From Accuweather:
A recent pattern change has allowed stifling heat to build gradually across the southwestern United States. As the week continues, meteorologists caution that the heat will further intensify over the region, which will allow numerous daily records from Sacramento to Phoenix to be challenged.
Temperatures began to climb on Sunday with some locations in the Sacramento Valley of California reaching 10 degrees Fahrenheit above normal. In anticipation of the hot weather, which will result in widespread triple-digit temperature readings, numerous heat advisories, excessive heat watches and excessive heat warnings have been put in effect from California to Nevada and Arizona.