A Bay Area heat wave that was presupposed to be relatively short before lengthening after which lasting a couple of days longer than originally expected is finally, truly, coming to an end.

This was stated by the National Weather Service, but with one caveat.

“What you'll see (on Monday) is just a little relief,” said NWS meteorologist Brayden Murdock. “There'll be a lot more on Tuesday.”

Relief from Monday's heat will mostly profit those living near the coast on the peninsula and in cities along Interstate 880, where temperatures are expected to stay at 70 degrees. In the South Bay, temperatures will range from just above 80 degrees (in San Jose and Santa Clara) to only above 80 degrees (in Morgan Hill).

Only the far reaches of the East Bay are expected to see temperatures of 90 degrees or more. The hottest spot is prone to be Livermore, where 92 degrees are forecast.

All these temperatures are lower than those of the previous week, when the thermometer often rose above 38 degrees Celsius in the inside and hills and above 32 degrees Celsius or more elsewhere.

A high pressure system that proved more persistent than strong caused the warmth to last more than originally expected, Murdock said.

“If it had been a strong system, it would have been much hotter,” he said. “It just didn't have enough force or wind to push it far enough out of the area to feel the difference.”

According to Murdock, that advance was already underway Monday morning. The marine layer was far less compressed, providing some relief from the dense fog affecting the Monterey Bay coast and northern Salinas Valley.

The winds also showed signs of change, he said.

These aspects are only prone to be exacerbated as a low-pressure system from the Gulf of Alaska moves through the region. This low-pressure system is moving with barely less speed and force than originally expected, in response to Murdock.

“It's finally going to get here,” he said of its expected arrival on Tuesday. “From then on, our weather pattern will keep us rather cool for the foreseeable future.”

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.”