Inland Empire note: The valley floor (San Bernardino, Riverside, Ontario, Fontana) runs significantly hotter than coastal Southern California through summer — expect 100°F+ highs from June through September. Mountain communities (Big Bear, Lake Arrowhead, Crestline) typically run 20–30°F cooler. The High Desert (Victorville, Hesperia, Apple Valley) sees wider day-to-night swings and stronger wind.