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Originally published Friday, April 29, 2011 at 5:41 PM

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Scientists investigate twisters as crime-scene detectives

Weather scientists are retracing the footprints of this week's monstrous tornadoes the way detectives would investigate a crime scene: talking to witnesses, watching surveillance video and taking the measurements of trees ripped from the ground.

The Associated Press

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TUSCALOOSA, Ala. — Weather scientists are retracing the footprints of this week's monstrous tornadoes the way detectives would investigate a crime scene: talking to witnesses, watching surveillance video and taking the measurements of trees ripped from the ground.

The result will be a meteorological autopsy report on the disaster, revealing how many twisters developed and how powerful they were.

"First priority is finding the dead and taking care of the injured and getting utilities back up," said John Snow, dean emeritus of the College of Atmospheric and Geographic Sciences at University of Oklahoma. "But in parallel to that, we want to get as much data and find as much data as we can."

Researchers have to be on the scene fast — usually within days — to keep the evidence as fresh as possible, Snow said.

In one of its first official assessments of the tornadoes' strength, the National Weather Service on Friday gave the worst possible rating, EF-5, to one that raked Smithville, Miss. That tornado — a 205 mph monster that left at least 13 people dead — was expected to be joined by "many more" of Wednesday's tornadoes that will receive the same rating, meteorologist Jim LaDue said.

With at least 328 confirmed dead throughout the region, Wednesday's outbreak surpassed a series of tornadoes in 1974 to become the deadliest day for twisters since 1932.

As they survey damage from the ground and air, researchers from the weather service and the national Storm Prediction Center are asking about the buildings that were destroyed. Were they brick or wood, or a combination? Were they secured to a slab or set on concrete blocks? What type of roofs did they have?

Answers will help explain how the strong the twisters were. For example, a mobile home will be demolished by winds of 110 to 135 mph. But a well-built home can withstand much stronger winds.

Scientists might ask families if they left the garage door open. An open door lets wind inside, where it can push on walls and the roof with tremendous pressure.

Walt Zaleski, a warning-coordination meteorologist for the weather service's southern regional headquarters in Fort Worth, Texas, likened a roof with a large overhang to a baseball cap with a brim: Wind blowing in your face will press on the brim and lift the hat off. The same can happen with a house.

As of Friday, weather-service teams had not found any tornado paths that were rated less than an EF-3, with winds of 140 to 150 mph.

Jim Stefkovich, the meteorologist in charge of the agency's Birmingham office, said he believes there are pockets of greater damage yet to be examined.

Zaleski compared the investigation of the storms to assembling a million-piece puzzle.

"It's very complex," said Zaleski, who has been participating in such analyses since the 1970s. "We will try to reconstruct and determine the intensity of the tornadoes," along with their width, path and other details.

Assessing damage becomes more complicated as investigators move along the track of a tornado. Once structures start to break apart, the wind collects debris, "and you have a moving grinder that impacts all downstream structures," he said.

Sometimes one tornado follows into areas where an earlier twister has passed, making it hard to determine which damage was caused by which tornado.

In addition, a large disaster tends to produce duplicate reports of the same twisters, which can be further complicated by tornadoes with multiple funnels.

People associate the most severe damage with tornadoes, but thunderstorms can generate two kinds of damaging winds, the straight-line downburst and the more sensational twisting tornado, Zaleski said. A downburst often will cause the same damage as a tornado, he said, with winds of 100 to 120 mph.

The picture that emerges will help forecasters better understand how killer systems develop. The final report on the disaster will become a part of the National Climate Database, a vast record of the nation's most severe weather.

"To have an event of this magnitude with a modern, integrated observing system like we have now is unique in the history of meteorology for a tornado forecaster," said Russell Schneider, director of the Storm Prediction Center in Norman, Okla., which is run by the National Oceanic and Atmospheric Administration.

Snow said researchers would be studying this storm for a long time. Scientists studied the 1974 disaster for 15 years. "More than one Ph.D. thesis will come out of this,"he said.

Associated Press writers Randolph E. Schmid in Washington, D.C., and John Christoffersen in Birmingham, Ala., contributed to this report.

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