Sunday, October 11, 2009

wet forecast

Station Fire burned area courtesy NASA/JPL/Caltech

By Guy McCarthy

A Pacific storm is expected to bring the first widespread rains to southwest California since the Station Fire burned 250 square miles in the San Gabriel Mountains above Los Angeles in August and September, according to the National Weather Service.

A strong storm system - especially for October - will begin moving into northern and central California late Monday, and "considerable remnant moisture from former western Pacific Typhoon Melor is expected to be pulled into this system," according to a special weather statement issued this afternoon by Oxnard-based forecasters.

Lower Big Tujunga Canyon on Oct. 1

As of Sunday afternoon, the storm appeared to be bringing significant rainfall to southwest California, along with the possibility of mud and debris flows across recent burn areas, the Weather Service stated.

The heaviest rainfalls over Ventura and Los Angeles counties can be expected Tuesday night and Wednesday, with preliminary estimates of 2 to 4 inches in the foothills and mountains. Locally higher amounts are possible in mountain areas, according to the Weather Service.

Along with heavy rainfall, strong southeast to south winds Tuesday and Wednesday may bring gusts up to 55 mph in some mountain areas.

Detail map of burn areas including portion of Station Fire,
courtesy Los Angeles County Department of Public Works

"With the expected heavy rainfall across Southern California, there will be a threat of flash flooding and debris flows near recent burn areas," the Weather Service stated. "Flash flood watches for the burn areas may be issued within the next 24 hours."

Residents of southwest California, especially those with property in and around recently burned areas, should stay tuned to weather forecasts and statements as this Pacific draws closer to the region, the Weather Service advised.

Rainstorms this year in the area burned by the Station Fire have the potential to trigger debris flows that may impact neighborhoods at the base of the San Gabriel Mountains, as well as areas in Big Tujunga Canyon, Pacoima Canyon, Arroyo Seco, West Fork of the San Gabriel River, and Devils Canyon, according to an assessment released five days ago by the U.S. Geological Survey.

Triggered by storm rainfall, debris flows can travel faster than a grown person can run, creating a dangerous situation that may occur with little to no notice, according to USGS Research Geologist Susan Cannon. The powerful force of rushing water, soil, and rocks can destroy culverts, bridges, roadways, and structures and can cause injury or death.

The USGS assessment found that some watersheds in the Station Fire burn area could generate debris flows containing up to 100,000 cubic yards of material — large enough to cover an American football field 60 feet deep with mud and rock.

The deadly debris flows that occurred following the 2003 Old and Grand Prix fires in San Bernardino County are an example of what could happen this year in or below Station Fire burned areas, according to the USGS.

"People may remember that 16 people were killed by debris flows during the Christmas Day storm in 2003, but few realize that those were only two debris flows out of the hundreds that were triggered from the burned area," Cannon said.

"Nearly every burned watershed produced destructive debris flows or floods in response to that storm," Cannon said. "Some of the areas burned by the Station Fire show the highest likelihood for big debris flows that I’ve ever seen."

The full USGS report on Station Fire debris flow hazards is here.

The USGS report "Emergency Assessment of Debris-Flow Hazards from Basins Burned by the Grand Prix and Old Fires of 2003, Southern California" is available here.

Survivors' accounts from the Christmas 2003 tragedies are here.


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