The National Weather Service says an increasingly unstable airmass and moisture arriving over sweltering Southern California is bringing a risk of afternoon thunderstorms and flash-flooding in the mountains and high desert.
Flood watches have been posted through 6 p.m. Thursday for most of the mountains in Los Angeles, Santa Barbara and Ventura counties, and the in the high-desert Antelope Valley north of Los Angeles.
Relatively light winds in the upper atmosphere mean any thunderstorms that develop will move very slowly, increasing the risk of flash-flooding.
Forecasters say people near areas burned by wildfires should be on heightened alert for flooding and debris flows.
The weather service notes that the Santa Monica Mountains are not included in the watch area.
Many locations have reported temperatures in the 90s to over 100.
Copyright 2010 The Associated Press.