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 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 – 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.

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. Cooler air at high altitudes and warmer moist air coming from the Gulf of Mexico are combining and settling over the region.

“You get day after day of severe weather and day after day of tornadoes until the pattern changes,” Houston said.

But why that happens, Houston doesn’t know. While scientists can forecast hurricane seasons, predicting their land-bound cousins is much harder, Brooks said. While tornadoes, like hurricanes, rely on large-scale weather phenomena, the crucial triggers are extremely local weather conditions.

On top of that, tornadoes have a “Goldilocks” issue. To make a tornado, the conditions have to be just right. Too much or too little of one ingredient and there is no tornado. For example, wind shear – when upper and lower winds are at different speeds or coming from different directions – is crucial to create a funnel cloud. Too little and there is no spin. Too much and the tornado falls apart.

And tornadoes form most often in late afternoon, between 5 and 9 p.m., so if a thunderstorm starts up early in the morning, it’s far less likely to throw off a tornado, Brooks said.

As for why so many people are getting killed, Brooks suggests thinking of the landscape as a dartboard: “We’re throwing more darts and throwing bigger darts than normal.”

More people are living in mobile homes in the past few decades, and that has shown up in tornado fatality statistics. In 1970, about one-quarter of all tornado deaths were in mobile homes; now it’s about half, Brooks said. In 1970, Census data showed that 3 percent of the U.S. population lived in mobile homes; now it is 7.6 percent, with a higher rate in the Southeast and other parts of Tornado Alley, such as Oklahoma, Brooks said.

But as deadly as this year has been, it used to be far worse in the United States. In 1925, tornadoes killed 794 people. From 1916 to 1936, tornadoes killed an average of nearly 280 Americans a year. That’s because tornado warnings were not as good, people couldn’t hear them and housing was not as sturdy, Brooks said.

Even with a busy tornado year, meteorologists are getting the word out. Of the 110 deaths so far this year, 101 came while there was a tornado watch in effect, according to the National Weather Service.