(CNN) - Thousands of Southern Californians were ordered to evacuate Monday amid fears that 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.
Ventura County also issued both mandatory and voluntary evacuations for several communities.
In Santa Barbara, DePinto said the Thomas fire burned 17 major canyons that span 40 miles, destroying the county's watershed.
The Thomas Fire is the largest wildfire in state history, having burned more than 281,000 acres since it began on December 4. It was 92 percent contained Monday; officials don't expect full containment until later this month.
"If you think of mud, trees, rocks, huge boulders coming down; there's nothing to stop it because it's completely burned," DePinto said.
Homeowners in charred areas were advised to take precautionary measures like stacking sandbags to divert the flow of water and mud, CNN affiliate KABC reported.
Crews in Ventura and Santa Barbara counties had already been frantically clearing out debris basin and storm drains. Crews were taking no chances in flood channels, hauling away anything that could block the drain and cause pooling, then flooding.
While such cleanup occurs before every rainy season, the scale of the Thomas Fire increased the intensity.
"Generally, we are dealing with hundreds of acres (burned), not in the thousands or hundred thousands," Jeff Pratt, director of public works in Ventura County told CNN last week. "This is an order of magnitude or two greater than anything we've ever dealt with."
Up to 4 inches of rain was expected overnight in Santa Barbara and Ventura counties, according to CNN meteorologist Michael Guy. Two inches of rain was expected to fall in Los Angeles, with gusty winds, heavy precipitation and thunderstorm possible through Tuesday, Guy said.
The heaviest rainfall is expected is between 3 a.m. and 5 a.m. in the three counties, Guy said.
About a half an inch or rain per hour is enough to start to produce mudslides, according to Robbie Monroe of the National Weather Service Oxnard office.