National Weather Service Predicts Impending Return of “La Niña” Conditions

On July 8, following its monthly El Niño/Southern Oscillation (ENSO) Diagnostic Discussion, the National Weather Service’s Climate Prediction Center announced its opinion that La Niña conditions have begun to develop and will last through early 2011.

According to the National Oceanic and Atmospheric Administration (NOAA), La Niña, like its opposite phenomenon El Niño, “results from interaction between the surface of the ocean and the atmosphere in the tropical Pacific.”  In the case of La Niña, the result is cooler sea-surface temperatures that alter weather patterns around the world.

In the U.S., the effect of La Niña varies from region to region.  NOAA says that La Niña, “often features drier than normal conditions in the Southwest in late summer through the subsequent winter. Drier than normal conditions also typically occur in the Central Plains in the fall and in the Southeast in the winter,” and, “In contrast, the Pacific Northwest is more likely to be wetter than normal in the late fall and early winter with the presence of a well-established La Niña. Additionally, on average La Niña winters are warmer than normal in the Southeast and colder than normal in the Northwest.”

In many cases these conditions have meant drought conditions for California.  The New York Times reported in 2009 that, when La Niña conditions occurred in 2008, the rainy season ended in February rather than in April, and runoff from the Sierras was just 57 percent of normal.

July’s ENSO Diagnostic Discussion summary can be found here.

NOAA’s “Answers to La Niña Frequently Asked Questions” is available here.