The rain arrived in Southern California on Sunday, in what was expected to be the start of a wet week.
VIDEO: Rain footage from across Southern California
Residents placed sandbags to protect properties in foothill cities northeast of Los Angeles, where some communities below steep mountains have long lived with concrete barriers lining streets in hopes of keeping debris flows out of homes.
Glendora resident Cory Hansen piled sand bags around his home.
"Hundreds, hundreds of them," Hansen said. "We keep going down to the city yard picking up more, as much as we can. We don't know what's going to happen in the next few days."
Mary Waldusky and her husband have spent thousands designing their backyard, complete with outdoor living areas and a pool. But these days, the Glendora couple is more concerned about protecting it than enjoying it.
"We have a pallet of sandbags ready to go, we have plywood ready to cover our windows and our doors," Waldusky said.
January's Colby Fire stripped the hillsides above Glendora of their soil-anchoring vegetation.
A surprise rainstorm two weeks ago sent torrents of debris hurtling toward homes. Glendora city officials have pre-filled 12,000 sand bags to help residents prepare, and K-rails are in place to divert floodwaters away from homes.
Tuesday's storm system is expected to trigger even more flows, something L.A. County officials say they're prepared for.
"The system is up and running and the debris basins are cleaned out so that we have the ability to handle anything that is coming our way," said Bob Spencer, L.A. County Department of Public Works spokesman.
City officials raised the alert status to "yellow" for residents near the burn area Monday. The alert imposes restrictions requiring residents to remove vehicles, trash bins and other obstructions from the street to ensure emergency crews can access the area -- and to prevent any damage from mudflows.
The Glendora alert level will be raised to "orange" at 6 a.m., urging residents to voluntarily evacuate areas endangered by flooding and debris. An evacuation center will be set up at the Crowther Teen & Family Center, 241 W. Dawson Ave. The Inland Valley Humane Society will be available at the center to offer assistance with pets. Horses can be taken to the Pomona Fairplex, 1101 W. McKinley Ave.
Sandbags are available for residents free of charge at the City Yard, 440 S. Loraine Avenue.
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Meantime, fire officials in Monrovia are also not taking any chances. Monrovia Fire Chief Chris Donovan says they are particularly concerned about the homes on the roughly 8 to 10 blocks beneath the Madison Fire area.
Dozens of K-rails still line several streets since they were first put in place months after the 2013 Madison Fire that scorched 120 acres above the neighborhood. With no vegetation, the mud and debris flows from this storm are expected to be significant and dangerous to homeowners. On Monday night, residents filled bags with gravel, placing them strategically in front of their homes.
"The actions that our residents are taking here in Monrovia are prudent actions to take, filling the sandbags and stacking the sandbags, the information that we've shared with them for several years now, and more importantly tomorrow when the rains start, monitoring local news and radio stations, and if evacuation orders are issued to leave the area," Donovan said.
Voluntary evacuations ordered for Camarillo residents
In Ventura County, Camarillo residents living near the hillside that burned in last year's Spring Fire are also being encouraged to voluntarily evacuate amid an impending storm. Crews are trying to prevent a repeat of the mud flow one month ago, when the rain sent several feet of mud into backyards and severely damaged one home.
Meantime, Pacific Coast Highway remains closed between Las Posas and Yerba Buena roads. Sunday's rain resulted in mud cascading over the K-rail, forcing drivers to take a detour.
Silverado Canyon braces for possible mud flows
In the fire-scarred areas of Silverado Canyon in Orange County, residents were also preparing for possible mud flows. Emergency officials issued a warning to residents and even suggested voluntary evacuations beginning at 7 a.m. Tuesday for homes east of 30311 Silverado Canyon Road.
Sandbags for canyon residents are available at OC Fire Authority's Station 14, 29402 Silverado Canyon Road, as well as at an OC Public Works' Maintenance Yard, 20811 El Toro Road in Lake Forest.
CNS contributed to this report.