‘May gray’ arrives in Bay Area with 1,500-foot-thick marine layer

After days of hot and dry weather, the Bay Area will see a major shift to start the Memorial Day weekend as a low-pressure system moves in, weakening the ridge of high pressure above the state.

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“An upper level ridge over the northern Pacific that extends into the Pacific Northwest is continuing to flatten under the influence of a trough centered over Alaska, allowing a gradual cooling trend to begin today,” read the National Weather Service’s forecast discussion.

Brayden Murdock, a meteorologist with the weather service, said the Bay Area region will see foggier skies and cooler temperatures, especially along the coast, thanks to the new weather pattern.

“We’re back to onshore flow, which means most of our air is coming almost directly from the coast and pushing inland, so for that, we get a good opportunity for the marine layer to reestablish and then push further inland,” Murdock said. “Even areas away from the marine layer, a lot of that wind is that cooler ocean air, which is sitting around the upper 50s right now, and that gets a chance to push inland to help cool off some of those spots.”

On Friday morning, the marine layer was about 1,000 to 1,500 feet thick, resulting in gray skies along the coast. As the temperature cools this weekend, the foggy skies will push farther inland, Murdock said.

“Certain areas aren’t going to be cooking as long in the sun as they have been, and then as we continue through the weekend, we’re going to continue to see that set up,” he said. “Areas like San Jose [are] starting off with cloudier conditions tomorrow, so that helps things stay cool.”

The shift will result in cooler weather for the Memorial Day holiday. Oakland is expected to see highs in the mid 60s by Monday. San Jose’s high is forecast to be 75. Downtown San Francisco is expected to see highs in the 60s, although areas near the ocean will be cooler.

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“If you’re right along the Pacific during the weekend, you’re probably only going to be seeing highs in the 50s,” Murdock said.

In Santa Rosa, the foggy weather means overnight lows are forecast to drop to the high 40s before a high of 72 during daytime hours on Monday.

The gray skies will also mean some slightly wet weather as a drizzle arrives on Sunday and Monday. The cooling trend is expected to peak next Tuesday, Murdock said, before warmer weather starts to make its way back in.

“May gray going into June gloom,” Murdock said of the weekend weather. “We’re going to see plenty of that in the next few days.”