San Diego County hasn’t had significant rain in three weeks, and a brief hot spell will arrive on Friday and likely be followed by another one next week, the National Weather Service says. While much of the nation is digging out from snow, San Diego’s daytime high will be 78 on Friday, 80 on Saturday and 73 on Sunday. The seasonal high is 68. Once again, a large high-pressure system is blocking storms from the Gulf of Alaska from sliding down the length of the West Coast. “That’s not unusual during the winter,” said Adam Roser, a weather service forecaster.