‘June Gloom’ Creeps Ashore

The Los Angeles area’s brief flirtation with summer-like temperatures appeared to be over Friday as an invasion of dank morning weather from the Pacific signaled the beginning of what probably will be about a month of typical “June gloom.”

Forecasters said skies during the Memorial Day weekend should be mostly foggy and cloudy in the mornings, clearing before noon in the inland valleys but not until afternoon along the coast.

Afternoon temperatures could reach the mid-80s in places such as Woodland Hills, Monrovia and San Bernardino, but thermometer readings probably won’t get much above the mid-60s in Santa Barbara, Santa Monica and Huntington Beach.

In one of those strange twists of meteorological irony, it is getting cooler near the coast because it is getting hotter in the desert.

Tim McClung, a meteorologist with the National Weather Service, said ocean temperatures off Southern California are still relatively cool, which keeps the air temperatures there cool too. On Friday, the air and water temperatures at the beach in Santa Monica were in the low 60s.

On the other hand, the longer days, as the June 21 summer solstice nears, are rapidly heating up the inland deserts. In Death Valley on Friday, the thermometer peaked at 117 degrees.

Cool air is denser than hot air, so air tends to flow from cooler areas to warmer areas. With a temperature differential of 50 degrees or more at this time of year, the onshore flow of cool, damp air from the Pacific is especially persistent.

This dank air piles up against the coastal slopes of Southern California as fog and clouds. Morning sunshine starts burning away the overcast, but the closer to the coast, the later it will be before the skies turn sunny and temperatures begin to rise.

Compounding the effects of the onshore flow is a Southern California phenomenon known as the “Catalina eddy,” McClung said.

Winds tend to move down the West Coast in a generally southeast direction, paralleling the flow of the ocean’s Japanese Current. Off Southern California, in the general area of Santa Catalina Island, those winds start encountering the persistent eastbound breezes that circle the globe north of the equator.

McClung said those breezes deflect the air flow in an arc that is intensified and tightened by the rotation of the Earth, strengthening the onshore movement of moist marine air into the Los Angeles Basin.

By the Fourth of July, the coastal water and the air above it will have warmed considerably, lessening the temperature differential and weakening the onshore flow. There’s still morning fog along the coast in July, August and September, but it seldom moves more than a few miles inland and tends to burn off early.

Shorter days in the fall, winter and early spring allow the deserts to cool off, keeping the temperature differential--and onshore flow--relatively low.

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Gray Days of Summer

A weather condition that is common in Southern California, called a Catalina eddy, enhances “June gloom.” The eddy brings moist air ashore, and because of a bigger difference between sea and land air temperatures in May and June, more cloud cover results.

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JUNE GLOOM: When cold air meets warm Cold ocean air

Chilled, moist air flows onshore, pushes warm air up and forms a cap

At other times of the year, the ocean breezes and air over land are closer in temperature and no cap forms.

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The Influence of Catalina Eddy

Normal fog and clouds along the coast retreat toward the ocean during the day.

Onshore flow of fog, clouds increases as eddy circulation intensifies. Marine layer reaches farther inland

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Sources: Scripps Institution of Oceanography and UC San Diego, American Meteorological Society, University of Wisconsin-Madison, Jet Propulsion Laboratory

Reported by BRADY MacDONALD/Los Angeles Times