An already dangerous weekslong heat wave will only worsen this weekend as a heat dome intensifies and reaches peak strength over parts of the Western United States.

The heat dome is so formidable the National Weather Service in Phoenix called it “one of the strongest high pressure systems this region has ever seen.” Around 100 heat records could fall today through the weekend as it intensifies, piling onto the more than 1,000 high temperature records broken in the US since June.

More than 90 million people are under heat alerts after the heat dome expanded into places like California, which is now experiencing its first extreme heat wave of the year.

It has already been dangerously hot for weeks in Texas, Florida and Arizona, where Phoenix is in the middle of a likely record-breaking streak of consecutive 110-degree days, forcing many businesses and parks to close or readjust their hours. The low temperature in Phoenix might not drop below 90 degrees for eight consecutive days, another record.

The heat will be so intense, forecasters suggested residents of Las Vegas avoid the outdoors between 9 a.m. and 6 p.m., with the city forecast to challenge its all-time high temperature of 117 degrees on Sunday. It won’t get much cooler at night, with low temperatures nearing 90 degrees – a particularly dangerous side effect of the climate crisis.

This kind of extreme heat is one of the hallmarks of human-caused climate change, the symptoms of which are tallying up this year into a global record box score of sorts: “unprecedented” ocean heat off the coast of Florida and in the North Atlantic; record heat in Beijing, in what could be one of the hottest summers in China; record energy demand and heat in Texas; and an ongoing “Cerberus” heat wave, threatening to topple European temperature records.

It all adds up to what could be the hottest year on record.

Businesses, parks close as temperatures climb

Skyrocketing temperatures across the western US have forced some businesses and parks to close or change their hours, such as the Sacramento Zoo, which shortened its hours due to the heat, closing early Friday, Saturday and Sunday at 1 p.m. PT as temperatures are forecast to reach 109 degrees Fahrenheit this weekend.

In Bakersfield, California, where the highest temperature this weekend is set to reach 114 degrees Fahrenheit, the Kern County Museum announced it would be closed “due to extreme heat” this weekend, according to a museum social media post. In Southern California, the Cleveland National Forest announced on its Facebook page that four of its trails – the San Diego River Gorge, Cedar Creek Falls, Three Sisters Falls and Eagle Peak – will be closed through the weekend due to “excessive heat.”

Other businesses in California, such as the California Gray’s Flower Garden in Quincy and the Nascere Vineyards in Chico announced plans to close their doors over the weekend as temperatures are expected to reach over 100 degrees Fahrenheit.

The Bison Café in Quitaque, Texas said it was limiting its hours due to higher temperatures that are making the kitchen “very uncomfortable” for cooks.

Separately, in Phoenix, the Arizona Animal Welfare League decided to close its doors for the weekend. “The safety of our pets is our top priority and with temperatures expected to hit over 115, our staff’s main focus will be on keeping our animals cool and comfortable during this time,” a Facebook post from the organization reads.

A ‘perfect storm’ of deadly heat

The records also add up into something more serious for human health, doctors say.

The extreme temperatures could trigger heat illness in as little as 20 to 30 minutes for people doing anything strenuous outdoors, because heat acts as a “perfect storm,” which overloads the body until it eventually short-circuits and then shuts down, Levy said. The time frame would be even shorter for those most vulnerable to heat, like the elderly and those with preexisting health conditions.

Heat illness is a serious concern this weekend for the millions of residents in major metro areas like Phoenix and Las Vegas.

People across the Southwest and California’s Central Valley are under a rare “extreme” level of heat risk on Saturday, Sunday and Monday, according to the weather service.

The “extreme HeatRisk” is the highest possible risk level for heat, akin to a “high risk” for tornadoes, and is meant to warn of significant heat impacts requiring preparation.

These areas should prepare for spikes in heat-related ER visits, potential power outages from demand and temperatures high enough to turn deadly, especially for outdoor workers and those without reliable cooling.

Levy said workers can stay safe by taking frequent hydration breaks, wearing clothing able to reflect the sun and having a “buddy system” so no one is left in the heat alone when illness strikes. For those who don’t have reliable cooling, he recommends finding a cooling center and having a plan to get there before the need arises.

Before the latest wave, heat has already killed at least 12 people in Phoenix’s Maricopa County this year, and killed 425 people last year. The city has opened “respite centers” to help provide relief and the state has asked residents to keep their vehicles stocked with water.

The heat won’t go away after this weekend, even if temperatures peak: Longer term temperature outlooks from the Climate Prediction Center point to above normal temperatures across Southern California, the Southwest, South and Florida through next week.