WASHINGTON — For government scientists usually cautious about going out on a limb, a "special report" issued Wednesday minced few words: Last April saw an unprecedented onslaught of extreme tornadoes, flooding, drought and wildfire, they concluded.
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"While similar extremes have occurred throughout modern American history," the report by experts at the National Climatic Data Center stated, "never before have they occurred in a single month."
Drought "during April across the Southern Plains stood in stark contrast to the record precipitation across the Ohio Valley, the record floods along the Mississippi River, and the severe weather outbreaks across the Southeast," the experts added.
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, Center Director Tom Karl said at a briefing. The 30-year average for tornadoes in April is 135.
The tornado death toll for the year is 536 so far, and the vast majority were in April during two outbreaks across the Southeast.
The toll already makes 2011 the sixth deadliest year on record, said Harold Brooks of the Storm Prediction Center in Norman, Okla. That might 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 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 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."
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.
And it wasn't just April that saw extreme weather. The March-May spring tally posted by government scientists includes:
- 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.
- The wettest April on record for Illinois, Indiana, Ohio, Pennsylvania, Kentucky and West Virginia.
The last time any spring looked like this year's was in 1927, which also had a lot of tornadoes and flooding, said Brooks.
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.
The Associated Press contributed to this report.