Washington, D.C. — It was a spring to remember, with America pummeled by tornadoes, floods, wildfire, snowmelt, thunderstorms and drought.
Government weather researchers said Wednesday that, while similar extremes have occurred throughout modern American history, never before have they occurred in a single month.
The last time anything remotely looked like it was the spring of 1927, which also had a lot of tornadoes and flooding, said Harold Brooks of the Storm Prediction Center in Norman, Okla.
The tornado outbreak, floods and drought during April were comparable to extreme events in the past, but never so close together, 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 is expected to approach the single-month record of 542 set in May, 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 6th deadliest year on record. That may still rise somewhat, he added, though typically most annual tornado deaths occur by mid-June.
The researchers explained that April brought an active weather pattern across the 48 contiguous U.S., 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 Nina 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.
But Karl cautioned against focusing on any single cause for the unusual chain of events, "clearly these things interconnect."
Nonetheless, April lived up to poet T. S. Eliot's description as the cruelest month, and March and May contributed to the battering in the three months of climatological spring.
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 consuming millions of acres.
— Consecutive dry months caused drought that extends across much of the Southwest and South from Arizona and New Mexico across Texas to the Gulf Coast and southern Georgia.
— Yet it was the wettest April on record for Illinois, Indiana, Ohio, Pennsylvania, Kentucky and West Virginia.
So far this year the United States has suffered eight disasters costing $1 billion or more and the total damage to date is $32 billion and rising, Karl said. If no more disasters occurred, 2011 would still rank in the top 25 percent of years for disaster costs, he said.
La Nina is marked by a cooling of the tropical Pacific Ocean and has now returned to more normal conditions. However, when it is under way it sets up a storm weather track that brings storms into the upper Midwest and then south into Ohio Valley. This resulted in the extremely thick snowpack in some areas that contributed to the spring flooding and also brought dry, windy conditions to the southern Plains.
The spring warming then brought the warm, moist southerly flow of air in from the Gulf of Mexico, contributing energy to the storms that developed into the outbreaks of tornadoes and other severe storms.
Years with more tornado deaths than 2011 were:
—1925 with 794;
—1936 with 552;
—1917 with 551;
—1927 with 540
—and 1896 with 537.