[californiadisasters] Atmospheric River Deluge (Pineapple Express 2010)

Atmospheric River Deluge

By Eric Boldt ~ NWS Los Angeles/Oxnard

A relatively dry December 2010 changed dramatically starting on the 17th as a blocking high pressure system over the eastern Pacific Ocean retrograded westward allowing low pressure to dominate off the west coast. This low pressure system soon tapped into a deep moisture source originating west of Hawaii and undercut the high pressure system to the north. Even a tropical storm briefly formed in the main low pressure region west of Hawaii. The long conveyor belt of moist, subtropical air (a.k.a., atmospheric river)  languished over southern California for more than 100 straight hours breaking daily and monthly all-time precipitation records in many areas (see Super Soggy December, page 1).

Most valleys and coastal areas received from 5-10 inches of rain during the period, with foothill and mountain areas recording from 15-25 inches. Snow levels remained fairly high throughout the majority of the precipitation event (above 7000 feet) until the final day on December 22nd when colder and more unstable air moved over the region behind a cold front. Precipitation totals were the highest for a single storm event in the last 5-10 years.

However, no major flood problems were reported in southwestern California (San Luis Obispo, Santa Barbara, Ventura, and Los Angeles Counties). Even so, the Sisquoc River made a significant jump above monitor stage on Dec. 19 when higher intensity rainfall of 3-5" in 24-hours occurred across Santa Barbara and San Luis Obispo Counties. There was also flooding in low-lying areas and along Arroyo Grande Creek during the event which caused the evacuation of several homes near the Oceano airport (San Luis Obispo Co). The Santa Maria River overflowed dikes and lead to minor damage to buildings in Santa Maria (Public Health Dept) and high water across roads in the Guadalupe area.

On Dec. 21st and 22nd, heavy rain over the San Gabriel Mountains drained across the desert and filled normally dry arroyos that impacted roads and highways with low-water crossings. Several cars were stranded along Avenue H to the east of Lancaster on Dec. 22nd, with 5 individuals and 1 dog rescued. In a few locations along the PCH in the Santa Monica Mountain Recreational Area, rock slides blocked the highway which caused a shutdown of the freeway for several days. There was one significant rock and mud slide near some homes in the West Hollywood Hills during the rain event, but no significant damage was reported.

In the recent burn areas (Station, Morris, Crown, Jesusita), no major problems were experienced from debris flows or flash flooding. LA County DPW reported that many debris basins were filled only 20-25% of capacity by the end of the rain event. A mandatory evacuation order was in place for 232 residents of La Canada Flintridge and La Crescenta on Dec. 21-22. And finally on Dec. 22nd, following a 1-2 hour period of heavy rainfall, a small pop-out landslide occurred in La Conchita. A tear in the hillside about 100' high by 120' wide occurred and came within 10 feet of the back of one house. There was no damage except some trees were knocked over and 3 homes were evacuated.

Source: http://www.wrh.noaa.gov/lox/spotter/documents/Newsletter_april2011.pdf

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