SAN DIEGO — An excessive heat watch will be in effect for the mountains and deserts of San Diego County Friday morning through Sunday evening, as temperatures soar well into the triple-digits, meteorologists said. Starting Friday, weekend highs in the deserts will be 115 to 120 degrees and 100 to 105 degrees in mountain areas near 3,000 feet, according to the National Weather Service. “Exceedingly high temperatures can cause heat-related illness or death,” the agency said. “Those working or playing outdoors, and those without air conditioning are most vulnerable.” The hottest days are expected to be Saturday and Sunday. For the rest of the county, the marine layer and weak onshore flow will keep areas near the coast out of the triple digits. Highs in coastal areas this week are expected to be 75 to 85 degrees and 88 to 98 degrees in the valleys.