Fire in Monterey County, which has burned about 34,000 acres and was only 15 percent contained as of midday Saturday.

On the other hand, lightning accompanying these events can spark wildfires and produce power outages.

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Lightning can cause power outages. Take steps now to stay safe in the event of a power outage: Keep a battery-operated flashlight and radio within easy reach. Use safer LED candles. Wax candles are not recommended. Plan for another way to communicate. Don’t depend on a phone that requires electricity. Keep a standard handset or mobile phone ready as a backup. Store water-filled plastic containers in your freezer. You can use them as blocks of ice to prevent food from spoiling. For more PG&E safety tips, visit www.pge.com.

John Lindsey is PG&E’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John for weather and other useful information.

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