Even as the temperatures began to build in the Bay Area early Friday, the National Weather Service said the process already was in place to bring cooler air to the region and lower temperatures beginning Saturday and into next week.
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Such a development is kind of par for the course in the early part of the 2025 summer.
“Right now, we’re under the effect of a short-wave high-pressure ridge,” NWS meteorologist Nicole Sarment said. “But that ridge already is starting to be flattened by upper-level trough activity. That trough is undercutting the ridge itself, so the high pressure can’t develop into a true long-wave ridge. It’s the same process we’ve been seeing.”
The difference this time is that the weather pattern itself is a bit different than the previous ones. This upper-level trough is a different one in its movement than the trough pattern that hung out in the upper atmosphere for much of June. Sarment said the difference in the pattern’s direction could lead eventually to a long-term building of a ridge.
That said, such a long-term ridge is nowhere on the immediate or even distant horizon, according to Sarment. Temperatures in the long term are expected to stay at below-normal levels.
“It’s not completely unusual,” Sarment said. “It’s still relatively early in the summer, and the best chance for a thicker ridge and longer-term event might be the end of the month. But the farther out you get in the forecast, the harder it is to know for sure.”
What Sarment said the NWS did know on Friday is that temperatures in some areas of the interior East Bay will exceed 100 degrees. Brentwood is expected to pace the region with a high mark of 105, while Antioch is expected to reach 103. Livermore is expected to hit 98 degrees and be the hottest spot in Alameda County, while Pleasanton is likely to reach 95.
In other areas, the thermometer will be warmer than it’s been but not quite that intense. Morgan Hill is likely to pace the Santa Clara Valley with high marks of 91 degrees, while San Jose is forecast to reach 89. Oakland is expected to hit 74 degrees and San Mateo 75.
On Thursday, Concord reached an official high temperature of 99 and Livermore also was 99, according to the weather service. Sarment said the weather service’s official figures showed the high temperature in San Jose at 86, Oakland 77 and San Mateo 76.
“Thursday was wide-spread the hottest day,” Sarment said. “(Friday) will be the hottest day for the interior East Bay, but the cooldown will start everywhere else.”
The interior cooldown will begin on Saturday, she said. On Monday, another longer-range high-pressure ridge will begin to build, but it will be stymied in its growth by the upper-level trough and because its growth will occur over the Pacific Ocean rather than near land.
As a result, the high temperatures in in Brentwood, Antioch and the other hottest spots are expected to fall drop 7-8 degrees on Saturday, according to the weather service. By Tuesday, the high temperatures in most of the interior East Bay won’t rise out of the 80s. Sarment said those cities that may reach the 90s on Tuesday are even less likely to do so on Wednesday.
“The ocean is cooler than normal, and that’s another factor in keeping anything long-term from building” she said in reference to a lasting spell of heat. “The overall pattern is just not there for it.”