As firefighters continue to battle the rapidly-growing Border 2 fire, a smoke advisory is in effect for the southwestern area of San Diego County.

Border 2 Fire initially started 2:31 p.m. Jan. 23 in San Diego County. After being active for 20 hours, it has burned 5,389.4 acres, an increase of 1,139 acres since the last update. A fire crew of 692 effectively contained 10% of the fire by Friday noon. The blaze's cause remains under investigation.

The fires come as San Diego County mountains and valleys, along with other parts of Southern California, remain under a red flag warning.

The last time it was this dry in January, there were barely more than 500 people in San Diego. The year was 1850, and only a quarter-inch of rain had fallen in six months. It made the region ripe for wildfire — but that wasn’t much of a concern. The city’s population was clustered along the coast, far from the backcountry, where most fires burned.

Over 1,100 firefighters were “strategically pre-positioned” across Southern California to address "ongoing critical fire weather," Cal Fire said.

The Gilman Fire broke out in the La Jolla neighborhood today, while a brush fire at the border with Mexico, has engulfed 20 acres. Follow Newsweek's live blog.

Firefighters are battling a new brush fire that erupted in San Diego County near the U.S.-Mexico border, as they also race to contain the Hughes Fire near Castaic Lake.