The final week of summer in San Diego will start out a little hotter than average, then cool off a bit, says the National Weather Service. But high temperatures and gusty winds will raise the risk of wildfires Tuesday through Thursday in the region’s mountains and deserts. The fall equinox occurs at 6:54 p.m. on Saturday. But for reporting purposes, the weather service defines summer as June, July and August. This year, the mean temperature during that period was 73.6 degrees — making it the third hottest summer on record in San Diego.