Weather Watch

A monsoon can happen in August

August is dry, averaging only about 0.05 inches of rain in San Luis Obispo. As we’ve seen most years, it can be quite overcast and foggy along our coastline.

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 desert Southwest heats up during the summer months, creating a thermal low. This low-pressure zone can change the direction of the jet stream, which steers 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 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 it feels outside.

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.

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 until it peaks during the month of September.

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, so firefighters must be especially alert.

Today’s forecast

The story for most of the summer has been a persistent upper-level, low-pressure trough over California. This trough will produce much cooler than average temperatures through Wednesday. This morning’s deep marine layer will produce pockets of drizzle along the beaches and into the coastal valleys. The coastal clouds will retreat to near the coast by late this morning. The coastal stratus will burn off from beaches by this afternoon. All the while, we’ll continue to have gentle to moderate (8 and 18 mph) northwesterly (onshore) winds.

Today’s temperatures will range between the high 50s to the low 60s along the northwesterly facing beaches (Los Osos and Morro Bay) and low to mid-60s along the southwesterly facing beaches (Avila Beach and Cayucos) today.

The coastal valleys will range between the high 60s to the low 70s, with the North County only hitting the high 70s. To the east, California Valley is forecast to reach the low 90s.

Afternoon and evening thunderstorms will develop in the Sierra Nevada Mountains through Monday, but most of the activity will be focused in higher-elevation areas.

Starting around Thursday, temperatures will climb back closer to average, especially inland.

Surf report

Today’s surface charts and models are not indicating much storm activity in both the Northern and Southern Hemisphere.

This morning’s 3- to 5-foot northwesterly (315-degree deepwater) swell (with a 7- to 9-second period) will remain at this height and period all the way through Friday.

Arriving from the Southern Hemisphere:

Today’s 1- to 3-foot Southern Hemisphere (210-degree deepwater) swell (with a 14- to 16-second period) will continue at this height and period through today, gradually decreasing Monday.

Seawater temperatures will range between 56 and 58 degrees Monday, increasing to 57 and 59 degrees Tuesday through Friday.

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John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 23 years. If you would like to subscribe to his daily weather forecast or ask him a question, send him an e-mail at pgeweather@pge.com.

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