TheCatalina eddy wind pattern, also called the "coastaleddy" or "marine layer," is a localizedweatherphenomenon that occurs in theSouthern California Bight, the mostly concave portion of the Southern California coast running fromPoint Conception toSan Diego. The Catalina eddy leads toJune Gloom, which is so much a part of the late spring and early summer weather in Southern California.^[1] The eddy is named forSanta Catalina Island, one of theChannel Islands offshore between Los Angeles and San Diego.^[2]

Though the coastal marine layer can develop at any time of the year, predominantly these eddies occur between April and September with a peak in June.^[3] During these months, upper-level northwesterly flow along the California coast is directed onshore by the Channel Islands. When the flow is blocked by the mountains that ring theLos Angeles Basin to the east and north, a counterclockwisevortex is created. As temperatures drop after sunset, themarine layer deepens and coastalstratus 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 southwesternMojave 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 thedeserts causes air to rise, the resultingpressure 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."

• Maas, Clifford F. and Mark D. Albright, "Origin of the Catalina Eddy," Monthly Weather Review, American Meteorological Society, Volume 117, Issue 11 (November 1989)
• Glossary, National Weather Service, National Oceanic and Atmospheric Administration (NOAA)
• Gales, Donald Moore, "Handbook of Wildflowers, Weeds, Wildlife and Weather of the South Bay and Palos Verdes Peninsula", 4th Ed. Published by Foldarol Company 2003,ISBN 0-9620623-1-6

• Marine meteorology
• Climate of California
• Weather events in the United States

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