WDAY: The News Leader

Published August 09, 2010, 09:03 AM

NWS: Determining the strength of a tornado

Tyler, ND (WDAY TV) - The National Weather Service has rated tornado damage in the southern valley from Saturday's tornado. Near Tyler, North Dakota damage was an EF-3. Across the river near Doran, Minnesota they estimated EF4 damage with winds up to 175 miles per hour. How do they determine the tornado's strength?

It's a science all its own: scouring storm ravaged farmsteads.

GREG GUST/NWS GRAND FORKS: "You can see the harvested grain crop and then you can see the dark path coming in here."

Greg Gust from the National Weather Service in Grand Forks is near Tyler, North Dakota, studying each section of damaged property to tell the severity of Saturday's tornado.

GUST: "It's not what the tornado looked like at a distance, how wide it was, or how narrow it was. No one's out there measuring the wind speed with an anemometer. They would be probably seriously injured if they tried."

REPORTER: The damage here is so bad, they brought in 20 year veteran National surveyor Todd Heitkamp. At EF3 damage or worse they have to. It's like a "second pair of eyes."

TODD HIETKAMP/NATIONAL NWS: "It's kind of like playing CSI or Sherlock Holmes, put the pieces together to really find out what failed first and then what then caused other failures to occur."

One telling piece of evidence: a home or building. NWS officials weigh the tiniest of details from what a structure is built on to what it's made out of, and then how damaged it is and where it ended up.

TODD: "Because it takes a lot of strength and force to bring that structure out of where it was."

They're not only looking at homes and buildings, but how the tornado effected and ripped apart trees.

GREG: "The tornado is hitting it so hard, it's not even uprooting it, and it’s just peeling it."

The tiniest of details, an important measure to help them not only gauge this storm, but learn for the next one. The man who invented the science of rating tornados, Dr. Ted Fujita, rated his first tornado here in Fargo in 1957.