WASHINGTON -- Another week, another rumbling train of tornadoes that obliterates entire city blocks, smashing homes to their foundations and killing people even as they cower in their basements.
With the year not even half done, 2008 is already the deadliest tornado year in the United States since 1998 and seems on track to break the U.S. record for the number of twisters in a year, according to the National Weather Service. Also, this year's storms seem to be unusually powerful.
But meteorologists cannot explain exactly why this is happening.
"There are active years and we don't particularly understand why," said research meteorologist Harold Brooks at the National Severe Storms Lab in Norman, Okla.
The numbers for the U.S. so far this year: at least 110 dead, 30 killer tornadoes and a preliminary count of 1,191 twisters (which, after duplicate sightings are removed, is likely to go down to around 800). The record for the most tornadoes in a year is 1,817 in 2004. In the past 10 years, the average number of tornadoes has been 1,254.
Global warming cannot really explain what is happening, Carbin said. While higher temperatures could increase the number of thunderstorms, which are needed to trigger tornadoes, they also would tend to push the storm systems too far north to form some twisters, he said.
La Nina, the cooling of parts of the Central Pacific that is the flip side El Nino, was a factor in the increased activity earlier this year, but it can't explain what is happening now, according to Carbin.
A short-term answer is that the nation's heartland is stuck in a tornado rut with usually temporary weather conditions that can lead to tornadoes parked over the Plains, said Adam Houston, a professor of meteorology at the University of Nebraska-Lincoln.
"You get day after day of severe weather and day after day of tornadoes until the pattern changes," Houston said.