"Temperatures will start dropping tomorrow," forecaster Steve Anderson said Wednesday morning.
Daytime highs in the greater Bay Area are expected to be in the low 50s, and overnight temperatures will dip to about 35-40 degrees, he said.
"The overnight lows are not too crazy cold until you get out into the East Bay and North Bay valleys, where temperatures will be slightly below freezing," he said.
Anderson said a storm system is coming down from the Gulf of Alaska, and could bring rain to the Bay Area Wednesday night and Thursday morning.
"Behind it, it's dragging cold air down from Canada," he said of the system.
The skies will clear after Thursday, but the cold weather will linger through the weekend, Anderson said.
Early Friday morning, between about 2 a.m. and 8 a.m., a freeze warning will be in effect for North Bay valleys, and a frost advisory will be issued for the South Bay, he said.
Anderson advised residents to make sure small outdoor pets have warm shelter, and to take precautions with plants that are sensitive to the cold.
"If they're potted plants, bring them inside if they're small enough, or throw a cover over them to keep the frost off if needed," he said.
Temperatures should begin to warm up by Monday, he said.