In the past, I’ve seen the summer cycle of morning low coastal clouds broken when subtropical moisture from the south migrates across the Santa Ynez Valley.

That 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.

Those 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.

That low-pressure zone can change the direction of the jet stream, steering subtropical moisture toward the Central Coast.

When that occurs, we often experience higher relative humidity levels and dew-point temperatures.

The dew point is the temperature to 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.

A few years ago in July, 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.

In August 1976, the wettest August on record, two subtropical weather systems produced heavy rain throughout the Central Coast 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 Santa Ynez, the temperature reached the high 90s.

Kathleen produced gale-force winds and widespread flooding in many parts of the West, especially in California’s Imperial Valley.

Rains associated with that weather phenomenon are seldom destructive.

However, as the summer progresses, the chance of receiving that type of rainfall becomes greater until it peaks during the month of September. As the climate continues to warm, greater amounts of subtropical moisture are expected to stream farther northward in the future.

With that being said, monsoonal 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.

That type of rain can help Cal Fire extinguish wildfires. On the other hand, the lightning accompanying those events can spark wildfires.

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Lightning can cause power outages by striking utility poles. Take steps now to stay safe in the event of a power outage:

Keep a battery-operated flashlight and radio within easy reach.

Wax candles are not recommended. Use safer LED candles.

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

John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Follow him on Twitter @PGE_John.


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