Cooling occurs when the marine layer resurfaces

A four-day heat wave that gave the Bay Area its first taste of summer air gave strategy to a low-pressure system Friday morning that began to push the heat-generating ridge of high pressure out of the region and permit the region's natural air con system to do its job.

Fog returned to the coast, cool morning breezes reached much of the inside, and cities across the region were expected to see temperatures 8 to fifteen degrees cooler than earlier within the week.

“What we had overnight and into Friday was entirely due to the marine layer,” said Dylan Flynn, a meteorologist with the National Weather Service. “The ridge was compressed and stayed very close to the coast. But as the high pressure area moves east, the marine layer doesn't get as compressed. It can move outward, and a much larger portion of the area feels that.”

The migration of the 2 systems – the high pressure system moved east and the low pressure system got here from the coast – actually began on Thursday, with temperatures 5 to six degrees lower than the day gone by. In Concord, the thermometer reached 92 degrees. In Livermore, highs of 91 degrees were reached, in San Jose 83 degrees and in Redwood City 81 degrees.

They will drop significantly on Friday. A heat warning that had been in effect for big parts of the region since Tuesday and for the Santa Cruz Mountains since Thursday morning was now not valid on Friday morning.

The hottest spot within the region on Friday is anticipated to be Brentwood in far eastern Contra Costa County at 91 degrees. Highs of 84 degrees are expected in Livermore, 79 degrees in Concord and 82 degrees in San Jose. In Oakland and San Francisco, the mercury probably won't get above 60 degrees, Flynn said.

“We'll enjoy cooler weather all weekend,” he said. “We'll have clouds in the marine layer, and they could even produce a little drizzle, probably not even enough to wet the ground.”

Temperatures are expected to warm up again barely on Monday, Flynn said, but he added that the “small heat event” might be cooler than the one which follows it. This slight warming is anticipated to proceed until mid-week before temperatures “return more to their normal levels,” he said.

“This is really not far from what we would expect this time of year,” he said. “We have not reached our hottest yet.”

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