The heat peaked Wednesday with cooler conditions expected Thursday west of the mountains and temperatures forecast to be 5 to 10 degrees cooler, the National Weather Service said.
Records fell Wednesday in Oceanside Harbor, Vista and Chula Vista, according to the National Weather Service.
The high of 90 near the water in Oceanside beat the high of 82 from 1977, while in Vista the high of 98 outstripped the high of 95 almost 60 years ago, in 1964.
Chula Vista, also with a high of 98, saw the oldest record fall. The previous hottest Oct. 19 was 92, all the way back in 1940.
But temperatures will decrease through the end of the week, with the coolest conditions for most areas expected Sunday.
As the low-pressure system near the Pacific Northwest moves inland through the weekend, it will bring gusty west-to-northwest winds and a chance of precipitation in addition to cooler conditions.
Winds will be strongest over the mountains into the deserts on Saturday with gusts 60 to 65 mph.
There is a chance of showers in all parts of the county Saturday night into Sunday and even high-elevation mountain snow above 8,000 feet.
High temperatures on Sunday are forecast to be 5 to 10 degrees below average near the coast, 10 to 15 inland and 20 degrees below average in the valleys, mountains and deserts.
Low temperatures in the mountains could dip below freezing for the first time since May on Sunday and Monday morning, the NWS said.
Forecasters expect a little warmer and drier weather with the potential for another round of Santa Ana winds early next week.