Catalina eddy

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The "Catalina eddy"--also called the "coastal eddy"--is a localized weather phenomenon that occurs in the so-called "bight of California", the mostly convex portion of the Southern California coast running from Point Conception to San Diego. The Catalina eddy leads to the "June gloom" that is so much a part of the late spring and early summer weather in Southern California.

In spring and summer months, upper-level northwesterly flow along the California coast is directed onshore by the Channel Islands offshore. When the flow is blocked by the mountains that ring the Los Angeles Basin to the west and north, a counterclockwise vortex is created. As temperatures drop after sunset, the marine layer deepens and coastal stratus clouds thicken. While the vortex is relatively small--rarely more than 100 miles (160 km) in diameter--it can extend into inland valleys and even into the southwestern Mojave Desert. A very strong Catalina eddy can be as deep as 6000 feet (1.8 km).

A Catalina eddy is rarely prolonged: as the heat over the deserts causes air to rise, the resulting pressure gradient and increase in the normal onshore winds causes the vortex to dissipate. The result is the common local weather forecast calling for "late night and early morning low clouds and fog, followed by afternoon sunshine and sea breezes."

While a Catalina eddy can develop at any time of the year, it is most common between April and September and peaks in June. The eddy gets its name from Santa Catalina Island, the Channel Island closest to the Los Angeles-Long Beach area; the center of an eddy is often located above or near the island.

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