A heat dome in the western part of the US will subject more than 34 million people to extreme temperatures beginning Tuesday.

Forecasters are warning residents in California, Nevada, Utah and Arizona to prepare for temperatures to reach triple digits.

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

Similar temperatures last year killed at least a dozen people in the American Southwest.

What's the latest forecast?

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.

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

What is a heat dome?

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.