PHOENIX (AP) — A punishing heat wave has arrived in the Southwestern U.S. and brought temperatures that will approach 120 degrees in Arizona — a number not seen in the desert city in more than 20 years.

Arizona is seeing the most stifling temperatures, but the wrath of the heat wave is being felt across Nevada and California as well. Las Vegas was forecast to hit 117 on Tuesday on the first day of summer, and excessive heat warnings cover almost all of California.

Airlines canceled flights in Phoenix because the extreme heat makes it too difficult to take off. The major burn center in Arizona...