JURUPA VALLEY, Calif. (CNS) — A brush fire that erupted Monday in the Santa Ana River bottom between Jurupa Valley and Riverside expanded to 150 acres after a second fire branched off from the main one, and the combined wildfire was threatening homes and triggering evacuations amid windy conditions.

The main fire started about 11:10 a.m. in the area of Jurupa Avenue and Van Buren Boulevard, less than a mile north of Riverside Municipal Airport, according to the Riverside County Fire Department.

Crews had been making steady progress encircling the fire when a separate brusher sprang up about 3:30 p.m. ahead of the larger one, near the Western Riverside County Animal Shelter.

The fires blended into one under increasing southwesterly winds, and the consolidated fire was burning close to homes south of Limonite Avenue. Evacuation orders were issued between Downey Street to the west and Avenue Juan Diaz to the east, encompassing hundreds of residences.

A second mandatory evacuation order was issued about 5 p.m. within Riverside, impacting residents north of Jurupa Avenue, stretching from Van Buren to the west to Anza Narrows Park to the east.

A temporary evacuation center was set up Patriot High School, 4355 Camino Real, in Jurupa Valley.

Sheriff's deputies were also closing access points to the river bottom, except for emergency personnel.

At 4:20 p.m., Riverside Municipal Airport and Flabob Airport three miles to the northeast were shut down, and temporary flight restrictions were imposed, permitting only Cal Fire aircraft to come and go in the airspace.

Four Cal Fire water-dropping helicopters and two air tankers were called in to make runs on the brusher.

Multiple engine, truck and hand crews from the county, city of Riverside, Corona, Murrieta Fire & Rescue and Ontario were additionally working to encircle the fire.

No injuries were reported.

At 12:40 p.m., the initial fire turned north, immediately threatening properties near Kennedy Street and Riverdale Place in Jurupa Valley. A mandatory evacuation was ordered in the neighborhood, but at 1:50 p.m., the order had been downgraded to an evacuation warning.

A battalion chief at the scene described the winds as "erratic," hampering containment efforts. He said the fire had the potential to grow to 500 acres. The flames were concentrated in Jurupa Valley, with the back end of the fire in Riverside.

As of 5:30 p.m., the winds were beginning to let up, according to reports from the scene.

The fire was 5% contained as of about 7:10 p.m., according to a Riverside County Fire Department statement.

The cause of the fire was not immediately known.

The river bottom is dotted with homeless encampments, and during dry periods, fires from camping and cooking are a regular occurrence.