SAN DIEGO -  

Frost is likely in San Diego County's wind-sheltered valleys overnight as temperatures drop into the high 20s and 30s, forecasters said.

The frost could cause some overpasses and bridges to become icy and potentially hazardous for motorists, as moisture condenses and freezes on the road surface, according to the National Weather Service.

It also could pose a threat to sensitive plants and crops, forecasters said.

Officials with the weather service issued a frost advisory set to go into effect at midnight and extend until 8 a.m. Sunday.

The overnight temperature in the valleys was expected to be mostly in the 30s, but the coldest spots could be in the upper 20s briefly, forecasters said.

Forecasters expected frost again Sunday night into Monday morning in areas west of the mountains.