Weather Watch

Catalina eddy may bring a change to the Central Coast’s weather

A marine layer rolling into San Luis Obispo at sunrise.
A marine layer rolling into San Luis Obispo at sunrise.

The Catalina eddy and the cutoff low are two types of low-pressure systems that can make weather forecasting a challenge along the Central Coast.

Forecasting a Catalina eddy — a circular counterclockwise air flow — can be difficult because it usually covers a small area in the Southern California Bight, the coast from Point Conception to just south of San Diego. But when a Catalina eddy does form, it can quickly stop relentless northwesterly winds and allow extensive marine low clouds to develop over the coastal regions.

The formation of the Catalina eddy is still not completely understood. During a period of gale-force northwesterly winds, such as the unrelenting winds that blew this past week, coastal eddies seem to develop as these winds travel down the rugged California coast, move past Point Conception and interact with the topography of the Southern California coast. This interaction causes the winds to flow in a counterclockwise direction and creates an area of low pressure in the vicinity of Santa Catalina Island.

When coastal eddies develop, the winds will shift out of the south and usually produce a rapid increase in the depth of the marine layer, resulting in a thick blanket of fog and cooler temperatures. This condition also allows seawater temperatures to increase as upwelling diminishes and warmer water from the south moves north along the shoreline.

At the time of this writing, a few of the models indicate an eddy developing off the coastline Sunday night into Monday that should produce plenty of marine stratus clouds with areas of fog and drizzle. The Catalina eddy can occur all year long, but happens most often from May through July during the peak of our northwesterly winds.

Another weather forecast challenge is the cutoff low. The late great weather forecaster George Fischbeck of KABC-TV in Los Angeles used to say, “A cutoff low is a weatherman’s woe.” These cutoff low-pressure systems are notorious for being unpredictable and can humble the most confident meteorologist.

Later this week, an upper-level low-pressure system is expected to develop off the Northern California coast and break away from the jet stream, becoming a cutoff low. That means the jet stream will shift to a higher latitude and leave a circulating low-pressure system behind. This cutoff low is forecast to move toward central California’s coastline this Friday into Sunday with scattered rain showers and a chance of thunderstorms over San Luis Obispo and Santa Barbara counties.

At our latitude, the jet stream is typically a tubular ribbon of high-speed winds some 18,000 feet to 40,000 feet high, flowing in wavelike patterns from the west to the east for thousands of miles. The jet stream is the main mechanism that drives storms from the Pacific Ocean east toward the Central Coast. When a low-pressure system breaks away from the main flow, it becomes difficult or almost impossible to predict the speed and direction of these weather systems.

These lows have been known to stay nearly stationary for days, or even move west, back out to sea. Often when a cutoff low develops, the many numerical weather prediction models that meteorologists consult for guidance can diverge wildly, much like the cutoff lows themselves.

“A weatherman’s woe,” indeed.

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John Lindsey is PG&E’s Diablo Canyon meteorologist and media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.

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