July and August are our driest months, averaging only a few hundreds of an inch of rain at Cal Poly. As we’ve seen in most years, it can be overcast and foggy along our coastline.
However, this cycle of low coastal clouds can be broken when subtropical moisture from the south wanders 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.
This moisture can also produce some spectacular sunrises and sunsets.
This subtropical moisture is part of a seasonal pattern called the North American monsoon. The technical definition is simply a seasonal shift in the wind. At this time of the year, a thermal low pressure system can develop over the Colorado River Valley, east of San Diego, while high-pressure builds over the Four Corners area.
When this condition occurs, the mid-level winds at about 8,000 to 11,000 feet of altitude will often shift from a southwest direction to a southeasterly incoming direction. This, in turn, can steer subtropical moisture from the Gulf of California toward the Central Coast.
Even if a thermal low doesn’t develop, the “Four Corners High” often drives monsoon moisture from the south over Southern and Central California. When the monsoon arrives, relative humidity levels and dew-point temperatures increase.
The dew point is the temperature at which air must be cooled 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 and down.
Dew-point temperature tells how much moisture is in the air, regardless of the temperature, until you reach the dew point. In my book, dew point is a far more straightforward indicator of how sticky or muggy it feels.
When this subtropical moisture moves in from the south, thunderstorms can develop over the higher mountains of San Luis Obispo County due to a combination of orographic lift and surface heating.
If the subtropical moisture source is plentiful — for example, the remnants of hurricanes and tropical storms from the eastern Pacific Ocean and the Gulf of Mexico that travel to our area — periods of rain and thunderstorms can develop throughout the Central Coast.
This happened in August 1976, the wettest August on record. Two subtropical weather systems produced nearly 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 northward 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.
As the summer progresses, the chance of receiving this type of rainfall becomes greater. San Luis Obispo is located on the fringe of the North American monsoon. Rains associated with this weather phenomenon are seldom destructive. But lightning accompanying these events can result in damage and also spark wildfires.
“Wildfires are a huge risk in our service area of Northern and Central California, and PG&E is serious about working with Cal Fire and other state and local agencies to do our part to reduce that risk—including helping educate our customers about fire safety, operating our own equipment responsibly, and being prepared to respond,” said Barry Anderson, PG&E’s vice president of emergency preparedness and response.
To learn more about safety, please visit www.pge.com