Hot conditions were forecast through Wednesday in San Diego County, peaking Monday and Tuesday with widespread heat risk and isolated showers and a slight risk of thunderstorms over the mountains Sunday afternoon, the National Weather Service said.
Dangerous lightning and flooding were also possible over the mountains, the NWS said.
A heat advisory was issued from 10 a.m. Monday to 8 p.m. Wednesday for San Diego County coastal areas and valleys.
An excessive heat warning was issued until 11 p.m. Tuesday for San Diego County deserts, which could get dangerously hot with temperatures of 105 to 110 degrees. Extreme heat could significantly increase the potential for heat related illnesses.
Gradual cooling was expected for the latter half of the week.
Skies were mostly clear Sunday morning, with just a few patches of low clouds offshore.
High temperatures along the coast Sunday are predicted to reach the lower 80s, around 90 degrees in the western valleys, in the upper 90s near the foothills, the lower 90s in the mountains and 104 to 109 in the deserts, according to the weather agency.
Drier air was predicted to move in Monday and lead to dry conditions which were favored to last through the end of the week. Some rain in the mountains was expected Wednesday and Thursday ahead of an upper trough as moisture pulls back into the region.
The upper trough was forecast to lead to gradual cooling during the second half of the week with highs trending slightly below normal west of the mountains by next weekend.
Wind gusts were predicted near 20 knots across the outer waters and near San Clemente Island Sunday afternoon and evening and Monday. Otherwise, no hazardous marine weather was expected through Thursday.
City News Service contributed to this article.