Story highlights The first major storm of this rainy season is expected to drench area through Tuesday

Flash flood watches have also been issued for parts of Southern California

(CNN) Thousands of Southern Californians were ordered to evacuate Monday amid fears that heavy rainfall could trigger mudslides in regions charred by wildfires.

The wildfires, including several blazes last year, have burned acres of protective brush on hillsides, leaving little to no vegetation to prevent mudslides and debris flow, The first major storm of this rainy season was expected to drench Ventura and Santa Barbara counties and Los Angeles overnight Monday through Tuesday.

In Santa Barbara County , more than 6,000 people were under mandatory evacuation orders, including residents in parts of Carpinteria, Montecito and Goleta, located below areas scorched by the wildfires over the past year and a half. Those fires included the massive Thomas Fire that started in December and is still not completely out, according to Gina DePinto, a county spokeswoman.

Voluntary evacuation warnings were in effect for another 20,000 people, including others in those same communities, she said.

Los Angeles County officials issued mandatory evacuation orders for residents of Kagel Canyon, Lopez Canyon and Little Tijunga Canyon, areas affected by the Creek Fire. Police and fire officials in Los Angeles helped with evacuations in areas damaged by the Creek and Fish fires, officials said.