Weather forecasters and public officials in Southern California were growing increasingly concerned on Thursday that a complex storm system moving in from the north could stall, leading to days of heavy rain and the potential for landslides in areas that were recently burned by wildfires.
Just how much rain — and where and when it will fall — was still unclear on Thursday afternoon. Under one scenario, the Los Angeles area could record a third of its total annual rain, which would amount to about five inches, by the end of this weekend. Under another, far less rain would materialize because the storm causing it would have moved on more quickly.
Los Angeles County officials said an evacuation warning would be in place for some places from Thursday evening through Sunday, including areas that were burned during the Palisades and Eaton fires in January. A warning means that people should be prepared to leave quickly.
Here’s a look at how it’s all unfolding as the storm moves in.
Two waves of rain are expected.
Two waves of rain are expected to sweep Southern California’s coastal plain between San Luis Obispo and San Diego Counties. Los Angeles County falls within the bull’s-eye of where the most rain could fall over the next four days.
Forecasters are confident in the timing and amount of rain in the first wave, from Thursday night into Friday. Most areas are expected to see about a quarter inch to a half inch of rain.
The second wave is predicted to arrive Friday night and continue into Saturday, with the heaviest rain most likely Saturday morning. But “there’s still a lot of uncertainty,” said Rich Thompson, a meteorologist with the National Weather Service.
“At this point, the storm could stall, and this could become problematic with it dumping heavy rainfall over the same area for a prolonged period of time,” Mr. Thompson said early Thursday.
We’ll get to why things are uncertain in a minute. But first:
How much rain will fall?
Forecasters are pretty confident that coastal Southern California will be wet through Saturday, with the northern half of the region and Los Angeles County most likely getting more rain than the southern half, and San Diego County. They are less confident of how much rain will fall over the next four days.
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The most likely scenario: A majority of weather models show coastal and valley areas, including downtown Los Angeles, recording one and a half to four inches of rain, and mountains and foothills recording four to seven inches.
That’s still a wide range, and it would bring “plenty of urban flooding,” said Mr. Thompson, particularly in low-lying areas with poor drainage, like underpasses. Mudslides and landslides are likely on mountain roads, such as in the Santa Monica Mountains.
- The least extreme scenario: Most locations get half an inch to two inches of rain.
- The most extreme scenario: Downtown Los Angeles records five to six inches with over seven in the mountains. “This doesn’t have a very high probability of occurring,” said Bob Oravec, a meteorologist with the Weather Prediction Center.
Why are burn scars such a concern?
In heavy rain, wildfire burn scars, especially those from the last one to two years, are prone to landslides, mudslides and debris flows.
Wildfires disrupt the landscape, eating up vegetation and cooking the soil, which turns hard and develops a waxy consistency that repels water.
The debris flows start as a flood of water from heavy rains that picks up sediment, moving quickly across a hardened landscape without any vegetation to stop it.
It typically takes about a half inch of rain per hour to set off a debris flow in a burn scar.
Why is there so much uncertainty in the forecast?
Meteorologists use forecasts from many different models to arrive at the most likely scenario for an event. The more the models agree, the more likely a forecast is to be accurate. But they also pay close attention to the outliers, the forecasts that disagree with the consensus.
On Thursday afternoon, before the rain arrived, models were showing a wide range of possibilities around how long the second wave of rain would last, disagreeing by several hours, said Chad Hecht, a meteorologist with the Center for Western Weather and Water Extremes at the University of California, San Diego. The longer the rain lasts, the higher the totals become, and the more likely the storm is to cause the types of hazards that have public officials concerned.
On Thursday morning, the computerized models that forecasters use to help them predict the weather were suggesting that the storm would most likely sit over Los Angeles and Orange Counties — but the storm’s trajectory could change, Mr. Thompson warned.
This storm is particularly difficult to forecast because it has broken off from the jet stream, the conveyor belt of winds that transports storms through the atmosphere. It has spun off like a top toward Southern California.
“It’s going to sit there and wobble around for a little bit,” said Todd Hall, a meteorologist with the Weather Service. “And that’s what causes all the uncertainty.”
Amy Graff is a Times reporter covering weather, wildfires and earthquakes.
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