A chance of thunderstorms will persist Thursday in most parts of San Diego County as the mercury continues to rise heading into the weekend. Thunderstorms will be possible in the inland valleys, the deserts and the mountains Thursday afternoon, according to the local office of the National Weather Service. Partly cloudy skies are expected in the coastal areas. The skies are expected to clear throughout the county by Friday morning, meteorologist Jimmy Taeger said. The chance of measurable precipitation has been set at 20 percent Thursday for the inland valleys, the mountains and the deserts, according to the weather service.