92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Wednesday, 25 January 2012
The Unusually Wet 2010-11 La Niña Winter in California
Hall E (New Orleans Convention Center )
Steve LaDochy, California State University, LA, Los Angeles, CA; and P. Ramirez, W. C. Patzert, and J. Willis
Manuscript (658.9 kB)

The 2010-11 La Niña is considered a moderate to strong event. SSTs in the ENSO regions averaged 1.5- 2 0C below normal. Climate forecasters expected the typical La Niña climate for winter 2010-11. Forecasts were not too bad overall, but missed terribly in California, which broke December rainfall records in southern California. Unusually high snowfall totals also occurred in the Sierra Nevada Mountains. In December a river of atmospheric moisture poured copious amounts of rainfall over the state during a 6-day period (December 17-22). The south coast climate division and the Los Angeles area experienced their wettest December in the 116 year precipitation record with 11.80” (493% of normal) and 10.23” (536%), respectively. An average southern California December would have been drier than normal for the year. Heavy rains, coupled with exposed soil from the massive Station Fire of 2009, led to flooding and mud and debris flows in the Los Angeles region. At higher elevations, especially the Sierras, winter of 2010-11 snowfall amounts rivaled the snowiest 1982-83 winter in California, which occurred during a strong El Niño year. Why did this La Niña event turn into such a powerful rain and snow machine? Not all La Niñas produce drier conditions in California. For southern California, only 9 out of 12 La Niña years resulted in below normal rainfall. For strong La Niñas, Los Angeles was drier in 2 out of 4 cases (Monteverdi and Null 1998). Contrary to opinion, heavy rainstorms in southern California are not preferentially linked to El Ninos, but occur under La Niña and neutral ENSO conditions as well. Snowfall amounts in the Sierras benefited greatly from extreme moisture in the December storms, which put snowfall levels briefly above the record 1982-83 winter. These mountains also received large snow quantities from March storms that swept through northern California. Atmospheric circulation patterns contributed to the unusual precipitation totals, with strong southwest winds from lower latitudes, an MJO Index peaking in November 2010, and an enhanced subtropical jet stream. Increased convection in the equatorial Pacific and west of Hawaii in late November-early December also contributed to abnormally high precipitation totals. When all of the aforementioned interacting and contributing factors combine, as they did in 2010-11, unusually heavy precipitation results in California.

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