The "Catalina eddy," also called the "coastal eddy

Eddy (fluid dynamics)

In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid flows past an obstacle. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object...

," is a localized weather

Weather

Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...

 phenomenon

Phenomenon

A phenomenon , plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'...

 that occurs in the so-called "Bight of California

Southern California Bight

The Southern California Bight includes coastal southern California, the Channel Islands and part of the Pacific Ocean.Within the Southern California bight lie the traditional territories of the Chumash and the Gabrieliño. These two cultures are considered the epitome of hunter-gatherer complexity...

", the mostly convex portion of the Southern California

Southern California

Southern California is a megaregion, or megapolitan area, in the southern area of the U.S. state of California. Large urban areas include Greater Los Angeles and Greater San Diego. The urban area stretches along the coast from Ventura through the Southland and Inland Empire to San Diego...

 coast running from Point Conception

Point Conception

Point Conception is a headland along the Pacific coast of U.S. state of California, located in southwestern Santa Barbara County. It is the point where the Santa Barbara Channel meets the Pacific Ocean, and as the corner between the mostly north-south trending portion of coast to the north and the...

 to San Diego. The Catalina eddy leads to the chilly "May Gray" and "June gloom

June Gloom

June Gloom is a southern California term for a weather pattern that results in cloudy, overcast skies with cool temperatures during the late spring and early summer. June Gloom in southern California is caused by the marine layer effect common to the West Coast, and is enhanced by the Catalina eddy...

" that is so much a part of the late spring and early summer weather in Southern California. The eddy gets its name from Santa Catalina Island

Santa Catalina Island, California

Santa Catalina Island, often called Catalina Island, or just Catalina, is a rocky island off the coast of the U.S. state of California. The island is long and across at its greatest width. The island is located about south-southwest of Los Angeles, California. The highest point on the island is...

, the Channel Island

Channel Islands of California

The Channel Islands of California are a chain of eight islands located in the Pacific Ocean off the coast of Southern California along the Santa Barbara Channel in the United States of America...

 closest to the Los Angeles-Long Beach area; the center of an eddy is often located above or near the island.

While a Catalina eddy can develop at any time of the year, it is most common between April and September and peaks in June. During these 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

Los Angeles Basin

The Los Angeles Basin is the coastal sediment-filled plain located between the Peninsular and Transverse ranges in southern California in the United States containing the central part of the city of Los Angeles as well as its southern and southeastern suburbs...

 to the east and north, a counterclockwise vortex

Vortex

A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...

 is created. As temperatures drop after sunset, the marine layer

Marine layer

A marine layer is an air mass which develops over the surface of a large body of water such as the ocean or large lake in the presence of a temperature inversion. The inversion itself is usually initiated by the cooling effect of the water on the surface layer of an otherwise warm air mass...

 deepens and coastal stratus cloud

Stratus cloud

A stratus cloud is a cloud belonging to a class characterized by horizontal layering with a uniform base, as opposed to convective clouds that are as tall or taller than wide . More specifically, the term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in color...

s 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

Mojave Desert

The Mojave Desert occupies a significant portion of southeastern California and smaller parts of central California, southern Nevada, southwestern Utah and northwestern Arizona, in the United States...

. 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 desert

Desert

A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...

s causes air to rise, the resulting pressure gradient

Pressure gradient

In atmospheric sciences , the pressure gradient is a physical quantity that describes in which direction and at what rate the pressure changes the most rapidly around a particular location. The pressure gradient is a dimensional quantity expressed in units of pressure per unit length...

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."