An American flag waves atop a home in ruins Tuesday in Parkersburg, Iowa. On Sunday, an extraordinarily powerful twister ripped apart Parkersburg, destroying 288 homes in the town of about 1,000 residents.

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 like someone who has lost all his worldly possessions to a whirlwind, 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.

Over the weekend, an extraordinarily powerful twister ripped apart Parkersburg, Iowa, destroying 288 homes in the town of about 1,000 residents, said Gov. Chet Culver. At least four people were killed there. Among the buildings destroyed were City Hall, the high school, and the lone grocery store and gas station. Some of those killed were in basements.

The brutal 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.

“Right now we’re on track to break all previous counts through the end of the year,” said warning meteorologist Greg Carbin at the Storm Prediction Center, also in Norman.

And it’s not just more storms. The strongest of those storms – those in the 136- to 200-mph range – have been more prevalent than normal, and lately they seem to be hitting populated areas more, he said. At least 22 tornadoes this year have been in the top part of the new Enhanced Fujita scale, rating a 3 (for “severe”) or a 4 (“devastating”) on the 1-to-5 scale.

The twister that devastated Parkersburg was a 5 – the first in the U.S. since a tornado nearly obliterated Greensburg, Kan., just over a year ago. The Parkersburg tornado was the strongest to hit Iowa in 32 years.

So far, more than 50 of the deaths this year have been in mobile homes, the wrong place to be during a tornado. They have been a factor in nearly half of all tornado fatalities in recent years.

And if that’s not bad enough, computer models show that the conditions that make tornadoes ripe are going to stick around Tornado Alley for about another week, according to Brooks.

The nagging question is why.

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 Niña, the cooling of parts of the Central Pacific that is the flip side El Niño, was a factor in the increased activity earlier this year – especially in February, a record month for tornado activity – but it can’t explain what is happening now, according to Carbin.

Carbin explained the most recent tornadoes with just one word: “May.” May is typically the busiest tornado month of the year.