For days, weather experts waited for low pressure to develop and move in from the Gulf of Alaska to dissipate a persistent high-pressure bubble that has caused a week-long heat wave in the Bay Area. For days, they were wrong about when the transition would occur.

By Monday afternoon, they were not only certain that cooler days were coming, but also that the transition that would bring relief was already underway.

“We can see radar activity from that trough up there, and there’s a little bit more wind and less moisture far inland,” National Weather Service meteorologist Dial Hoang said around noon. “By this time (Tuesday), it could be 10 degrees cooler than it is now.”

The entire region will find Tuesday cooler than it has been since the Labor Day holiday. The greatest relief will be in the far interior, where it is typically warm. In Livermore, generally one of the warmest places in the region, the thermometer is expected to be no higher than 78 degrees on Tuesday.

On Monday, a temperature of 33 degrees Celsius was recorded in Livermore at 1 p.m., the expected maximum temperature for the region.

“People in Livermore are not going to believe the difference,” said meteorologist Brayden Murdock. “Especially considering the week they just had.”

According to the weather service, temperatures reached at least 99 degrees for six days in a row, and 100 for four days in a row in Livermore. Elsewhere, Concord reached 96 degrees during that six-day period from Sept. 3 through Sunday. San Jose also reached 90 degrees or higher on all six of those days.

A high pressure system that has proven more persistent than strong kept the heat lingering longer than originally expected, Murdock said. Forecasters initially said that when the heat increased after Labor Day, it would likely last only two days.

“If it was a strong system, it would have gotten a lot hotter,” Murdock said. “There just wasn’t enough force coming through or the wind wasn’t strong enough to push it far enough out of the area to feel the difference.”

Now they do. Temperatures dropped everywhere Monday and will do so again Tuesday, the weather service said; the further inland you go, the more dramatic the drop on the thermometer. Areas closer to the ocean on the peninsula have been feeling the cool air since Friday, and cities along Interstate 880 closer to the bay saw a chill Saturday.

Murdock also said the marine low was much less compressed, helping to alleviate some of the dense fog plaguing the Monterey Bay coastline and the northern Salinas Valley.

The trough isn’t expected to bring rain, and inland areas could see temperatures rise above 90 and possibly even 95 for a few days later in the week, according to the weather service. But the warming is expected to be brief and limited to those areas.

Murdock said the region will remain generally cool and temperatures are likely to be below seasonal averages.

“Our weather pattern will keep it cool for the foreseeable future,” he said.

Originally published: September 9, 2024 at 7:37 am