A heat wave ramped up across the Western United States over the weekend, when millions of Americans were expected to see record-breaking temperatures.

With heat alerts extending well into the week ahead, West Coast and Southwest residents may not see relief for days.

About one-third of Americans were under excessive heat warnings, watches or advisories over the weekend, according to the National Weather Service, after a persistent heat dome hovering over Texas expanded to California, Nevada and Arizona.

The oppressive heat was expected to peak on Sunday in those states, with some desert temperatures forecast to climb close to 130 degrees Fahrenheit.

"We've been talking about this building heat wave for a week now, and now the most intense period is beginning," the weather service said Friday.

Local public health officials across the country are urging people to seek cool shelter and to check up on neighbors.

Some of the hottest places in the U.S. may see their hottest day ever

While Las Vegas residents are used to scorching temps, meteorologists say they shouldn't underestimate the risks of this days-long heat wave.

"This heatwave is not typical desert heat due to its long duration, extreme daytime temperatures and warm nights. Everyone needs to take this heat seriously, including those who live in the desert," the National Weather Service in Las Vegas warned in a tweet.

Las Vegas could reach a high of 117 degrees Fahrenheit on Sunday, tying the city's all-time record.

The heat wave there has already sent people to hospitals. An emergency room doctor reported treating dehydrated tourists, as well as a passed-out elderly resident who kept his thermostat at 80 degrees to keep down electricity costs. Local health officials have seen at least seven heat-related deaths this year.

In nearby Death Valley, Calif., one of the hottest places on Earth, the temperature had reached 128 degrees on Sunday afternoon, surpassing the daily record by a single degree.

In west Texas, an extreme heat advisory remained in effect at Big Bend National Park. "These are extremely dangerous/deadly temperatures! Hikers should be OFF TRAILS in the afternoon," officials said.

Following days of brutal heat, Phoenix hit 118 degrees on Saturday, breaking the daily temperature high record by one degree. Phoenix looks headed to break its longest recorded stretch of 110-degree or higher heat — a period of 18 days, recorded in 1974 — with a forecast matching or topping record daily highs through at least Wednesday.

"This weekend there will be some of the most serious and hot conditions we've ever seen," David Hondula, the city's chief heat response officer, told the AP. "It's a time for maximum community vigilance."

So far this year, the heat has killed at least 12 people in Phoenix.

Maricopa County, where Phoenix is located, is home to about 200 cooling centers, hydration stations and respite sites. Hondula said in an interview on Friday with NBC's Meet the Press that cities like his could benefit from federal funds to keep the centers better staffed and open for longer.

Central and Southern states continue to endure dangerously high temperatures. A large swath of South Texas was under a heat advisory on Sunday, while the coastal region surrounding Corpus Christi was under an excessive heat warning.

In South Florida, Miami-Dade County was put under an excessive heat warning on Sunday, where temps in the coastal area were expected to hit 110 degrees. It's first time in history such a warning has been issued for the region, according to the county's chief heat officer.

The U.S is now experiencing temperatures similar to those felt during last year's heat waves in Europe. More than 61,000 people died as a result of the heat in Europe during that period, according to a study published last week.

What's causing this latest heat wave?

A couple of factors are driving the hotter temperatures. The recent arrival of El Niño, a natural climate pattern marked by warmer temperatures in the Pacific Ocean, is bringing hotter weather around the world.

El Niño, which coincides with the hottest years on record, exacerbates the effects of climate change, including warmer temperatures, caused by the burning of fossil fuels and other greenhouse gas emissions that warm the planet.

Climate change can make heat domes — a phenomenon that creates heat waves like this one — more frequent and more intense. A heat dome occurs when high pressure in the Earth's atmosphere traps hot ocean air from the sun like a hot-air balloon.

NPR's Nathan Rott contributed to this report.