A flash-flood warning was issued this afternoon for portions of northern Los Angeles County.

The National Weather Service said a storm system capable of producing heavy rain, hail and 50 mph winds was moving into the Antelope Valley. The forecast said California Highways 18 and 138 could be hit by the storms and urged those who notice bad weather to take shelter.

It's turning out to be a day of extreme weather around the region. Excessive-heat warnings have been issued for some parts of Southern California, and high surf has been pounding the coast.

The weather service said the storms could continue through the afternoon. Forecasters say the heat wave should give way to cooler temperatures later this week.

 -- Shelby Grad

Photo: A surfer stays ahead of a large wave at The Wedge in Newport Beach on Wednesday. Credit: Luis Sinco / Los Angeles Times