Near freezing temperatures and frost are expected early Monday and again late Monday night in parts of San Diego County.
A National Weather Service frost advisory for the valleys and deserts is set to remain in effect until 8 a.m. Tuesday. Temperatures could drop below freezing in the coldest spots.
Low temperatures of 33 to 40 degrees are forecast for the valleys early Monday morning and 31 to 38 degrees late Monday night. In the deserts, temperatures are expected to fall to between 33 and 38 degrees.
At about 3:45 a.m., the weather service reported temperatures of 31 degrees in Ramona, 33 degrees at Camp Pendleton, 37 degrees in Campo, 39 degrees in Oceanside, 43 degrees in Otay Mesa, 43 degrees in Carlsbad, 48 degrees at Miramar and 54 degrees in San Diego.
The low temperatures combined with clear skies and light winds are expected to produce areas of frost in wind-sheltered portions of the county, including Borrego Springs, according to the weather service.
Forecasters warned that the cold may harm unprotected cops, sensitive outdoor plans and pets.
Although more cold nights are in the forecast this week, daytime temperatures will gradually warm up. Cooler conditions, gusty wind and a slight chance of rain will follow later in the work week, then a drier and warmer weekend as high pressure rebuilds, according to the weather service.
