The National Weather Service says a strong, upper-level high centered over the Great Basin will bring triple-digit heat to the valleys and desert areas beginning Monday and peaking on Tuesday and Wednesday. Excessive heat warnings will be in effect most of Tuesday across the deserts.

Forecasters also warn that the heat, combined with very low humidity levels, will elevate concern about fire danger in the interior of southwest California.

Dry lightning will add to fire worries as a flow of monsoonal moisture arrives from the southeast later in the week, affecting Southern California and the southern Sierra Nevada.