More than 50,000 people were under evacuation orders and warnings Wednesday after a huge and fast-moving fire burned in the mountains north of Los Angeles, while two large fires already ignited in Southern California were still smoldering as strong winds made them harder to extinguish.

The blaze, called the "Hughes Fire", broke out late in the morning and in just a few hours burned more than 39 square kilometres of trees and brush, sending up a plume of black smoke near Castaic Lake. The fire has been burning for the third week around the lake.

LA County Sheriff Robert Luna said more than 31,000 people have been ordered to evacuate and another 23,000 have been warned to evacuate.

Five interstate highways are closed

LA County Fire Chief Anthony Marrone said the fire remained difficult to control, but firefighters were working with full force.

Luna said the closed portions of Interstate 5 will reopen soon.

A 48-kilometre stretch of Interstate 5, the main north-south road, was closed as flames raced across hilltops and into forested valleys.

Crews in the planes tried to stop the wind-whipped fire from moving across the interstate toward Castaic.

The National Weather Service reported on XTV that wind speeds in the area were 67 kilometres per hour at noon, but were expected to increase to 96 kilometres per hour by evening and into Thursday.

To the south, Los Angeles officials began preparing for potential rainfall, while some residents were allowed to return to the burned-out Pacific Palisades and Altadena areas.

According to the National Weather Service, strong winds are expected through Thursday, and rain is expected through Saturday.

“Rain is in the forecast and the threat of mud and debris flows in our fire-affected communities is real,” Supervisor Kathryn Barger said at a news conference Wednesday morning.

Red flag warnings for severe fire risk in L.A. and Ventura counties were extended until 10 a.m. Friday. Officials worry the Palisades and Eaton fires could break their containment lines as firefighters monitor hot spots.

Los Angeles mayor warned

Los Angeles Mayor Karen Bass warned that winds were extremely strong and advised Angelenos to visit the city's website to learn how to protect themselves from toxic air during the Santa Ana fresh wind event. LA County Public Health Director Barbara Ferrer warned that the ash could contain heavy metals, arsenic and other harmful substances.