The first of three major rainstorms is expected to strike the Southland today, with the storms possibly unleashing as much as 5 inches of rain in Los Angeles and 10 inches in the mountains through Sunday.
Forecasters said the first major storms of 2008 could cause the most significant rainfall to hit Southern California since January 2005. That winter, near-record rains triggered dangerous mudslides.
The National Weather Service predicted that the most powerful storm system, which is expected to land Friday, would bring "strong and possibly damaging winds." Forecasters predicted gusts of up to 70 mph in the mountain areas and the Antelope Valley, and warned that downed trees and power lines were likely.
In Orange County and the Inland Empire, forecasters expected up to 3 inches of rain, with up to 8 inches in the mountains through Sunday.
"It's three storms wedged one after another, with the middle one being the largest," said Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory in La Canada Flintridge.
Today's storm is expected to bring light to moderate rain tonight. The Friday storm is expected to bring light to moderate rainfall in the morning, with the downpour intensifying later in the afternoon and overnight.
"If this delivers as advertised, we haven't seen something like this for three years, so it will be a shocker," Patzert said.
Forecasters warned that burned areas possibly could be hit by mudslides beginning Friday night. They also said the heavy rain could cause urban and small streams to flood, and warned of potential "life-threatening flash flooding."
"Once we get into the rain on Saturday, that's when the problems could start because the ground is already wet," said Stan Wasowski, a weather service forecaster in San Diego.
Another inch of rain could fall Sunday.
The last time Los Angeles saw more than 3 inches of rain in a single month was in February 2005, when 11.02 inches of rain hit downtown Los Angeles. In January 2005, 9.32 inches of rain fell on L.A., the same month that a mudslide hit the coastal Ventura County enclave of La Conchita, killing 10 residents.
The 2004-05 rainy season nearly set a record, with more than 37 inches of rain falling on L.A., more than double the annual average of nearly 15 inches.
But weather experts say that they believe this winter's rainy season won't break any records. Since July 1, 3.76 inches of rain has fallen on downtown L.A.; the average for this time of year is 4.53 inches.
"The forecast is still for a dry winter, and that's largely based on the presence of La Nina this year," said Kelly Redmond, interim director of the Western Regional Climate Center at the Desert Research Institute in Reno.
Redmond said this year's La Nina, a cooling of ocean temperatures along the equator from the International Date Line to Peru, has helped keep the jet stream focused on the Pacific Northwest, which has been pelted by storms this winter.
"We've looked at this relationship over the past 75 years, and we haven't seen above normal rainfall years with La Nina," Redmond said. He added that the Central Sierra is seeing only 50% to 55% of the average year's snowpack.
Still, he said: "I've been surprised thus far with the vigor of the storms" in Southern California.
Meanwhile, authorities spent Wednesday preparing for the storm.
Los Angeles Department of Water and Power crews were assembling a special team to respond quickly to downed power lines, and additional crews and operators were placed on standby.
In Huntington Beach, crews cleared debris from storm drains to prevent urban flooding.
Kirsten James, water quality director of Santa Monica-based Heal the Bay, advised people to avoid going to the beach during and after the storms, as the rains flush trash and other debris into the creeks, bays and coastline.
"There will be all this bacteria that has built up and is being washed down," James said. "We advise [residents] to stay out of the water 72 hours after a storm event."
The storm also was expected to bring significant surf along the coast starting Friday. U.S. Coast Guard officials said they expected waves of up to 15 feet by Saturday.
"This is definitely not a time for new or inexperienced boaters to be out on the water," Coast Guard Lt. Andrew Munoz said.
Times staff writer Paloma Esquivel contributed to this report.