The amount of moisture in the air over southwestern California on Friday will be exceptional. (weatherbell.com)

(This story has been updated.)

Southern California is under siege from one of the strongest storms in years. Torrential rain, flash flooding and damaging winds are forecast into Saturday

Friday morning’s headline on the National Weather Service website for Los Angeles read “Dangerous Storm”.

Daniel Swain, a climate scientist at UCLA, tweeted the storm was “undergoing explosive deepening” and to expect damaging wind and torrential rain from Monterey to Los Angeles.

California’s Central Coast was met by an onslaught of heavy rain Thursday night into Friday morning, where one to two inches was reported. A little farther south, Santa Barbara had already picked up a quick three quarters of an inch of rain early Friday, with several more inches on the way.

The storm is also bringing ashore howling winds. At Diablo Canyon Power Plant near San Luis Obispo, winds were clocked at 45 mph and gusting to 63 mph at 9:00 a.m. local time.

Heavy rains and strong winds are predicted to reach Los Angeles by midday and make for a very difficult Friday afternoon commute.

“The storm looks to be the strongest storm to hit Southwest California this season,” the National Weather Service in Los Angeles said Thursday. “It is likely the strongest within the last six years, and possibly even as far back as December 2004 or January 1995.”


Storm timeline:

  • Friday morning/afternoon — Heavy wave of rain arrives, focuses on areas south of Santa Barbara; rainfall rates in this wave could reach or exceed 1 inch per hour; wind gusts exceed 40 mph along the coast, 60 mph in high elevations
  • Friday night — Heavy rain slides south to focus more on the San Diego area; flash flooding, mudslides, debris flow possible
  • Saturday morning — Rain begins to taper off
  • Saturday afternoon — Light rain, drizzle

A few things are coming together to make this storm particularly hazardous for the Los Angeles and San Diego region.

Greg Postel, a meteorologist for the Weather Channel, dug into the records for this region and found that at 988 millibars, this particular storm could become one of the strongest cyclones in this region by pressure — the lower the pressure, the stronger the storm.

The lowest February pressure recorded in San Diego was 984.4 millibars in 2010.

As the storm moves onshore, low-level winds will come out of the south. This will basically scoop up warm, Baja moisture and transport it to the Southern California coast. Wind gusts will likely be severe in some areas, which will bring down branches and perhaps trees and lead to power outages.

This storm is another Pineapple Express moisture setup — named for its Hawaii origin — which California has become very familiar with this season. They tend to be common for the West Coast in wintertime (especially during El Niño), but over the past five years, this phenomenon has failed to materialize, deepening the severity of the drought.

The National Weather Service noted Friday morning that, even before the heaviest rain had moved ashore, this Pineapple Express plume of moisture was supporting 0.50 inch per hour rainfall rates. It predicted these rates would “intensify by at least a factor of two” in some of the higher terrain inland from the coast. Air flowing up over mountains rises, cools, and condenses, and the moisture is effectively wrung out, enhancing rainfall.

With all that in mind, torrential rain is predicted.

Computer models predict widespread rainfall totals of three to five inches between San Luis Obispo and Long Beach, with totals of up to 5-10 inches in higher terrain. That’s reflected in the forecast from the National Weather Service:

In Los Angeles, about four inches is predicted, which would be the most in a calendar day in more than a decade.

To the south, from around Laguna Beach to San Diego, generally about one to three inches is forecast.

Of particular concern is that the extreme rain falling over high terrain won’t stay there. It’s going to flow down the mountainsides with the potential to trigger mudslides and flash flooding. A flash-flood watch is in effect for a large area of Southwest California, including the San Diego and Los Angeles areas.

“There will probably be widespread urban roadway flooding, along with potential flooding of small streams and creeks,” the Weather Service in Los Angeles wrote Thursday. “There will also be a significant threat of rock and mudslides across all of Southwest California, especially near canyon roadways.”

Recent burn areas will be particularly susceptible to flash flooding and debris flow. The Santa Barbara County Sheriff Department issued an evacuation warning Friday morning in the Sherpa Fire Burn area due to the threat of flooding.

A major storm packing intense rain and winds is set to pound Southern California. It could be the strongest storm to hit the region in years. (Reuters)