More than a third of Americans were under extreme heat advisories, watches and warnings as a blistering heat wave that's been baking the nation spread further into California, forcing residents to seek out air conditioning or find other ways to stay cool in triple-digit temperatures. 

The wildfire season was ramping up amid the hot, dry conditions with a series of blazes erupting across the state, Wade Crowfoot, secretary of the Natural Resources Agency, said at a media briefing this week.

In Southern California's Riverside County, located east of Los Angeles, three large wildfires ignited Friday afternoon threatening homes and forcing evacuations. The CAL FIRE/Riverside County Fire Department reported that one of the fires grew from 20 acres to 600 acres in just a matter of a few hours.

The sweltering conditions were expected to continue to build through the weekend in Central and Southern California, where many residents should prepare for the hottest weather of the year, the National Weather Service warned. Highs in inland desert areas could top 120 degrees Fahrenheit during the day, and remain in the 80s overnight, offering little relief.

The city of Los Angeles Emergency Management Department said Friday that a heat advisory for the region is in effect until 11 p.m. Monday, warning that temperatures could reach 106 degrees in the San Fernando and San Gabriel Valleys. 

"Things really turn up this weekend in the Southwest," Weather Channel meteorologist Stephanie Abrams said Friday on "CBS Mornings." "Saturday, records start falling in New Mexico and Arizona. Sunday, we'll be close to tying our all-time record high in Vegas at 117 degrees. Death Valley is going to approach 130, with their lows around 100 degrees."

She added, "This type of heat is going to continue at least through next week."

In the desert city of Palm Springs — where Friday's high temperature was forecast to hit 116 degrees — many homeless people were left to contend with the heat on their own, with just 20 indoor beds at the lone overnight shelter.

Roman Ruiz, the city's homeless services coordinator, said homeless residents struggle daily just to find a place with enough shade.

"I don't know how anyone can do it really," he said. "I feel so bad, and yet there's not much I can do."

Elsewhere, officials prepared to repurpose public libraries, senior centers and police department lobbies as cooling centers, especially in desert areas.

The heat wave came as the California State Fair prepared to kick off Friday in Sacramento, forcing organizers to cancel planned horseracing events due to concerns for animal safety.

Forecasters said the long-duration heat wave is extremely dangerous, especially for older people, homeless residents and other vulnerable populations. The heat could persist into next week as a high pressure dome moves west from Texas.

• Expert advice to prevent heat stroke and other hot weather health hazards

"Excessive heat is the leading weather related killer in the United States," the National Weather Service warns.

Jeff Goodell, author of "The Heat Will Kill You First," says the risk increases the longer the heat wave continues.

"Our body has a pretty narrow range of temperatures which it can handle," he told CBS News, "and when it starts to get too hot, our heart starts pounding and it's pushing blood out towards the surface of our skin in a desperate attempt to kind of cool that blood down, which it does by, you know, our body starts sweating … and that sweating cools the blood and that, in theory, cools the body. But that mechanism only works so far."

"For anyone who has heart problems, circulatory problems, that mechanism begins to break down, and that's when you start moving into the land of heat exhaustion and heat stroke and ultimately, if it's too hot for too long, death," Goodell said.

In Las Vegas, regional health officials launched a new database Thursday to report "heat-caused" and "heat-related" deaths in the city and surrounding Clark County from April to October.

The Southern Nevada Health District said seven people have died since April 11, and a total of 152 deaths last year were determined to be heat-related. The tally includes deaths due to heat exposure or hyperthermia and cases with those reasons listed as "significant factors," district spokesperson Jennifer Sizemore said.

Phoenix hit the 110-degree mark for the 15th consecutive day Friday, putting it on track for a possible new record next week. The longest measured stretch of 110 degree-plus temperatures for the city is 18 days, recorded in 1974.

The overnight low temperature at Phoenix Sky Harbor International Airport Thursday morning was 95 degrees, which means temperatures may not be dropping far enough to allow people to recover after dark.

While there are some 200 cooling and hydration centers operated at libraries, community centers, churches and other public spaces across metro Phoenix, most close anywhere between 4 p.m. and 7 p.m., leaving people with few options for cooling off in the still-sweltering nights.

David Hondula, chief heat officer for the City of Phoenix, said some centers plan to close later over the weekend, including one downtown near a large encampment of homeless people that will stay open 24 hours.

Hondula suggested that the Federal Emergency Management Agency could play a role in the future to help keep cooling centers open longer.

"We'd certainly be interested to have that conversation," he said.

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Meanwhile in California, cooling centers in and around Sacramento planned to offer some extended evening hours. In the small Central Valley city of Galt, about 25 miles south of the state capital, the police department planned to open its air-conditioned lobby between 2 p.m. and 10 p.m. from Friday until Monday.

"We want to make sure that anybody who does not have the ability to find appropriate shelter, that they can have a place to go to keep themselves in a safe and cool environment," Lt. John Rocha said.

The same lobby served as a warming center during California's unusually wet, cold and snowy winter, demonstrating the weather whiplash the state has experienced this year.

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 California's Division of Occupational Safety & Health.

Global climate change is "supercharging" heat waves, Crowfoot added. California has instituted a $400 million extreme heat action plan to protect workers, help vulnerable communities and assist local communities in opening cooling centers.

"Fires are getting larger quicker and that's typical for a heat wave like this," Los Angeles County firefighter Tanner Renz told CBS News. "I think we're gonna have more acreage burn this year. ... It's concerning through the entire county and the entire state."

People looking to cool down in California's many rivers should be wary, UCLA climate scientist Daniel Swain said, noting that waterways swollen from the epic Sierra Nevada snowpack remain dangerous as there is still snow left to melt.

"Be aware that the water will still be icy cold despite how hot the air will be and could be flowing very fast, much faster than usual for mid-July," he said.

In:
• Weather Forecast
• California
• Heat Wave
• Palm Springs
• Las Vegas