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Extreme Weather Events Unprecedented, Scientists Say

by Randolph E. Schmid

WASHINGTON — Tornadoes, floods, wildfires, snowmelt, thunderstorms, drought — for Americans, it was a spring to remember.

A tornado makes its way across Baca county, Colorado, in May 2010. (Photograph: Willoughby Owen/Getty Images/Flickr) Government weather researchers said yesterday that, while similar extremes have occurred throughout modern American history, never before have they occurred in a single month, as they did in April.

The last time anything remotely like it happened was the spring of 1927, which also had many tornadoes and flooding, said Harold Brooks of the Storm Prediction Center in Oklahoma.

The tornado outbreak, floods, and drought during April were comparable to extreme events in the past, but never so close together in recorded history, agreed Deke Arndt, chief of the climate monitoring branch at the National Climatic Data Center in Asheville, N.C.

The preliminary tornado count was 875 for April, and even after duplicates are eliminated the final total for May is expected to approach the single-month record of 542 set in 2003, Tom Karl, director of the climatic data center, said at a briefing.

The tornado death toll for the year is 536 so far, Brooks said, making 2011 the sixth-deadliest year on record. That may still rise, he added, though most annual tornado deaths occur by mid-June.

The researchers explained that April brought an active weather pattern across the 48 contiguous United States, with strong storms moving through the center of the country, tapping into moisture from the Gulf of Mexico as they matured across the mid-Mississippi Valley.

Contributing to the thrashing were the La Niña conditions in the Pacific Ocean, unusually warm ocean temperatures in the Gulf of Mexico, and the increase of moisture in the atmosphere caused by the warming climate.

While Karl cautioned against focusing on any single cause for the unusual events, “clearly these things interconnect,’’ he said.

Nonetheless, April lived up to poet T. S. Eliot’s description as the cruelest month, and March and May contributed to the battering.

The tally included:

■ Heavy snowmelt in the upper Midwest combined with record rains in the Ohio River Valley produced floods along the lower Mississippi River equaling or surpassing the historic floods of 1927 and 1937.

■ Ideal wildfire conditions developed across the southern plains as rainfall encouraged rapid plant growth, followed by drought and hot weather to launch still-burning fires.

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