A classic summer heat wave will develop in San Diego County on Friday and Saturday with temperatures hitting 80 degrees at-or-near the coast and the upper 90s and low 100s across inland valleys, says the National Weather Service. “The week will start out mild, with temperatures going up a degree or two each day,” said James Brotherton, a weather service forecaster. “The big warmup will come right after the Fourth of July, with Friday and Saturday being the warmest days.”