After a historically wet winter and a cloudy spring, California’s summer was in full swing Thursday as a heat wave that’s been scorching much of the U.S. Southwest brings triple digit temperatures and an increased risk of wildfires.

Blistering conditions will build Friday and throughout the weekend in the central and southern parts of California, where many residents should prepare for the hottest weather of the year, the National Weather Service warned.

Midday highs were mostly expected to be above 100 degrees, and desert areas could reach 120, forecasters said. Little relief was expected overnight, when temperatures could remain in the 80s. An excessive heat watch was in effect through Sunday for interior Los Angeles, Ventura, Santa Barbara and San Luis Obispo counties.

“Please plan accordingly, this is not the time to be hiking or be outside for long durations,” the weather service’s LA office said on Twitter. “If you need to work outside, shift hours to the early morning, take frequent breaks and hydrate!”

Employers were reminded to adhere to regulations that require outdoor workers are given water, shade and regular breaks to cool off. The state will be performing spot checks at work sites to make sure the rules are being followed, said Jeff Killip with the Division of Occupational Safety & Health.

Across the U.S., more than 111 million people were under extreme heat advisories, watches and warnings, mostly in the Southwest, the National Weather Service reported Wednesday.

Forecasters said the long-duration heat wave is extremely dangerous, especially for elderly people, homeless residents and other vulnerable populations. The heat could persist into next week as a high pressure dome moves west from Texas. In Arizona, temperatures have hit 110 degrees for more than a dozen consecutive days.

Horse racing events were canceled at the California State Fair near the state capital over concerns for animal safety.

Meanwhile, California’s wildfire season was ramping up amid the hot, dry conditions with a series of blazes erupting across the state this week, said Secretary Wade Crowfoot of the Natural Resources Agency.

“As we get deeper into the summer and vegetation that grew up during the wet spring dries out, we are seeing an uptick in wildfire activity,” Crowfoot said Wednesday during a state media briefing.

Crowfoot said global climate change was “supercharging” heat waves. California has instituted a $400 million extreme heat action plan to protect workers, help vulnerable communities and assist local communities in opening cooling centers.

Officials said the state’s power system, which was strained to the point of widespread blackouts in recent years, has been fortified and should be able to withstand the latest heat wave. The California Independent System Operator, which runs the electricity grid, said battery storage capacity reached 5,600 megawatts on July 1 — enough to power more than 3.8 million homes for up to four hours before recharging.

“The batteries being added to the grid are charged during the day, when solar power is abundant, and dispatched primarily in the evening hours when demand is still high and the sun is setting and solar capacity diminishing,” Cal ISO said in a statement.

Heat wave to hit Southern California starting tomorrow