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La Niña Triggers Extremes in U.S. Weather

Posted by Brad Rippey, USDA Meteorologist in Conservation
Jun 27, 2011

No matter where you live in the United States, you’ve probably noticed that the weather just hasn’t been “normal” in recent months.  Our nation has experienced widespread flooding, relentless drought, expansive wildfires, and devastating tornadoes – sometimes all at once.

Some of the blame has to be directed at La Niña, a cooling of the waters of the central and eastern equatorial Pacific Ocean.  La Niña developed during the summer of 2010, leading to a profound influence on North American weather from the autumn of 2010 through the spring of 2011.  In typical fashion, La Niña forced the subtropical jet stream northward, resulting in drought development, expansion, and intensification in the South.  At the same, time jet stream disruptions induced by La Niña led to persistently cool, wet conditions across roughly the northern half of the U.S.

La Niña Triggers Extremes in U.S. Weather
La Niña Triggers Extremes in U.S. Weather

With cool weather across the North and warm conditions in the South, there has been – for many months – an unusually strong U.S. temperature gradient.  The semi-permanent boundary where the cool and warm air masses collide has been a focus for heavy precipitation, leading to heavy winter snowfall and extensive spring rainfall and flooding in the Mississippi, Ohio, and Missouri River basins.  The sharp temperature boundary was also an ingredient in some of the nation’s most destructive thunderstorms on record.  April 27, with some 313 fatalities, was the nation’s deadliest “tornado day” since records of that type were initiated in 1950.  And the tornado that leveled much of Joplin, Mo., on May 22, killing at least 151 individuals, became the deadliest single U.S. tornado since April 9, 1947.

Even though La Niña began to weaken during the spring of 2011, atmospheric disruptions associated with the phenomenon persist today.  By mid-June 2011, drought stretched from Arizona to the southern Atlantic States.  Arizona’s largest wildfire in modern history has charred nearly a half-million acres of vegetation near the community of Alpine.   In southern Georgia, a 200,000-acre fire is burning in the Okefenokee National Wildlife Refuge.  Nationwide, this year’s wildfires have claimed 4.3 million acres, compared to the mid-June average of 1.7 million acres.  On June 12, USDA/National Agricultural Statistics Service reported that at least 40 percent of the rangeland and pastures were rated in very poor to poor condition in ten Southern states, headed by Texas (81 percent very poor to poor) and New Mexico (79 percent).  At the same time, more than 40 percent of the cotton crop was rated “very poor” or “poor” in Texas, Oklahoma, Georgia, and Alabama.

Prospects for Southern drought relief could depend on an unlikely source: the tropics.  The National Weather Service expects an active Atlantic tropical season, with twelve to 18 named storms and 6 to 10 hurricanes.  Long-term averages are 11 named storms and 6 hurricanes.  Atlantic tropical activity typically peaks from August to October.  However, an active Atlantic hurricane season does not necessarily translate into storms making landfall.  In 2010, for example, none of the 12 Atlantic hurricanes made a U.S. landfall.  Farther north, relief from excessively cool, wet conditions will depend on the pace of the atmosphere’s recovery from La Niña, as well as other unforeseen factors that may come into play.


Category/Topic: Conservation