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“It’s really an unusual low-pressure system pulling in this moist, tropical air from Mexico,” Kittell said.

The effects of the cool, moist air will be felt along the coast, in downtown Los Angeles and in some areas of the San Gabriel Valley, Pittell said.

May gray is fueled by onshore breezes that push the layer of cool moist air that hangs above the Pacific Ocean further inland, creating persistent cloud cover.

“You might have some moist grounds, moist cars when you get up Tuesday morning — and even some drizzle possibly on your morning commute.”

Many in the tour group wore rubber boots as they stood on moist concrete in a chamber faintly lit by filtered sunlight, peering into the dark tunnel.