'Have An Evacuation Plan': When To Expect Heaviest Rainfall

LOS ANGELES, CA — As the Southland's biggest rainstorm of the season is expected to arrive Wednesday, officials are warning residents to prepare for a deluge so severe that it could prompt evacuations amid flooding and landslides.

"I want to really urge everyone to take the alerts seriously here," National Weather Service meteorologist Ariel Cohen said at a Wednesday morning news conference. "We have a return to some potentially dangerous weather conditions across the area."

Areas in and around the burn scars of recent wildfires are at risk of landslides and those areas are also expected to see among the greatest rainfall totals in SoCal. Meantime, flash flooding is possible is areas throughout the region, forecasters said.

The NWS expects the incoming storm system to be "the biggest precipitation producer so far this season" — much more intense than the rains over the last two weeks. As much as a half inch of rain is possible on Wednesday. The brunt of the storm is expected to arrive Thursday, when widespread rain is likely to impact the area for 36 hours.

In total, the storm is expected to bring 1.5 to 3 inches of rain across much of LA County, with 3 to 6 inches expected in the mountains, including near the Palisades and Eaton fire burn scars.

While most areas are unlikely to experience dangerous weather conditions, officials said all Angelenos should be prepared to evacuate if ordered to do so. That means signing up for evacuation alerts online and having a plan to get out quickly, because "seconds mean the difference between life and death when it comes to those orders and warnings," Cohen said.

Those who live in or near wildfire burn scars should consider relocating temporarily while the storm passes, NWS meteorologist Ryan Kittell told the Los Angeles Times.

There's a 50 to 70% chance of debris flows in the area. In a debris flow, water flows rapidly downhill — at speeds of up to 35 mph — picking up mud, rocks, branches and potentially even boulders and cars, the Times reported.

"The force of this rushing water and debris can also damage or even destroy things like culverts, bridges, roadways and even buildings that were left standing after the fires — even miles away from the burned area," meteorologist Damien Lodes told AccuWeather.

Pacific Coast Highway was closed indefinitely over the weekend in anticipation of the upcoming storm and because of ongoing cleanup of mudslides that occurred during last week's storm.

The NWS has issued a Flash Flood Watch for the bulk of the Southland that will be in place from Thursday afternoon through late Thursday night. The biggest timeframe of concern is between 2 p.m. Thursday and 2 a.m. Friday, meteorologists are expecting "bursts of heavy rain," Cohen said.

Motorists should avoid driving if possible on Thursday, Kittell said.

“Thursday is just not a great day to be on the road,” he told the Times. “If nothing else, [expect] lots of slick roads, lots of traffic accidents. There will be some roadway flooding. Not all areas ... but certainly more than we’ve seen this winter so far.

Health officials have already raised alarm bells about toxic wildfire runoff from earlier rainstorms. The LA County health department shut down ocean-water access across a 9-mile stretch of coast late last month.

City News Service contributed to this report.