Heavy rainfall drenching the Southland raised concerns over mud and debris flows near wildfire burn areas Thursday.

Los Angeles County was expected to see up to 3 inches of rain along the coasts and valleys and as much as 5 inches in the mountains. Portions of Orange Counties were forecast to get up to 7 inches.

The National Weather Service issued flash flood watches for portions of Los Angeles, Orange, Riverside and San Bernardino counties through Thursday.

Debris flows in fire-scarred areas are particularly hazardous because they can happen with little warning and gush down to neighborhoods, sweeping away objects in their paths, stripping vegetation and damaging structures.

In Orange County, a mandatory evacuation order was issued for Silverado Canyon, Williams Canyon and Modjeska in the Bond Fire burn area due to the possible debris flows at 8 p.m. Wednesday and was downgraded to a warning at 10 a.m. Thursday.

“Canyon residents are strongly encouraged to continue to preparations and evacuate, especially those with disabilities, access and/or functional needs, and/or with large animals,” Orange County officials said.

Roads were closed, blocking off the canyon entrances Wednesday night.

Two weeks ago, a storm brought severe flooding and damaged several homes around the Bond Fire burn scar. Storm-weary residents piled sandbags around their properties to protect their homes fas they prepared to evacuate Wednesday.

There were also mudflow concerns in Los Angeles County.

Officials warned of moderate mudflows for burn areas near the Bobcat, Lake, Ranch2, Grandview, Soledad, Tujunga and Equestrian fires through 9 p.m.

In the Santa Monica Mountains, Mulholland Highway was closed between Las Virgenes and Cornell roads because of heavy mudflow and rocks falling on the roadway.