Daily temperature records will tumble as sizzling early season heat from a summerlike heat dome sends thermometers skyrocketing into the triple digits in parts of California and the West this week.

The official start of summer is just a few weeks away, but it will feel like July in much of the West as temperatures climb 20 degrees or more above average, the highest temperatures of the year so far for many locations.

Excessive heat warnings are in effect for more than 17 million people in California, Nevada, Utah and Arizona this week. The warnings are the most extreme form of heat alert issued by the National Weather Service and are used when widespread, dangerous heat is expected.

The soaring temperatures are being caused by a heat dome, a large area of high pressure that parks over an area, traps air and heats it with abundant sunshine for days or weeks. The resulting heat becomes more intense the longer a heat dome lasts.