Strongest storm yet could bring flooding, tornadoes, hail and high winds to L.A. area

December 21, 2010 |  2:52 pm

The National Weather Service warned Southern California that the most intense in a series of storms will slam into the region before dawn Wednesday.
[Updated at 3:28 p.m.: L.A. County officials announced "residences in La Canada Flintridge and 85 residences in La Crescenta [would be evacuated] due to the possibility of debris flows resulting from forecasted rain storms in the area."
This storm could bring with it thunderstorms, hail, and even waterspouts and tornadoes along the Southern California coast early Wednesday, forecasters said.
The powerful storm could drop as much as 0.75 to 1.5 inches an hour in places, which could bring flooding not only in foothills and mountains but in neighborhoods all over L.A. County, said Stuart Seto, a specialist for the weather service in Oxnard.
“The ground will be permeated with a lot of rain, and it was a very, slow consistent rain for the past five days,” Seto said. “This is going to be more of a thunderstorm-type rain.
“This thunderstorm activity is very dynamic and intense,” Seto said. He said there’s a possibility of waterspouts along the coast, which become tornadoes if they hit land.
While the tornadoes could cause damage if they strike a house, they are generally nowhere near as powerful as the ones in the Midwest.
The coast and valleys could be pelted with 2 to 4 more inches of rain, and the foothills and mountains could see 4 to 8 additional inches, Seto said.
Wednesday’s storm will move through Southern California faster than earlier storms, but will be more intense. Forecasters said it is expected to arrive about 2 a.m. in Ventura County and sweep over to L.A. County by 4 a.m.
The storm is expected to hold over the region for six to eight hours.

Wednesday’s storm will be more energetic than the previous systems because it will result from the collision of a cool area of low pressure moving south from Seattle to Southern California and warmer, moist air from the western Pacific Ocean, beyond the International Date Line.
The collision between the two systems will produce thunderstorms, which will cause rain to fall more swiftly, Seto said. “The rapid rainfall coming on certain areas will run off a lot faster,” Seto said. “We’re going to see a lot more runoff with these storms.”
Heavy rainstorms have proved to be a problem not only in foothill communities bordering burned forest land, but also in low-lying areas of Long Beach and San Pedro, which can be overwhelmed if flood-control channels can’t handle bursts of significant rainfall.
Also a problem will be sustained winds of 15 to 25 mph along the coast and in valleys, along with gusts of up to 65 mph in the mountains. It’s possible more large trees, with their roots soaked in loose soil, could fall on Wednesday.
“We’ve already had trees go over,” Seto said. “I wouldn’t want to be parked under one.”
Downtown L.A. has already received about 6 inches of rain since Thursday -– about 40% of the precipitation the city receives on average every year.
Seto said that by the time the storm clears out late Wednesday or early Thursday, as much as 20 inches of rain may have dropped on the wettest areas.
Already, the storm systems of December have dumped tremendous precipitation around the state. The Mammoth Mountain summit has recorded 9 to 13.5 feet of snow in the last week, the weather service office in Reno said.
Partly cloudy conditions are expected to return on Thursday and Christmas Eve. A weak storm system may return to L.A. on Christmas Day, but the storm is only expected to drop less than half an inch of rain.
In San Diego County, the rain caused flooding, numerous traffic snarls and road closures. Mud slid down a hillside in La Jolla. Between midnight and noon, there had been 202 traffic accidents in the county, three times the amount of an “average” day, according to the California Highway Patrol.
Three suspected illegal immigrants were plucked from the rain-swollen Tijuana River, shivering and desperate for rescue. The San Diego Police Department is warning residents of the Tijuana River Valley to evacuate their homes and take their animals to higher ground as the water level continued to rise.
Pollution flowing into the ocean from the river caused county health officials to close the Silver Strand beach in Coronado. The main thoroughfare through Camp Pendleton -- Vandegrift Boulevard -- was closed near the base air station due to flooding.
The hardest-hit area in the North County was in the Palomar Mountain region, where numerous rock slides, mudslides and downed trees were reported. No injuries were reported.
In Orange County, four men stranded in Trabuco Canyon were rescued Tuesday morning when they were plucked from the flooded foothills by helicopter. The men were spotted soon after daybreak when the helicopter flew over the fast-moving Trabuco Creek and saw them inside their vehicle.
A woman who was swept away in her pickup while crossing a rain-swollen creek in the San Bernardino National Forest was rescued Monday night after a harrowing four-hour recovery effort, officials said.
The 29-year-old woman was crossing Lytle Creek north of San Bernardino shortly before 5 p.m. in her Ford pickup when the high water washed away the road and started carrying the vehicle downstream.
As water filled her cab up to the dashboard, the woman used her cellphone to call for help, officials said.
More than 5 inches of rain have already fallen in downtown Los Angeles this month, and the record of 8.77 inches for December is within reach. Mammoth Mountain has already recorded the highest December snow levels ever.
Authorities say the mountainsides near Sierra Madre are holding up amid the rains, but a potential evacuation alert is ongoing to keep residents prepared.
Aside from some blockage on the 800 block of Skyland Drive, police are reporting unobstructed water flow. Sgt. Kenneth Berry with the Sierra Madre Police Department credited efforts in the last two months to clear out debris basins.
“As of right now, the mountainside is holding up,” Berry said. The green flag alert, Berry said, is meant solely to keep residents prepared for a quick evacuation if the expected stormy weather does cause mud flow later in the day.
“We just have to have people prepared,” Berry said.
-- Rong-Gong Lin II and Robert Faturechi in Los Angeles, and Tony Perry in San Diego
Map: A giant area of low pressure is moving from the Pacific Northwest to Southern California. When the cool system from the Gulf of Alaska collides with moist, warm air from the subtropical western Pacific Ocean, Southern California could be hit with thunderstorms, hail and even waterspouts and tornadoes, the National Weather Service said. Credit: National Weather Service.