Environment & Science

LA's 'May Gray' unusual this year thanks to 'reverse clearing'

An overcast day in Los Angeles.
An overcast day in Los Angeles.
Stock photo by Dhrupad Bezboruah via Flickr Creative Commons

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Call it "May Gray," call it the marine layer, whatever you call it, Southern California is seeing a lot of it lately.

And while it's typical for the region to get a nice early morning cloud cover this time of year, recently those clouds are sticking around in some unusual places, like the valleys and foothills.

Normally, the cloudy marine layer blows in from the ocean toward the mountains then recedes. Think of it like a wave coming in early morning and going out as the day warms.

That means skies tend to clear first in the L.A. basin while it’s still overcast at the beach, but recently, the coast is seeing blue skies while places like Pasadena and Azusa stay gray.

It’s a weather pattern called “reverse clearing,” explains Curt Kaplan with the National Weather Service in Oxnard.

An example of "reverse clearing."

Kaplan says normally, heat in the deserts creates low pressure in the air close to the surface which causes wind to blow on shore from the ocean, where the pressure is higher. These ocean winds often carry clouds we call the marine layer, June Gloom or May Gray.

However, this year there is an atypical low pressure system in the high atmosphere parked over Southern California.

This system is helping the marine layer in lower parts of the air to get really tall and thick. In fact, Kaplan says these clouds are sometimes twice as high as usual, up to around four thousand feet.

As these heavy clouds blow in from the ocean they are piling up at the base of the mountains and are taking longer to dissipate. Meanwhile, the steady ocean breezes are still blowing strong, clearing the skies along the beaches.

"So that’s why the valleys or the foothills will kind of be lingering but then it’ll start clearing near the coast," Kaplan explained.

One benefit is that this reverse clearing means it stays grey over parts of L.A. that are often baked with sun this time of year. With a nice cloud cover, those valley and foothill areas stay a little cooler than usual.

For those who like this weather, Kaplan has some good news for you: This reverse clearing pattern is expected to stick around through this week and possible longer.

If the persistent clouds are bringing you down, remember, it might be sunny just a few miles to the west. Beach day, anyone?