WASHINGTON — More than 34 million Americans are preparing for extreme temperatures, as a heat dome descends on the western part of the US this week.

Forecasters are warning residents in California, Nevada, Utah and Arizona to prepare for temperatures to reach triple digits before Friday due to the weather phenomenon.

Later in the week, extreme temperatures are expected to reach parts of the Pacific Northwest.

Similar temperatures last year killed at least a dozen people in the American Southwest, and US Border Patrol in Texas has already issued a warning that four migrants attempting to enter the US died over the weekend due to heat-related illnesses.

Forecasts show temperatures could reach 108F (42.2C) across a wide swath of California, from Sacramento to Bakersfield.

The National Weather Service (NWS) issued an excessive heat warning for the state's Central Valley region on Tuesday.

Sacramento is expected to see its first triple-digit high temperature of the year on Tuesday. Last year, California's capital city didn't get that hot until the end of June.

On Wednesday that warning will expand further east from Las Vegas to the Lake Havasu City, Arizona, region.

The NWS also issued heat warnings to southern Texas. Some areas of south Texas saw temperatures reach 117F (47.2C) Tuesday.

Thursday is expected to be the worst day this week for excessive heat.

Forecasters expect temperatures to be 10 to 20 degrees Fahrenheit above normal.

Phoenix, Las Vegas, Albuquerque and Fresno could see previous records for high temperatures squashed.

Residents facing the heat wave have been advised to drink extra fluids, stay inside in air-conditioned rooms out of the sun and wearing loose-fitting clothing.

"Heat stroke can be DEADLY," the NWS office in Phoenix cautioned in a post on Tuesday evening.

California's Environmental Protection Agency added in an alert that outdoor workers, seniors and children are particularly vulnerable to extreme heat, which "is an invisible but dangerous consequence of climate change".

Firefighters across the region are also anxiously watching to see if the heat causes any wildfires to break out.

The Corral fire, which broke out on Saturday about 60 miles (95km) west of San Francisco — and briefly threatened a weapons-testing facility — is now mostly contained, officials said on Monday.

But there is concern the tmperatures could cause the fires to flare up again in the coming days.

Residents in the California Bay Area who may have hoped to escape the heat by visiting local beaches have been warned that a northwest ocean swell is expected to bring 15-20ft waves to the coast.

Inexperienced swimmers were advised to stay out of the water, according to weather officials, and beach-goers should be on the lookout for sneaker waves that can suddenly pull people into the ocean.

This new heat wave follows last year's sweltering summer, during which Phoenix saw 31 consecutive days of temperatures at 110F (43.3C).

This week's high temperatures are the result of a heat dome.

An area of high pressure pushes air towards the ground, trapping it and causing it to heat up. As a result, temperatures can rise and linger.

The pressure also prevents other weather systems that might cool down an area — such as rain clouds — from forming.

A heat dome that struck the southwestern US last July was described by the NWS as "one of the strongest" of its kind to hit the region.

A 2023 June heat dome that struck Louisiana and Texas killed 12 people and sent hundreds to the hospital for heat-related health issues.

While heat domes were once described as rare, heatwaves and heat domes are becoming more common and intense because of human-induced climate change, scientists say. — BBC