Unseasonably warm weather is expected again Thursday in San Diego County, then temperatures will drop sharply on Friday and rain showers will arrive this weekend, according to the National Weather Service.
A ridge of high pressure anchored over the Great Basin will give way to cold, wet conditions on Saturday and Sunday, forecasters said.
Humidity levels will fall to around 10% Thursday amid the warm conditions, causing an elevated risk of wildfires in the San Diego County mountains and inland valleys, according to the NWS.
High temperatures on Thursday are forecast to reach 83 degrees near the coast, 94 inland, 97 in the western valleys, 86 in the mountains and 95 in the deserts.
On Friday, a cold trough of low pressure will drop into Central California and bring a sharp drop in temperatures, along with scattered showers from Friday evening through Sunday night, forecasters said.
Coastal areas could get up to three-quarters of an inch by Sunday night and the inland valleys could get up to 2 inches, according to the NWS. The county mountains could see up to 2.5 inches of rain and if any rain falls in the deserts, it will be less than one-tenth of an inch.
Snow levels will drop to around 5,000 feet by Saturday night, meaning several mountain tops in San Diego County could get a few inches of snow by Sunday night, forecasters said.
Strong, westerly winds are also associated with the autumn storm, with gusts potentially reaching 55 mph near-desert slopes on Saturday, forecasters said.
Highs near the coast are expected to drop to 64 on Saturday and Sunday, while highs in the western valleys are forecast to top out in the low-to-mid 60s both days.