Millions of people in the Western and Southern United States will bear the brunt of the summer's first heat wave starting on Tuesday as temperatures are expected to soar above 100 degrees Fahrenheit (37.

Forecasters have issued excessive heat warnings on Wednesday lasting through Friday night for portions of southeast California, southern Nevada and Arizona.

With temperatures surging 15 to 20 degrees above normal, heat warnings span much of California and the Desert Southwest, including Las Vegas, Phoenix and Sacramento.

Temperatures soared in California and the Southwest ahead of what could be a brutal summer.

A heat wave lasting through Friday is sweeping across California, Arizona, Texas and the rest of the Southwest U.S., with record temperatures expected in some places.

A massive high-pressure system known as a heat dome that has stalled over the U.S. Southwest will push temperatures in the region well above 100 degrees Fahrenheit (38 C) on Thursday, leaving millions of Americans to swelter in the coming days.

California's first heat wave of the year is marking an abrupt transition toward summer, with record-breaking, triple-digit temperatures expected across the state's interior. The coast, however, will be largely spared.