On first full day of summer, a two-day heat wave settles into the Bay Area

The heat is expected to to subside beginning on Sunday.

The first full day of summer arrived on Friday, and along with it promised to be some powerful heat that expects to bake much of the Bay Area, according to the National Weather Service.

“The most noticeable change is going to be from Thursday into Friday,” NWS meteorologist Dalton Behringer said. “You’re looking at temperatures that are going to be on average 6-10 degrees warmer than they were 24 hours earlier. But the hottest day is going to be Saturday.”

As the summer solstice  — the official onset of summer — came at 12:50 p.m. Thursday, a high-pressure system was building along the interior of the state — as well as nationally. That ridge of high pressure is a thick one that will allow temperatures to build over much of the state’s interior, Behringer said.

The effect of that will be to lift temperatures in much of the Bay Area at least into the high 80s on Friday, while the hottest spots hit the 90s, Behringer said. By Saturday, some cities along the interior could see triple-digits.

Concord is expected to top out at 91 degrees on Friday, as is Livermore. Both are expected to reach 99 degrees on Saturday. In the South Bay, Morgan Hill is forecast to reach 89 degrees on Friday before spiking to 97 on Saturday. San Jose is forecast to max at 83 degrees on Thursday before getting up to 91 on Saturday.

“The good news is that we are still calling this a progressive pattern,” Behringer said. “It’s still moving, so it’ll move out of the region pretty quickly, and it’s still gonna stay pretty comfortable near the water.”

The Bay Area’s marine layer, is expected to fade for a couple of day, though winds will continue to be breezy. San Mateo is expected get up to 73 degrees on Friday and hit 81 on Saturday, according to the weather service. Oakland is forecast to have a high of 71 on Friday and 77 on Saturday, while San Francisco is not likely to escape the 60s on either day.

Come Sunday, the thermometer will be creeping back down, Behringer said. By Monday, “it’ll be 8-10 degrees cooler than you see on Saturday, and that’ll be followed by kind of a normal pattern in the days after that.”

Forecasters reminded people to stay hydrated in the heat by drinking fluids hourly and not just when it begins to feel hot. Those sensitive to the heat were urged to stay in cool areas, if possible. Safety officials added that children and animals should never be left in the car; heat-related illnesses and death can happen within minutes, they said.