Cooling begins, but some areas are still hot

For days, weather forecasters waited for the development and arrival of a low-pressure system in the Gulf of Alaska that would displace a remaining high-pressure bubble that was causing a week-long heat wave in the Bay Area. For days, they were wrong about the timing of the transition.

By midday Monday, they were not only certain that cooler days were ahead, but also that the change responsible for the relief was already underway.

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

Across the region, Tuesday is expected to be cooler than it has been since before Labor Day. The most noticeable relief will come in the inland regions, where temperatures are usually hot. In Livermore, normally one of the region's hottest spots, temperatures aren't expected to rise above 78 degrees on Tuesday.

On Monday, Livermore recorded a high of 92 degrees at 1 p.m., the expected high for the region.

“People in Livermore aren't going to be able to believe the difference,” said meteorologist Brayden Murdock. “Especially considering the week they've just had.”

According to the weather service, the thermometer in Livermore reached at least 99 degrees for six days in a row and 100 degrees for four days in a row. In Concord, the temperature reached 96 degrees during the six-day period from September 3 to Sunday. In San Jose, it was also at least 90 degrees on all six of those days.

A high pressure system that has proven more persistent than strong has caused the heat to linger longer than originally expected, Murdock said. When the heat picked up again after Labor Day, forecasters initially said it would likely last only two days.

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

Now it's happening. Temperatures dropped everywhere on Monday and will drop again on Tuesday, according to the weather service; the further inland you go, the more dramatic the drop in temperatures. In the areas closer to the sea on the peninsula, the air has been cool since Friday, and in the cities along Interstate 880 closer to the bay, there was a cooling effect on Saturday.

Murdock also said the marine layer was far less compressed, which had led to an alleviation of the dense fog that affected the Monterey Bay coast and northern Salinas Valley.

The low is not expected to bring rain, and inland areas temperatures could climb above 90 and perhaps even as high as 95 for a few days later in the week, according to the weather service. But any warming is expected to be short-lived and confined to those areas.

Overall, the region is expected to remain cool and temperatures will likely be below seasonal averages, Murdock said.

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

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