<img alt="" data-portal-copyright="Hayk_Shalunts / Shutterstock” data-has-syndication-rights=”1″ data-focal-region=”x1:2527,y1:1527,×2:3489,y2:2489″ src=”https://cdn.vox-cdn.com/thumbor/Rh-0UqIHxDFqIJwy9lpef4uqchg=/331×0:5686×4016/400×300/cdn.vox-cdn.com/uploads/chorus_image/image/59084719/hollywood_rain.0.jpg” />
“This is the big one”
The season’s biggest storm is expected to hit Southern California tomorrow, and, once again, it could spell trouble for neighborhoods scarred by recent wildfires.
“This is the big one,” says weather service meteorologist Eric Boldt. “This is the largest storm of the winter.”
The Los Angeles Fire Department is bracing for flash floods, issuing voluntary evacuations for most areas impacted by the La Tuna Canyon fire, including portions of Sun Valley, Sunland, and Tujunga. Mandatory evacuations were ordered for Kagel Canyon, Lopez Canyon, and Little Tujunga, areas ravaged by the Creek Fire, plus several blocks of La Tuna Canyon Road.
Voluntary and mandatory evacuation orders go into effect at 6 p.m., when the heaviest rain is forecasted to arrive.
“Rainfall rates up to 0.6 inches per hour are possible late this evening with rates possibly increasing to 0.75 inches per hour or higher at times Thursday,” according to the National Weather Service. “Rainfall of this intensity can produce dangerous mud and debris flows near recent burn areas.”
The Weather Service says Los Angeles County will see the biggest downpours on Thursday, with rain dissipating Friday. Santa Barbara and Ventura counties, which were devastated by the Thomas Fire, will get the brunt of the rain today.
— Hans Gutknecht (@HansGutknecht) March 21, 2018
— NWS Los Angeles (@NWSLosAngeles) March 21, 2018
The latest forecast calls for two to five inches of rain across Southern California’s coasts and valleys, and five to 10 inches of rain in the foothills and mountains. The weather service says LA County’s rainfall will likely be on the lower end of that range.
Still, the high rainfall totals could cause problems in areas that were recently burned.
Areas recovering from the La Tuna Canyon fire or the Creek fire are covered in ash and other material that doesn’t absorb water. Heavy rain runs off recently burned soil “as it would run off of pavement,” says the weather service. That can lead to issues with debris flow and mudslides.
How rapidly the rain falls is another factor in the damage it can do. Forecasters say there’s a 20 percent chance that an inch of rain could be falling per hour at the peak of the storm.
”If the 2-4 inches of rain falls gently over the whole 72 hour period, not a lot is going to happen,” Andrew Rorke, a senior forecaster with the weather service, tells KPCC. But if the rain falls in big surges, that could be a problem, “and unfortunately we are forecasting a couple big bursts of rain.”
According to the weather service, Downtown Los Angeles has seen just 3.4 inches of rain since October 1, the start of the wet season. That’s roughly a quarter of the 12.6 normal rainfall total.
Rapidly developing Pacific storm with #PineappleExpress connection looks truly spectacular on satellite imagery today. Main event not expected in #SoCal until later Wednesday and Thursday (slightly delayed), but no decrease in intensity. #CAwx pic.twitter.com/0znlA5FnQU
— Daniel Swain (@Weather_West) March 19, 2018