As of about 9 p.m. Tuesday, temperatures had dropped into the 50s along the coast, the 40s in many areas west of the mountains and into the 30s in a few spots, according to the National Weather Service. A frost advisory for the valleys is scheduled to expire at 8 a.m. Wednesday.
Forecasters said a dry, cold air mass would result in areas of frost as temperatures fall into the mid-20s to low 30s in wind-sheltered valleys.
Overnight low temperatures were forecast to be 21 degrees on Mount Laguna; 25 on Palomar Mountain and in Julian; 27 in Ramona; 33 in Alpine; 34 in Oceanside; 35 in Escondido and Miramar; 69 in Borrego Springs and 44 in San Diego.
Wednesday night will be slightly less chilly in most areas, but patchy frost will be likely in the coldest valleys, according to the weather service.
Forecasters urged residents to cover sensitive outdoor vegetation and bring pets inside.
Temperatures are expected to be noticeably warmer Thursday and Friday, but light rain may be possible over the weekend, according to the NWS.