NAPERVILLE, Ill. (CBS) — Even before the first responders’ pleas for help Sunday night, the forecasters at the National Weather Service Office already knew the storm storm was going to be trouble.

“On our radar signature you can see evidence of debris getting lofted into the sky. When you see that it is very sobering. We hope everyone is going to be OK,” said Eric Lenning, meteorologist in charge at the National Weather Service Chicago.

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Doppler radar identified the storm as a tornado well before the survey crew even arrived there. Monday it was a question of just how strong the storm was.

There are at least 28 indicators a surveyor can look for, and one of them is tree damage. Many of the yards and streets of Naperville are littered with either entire trees or large branches. The way they are laid out is one of the things Lenning looked at to determine just how strong a twister it was.

“So you can tell the damage is maybe convergent,” he said. “The path is going maybe in one direction, but the path all the damage is maybe deposited across the path rather than just exploding out with a burst of wind if it’s a circulation you maybe are going to see the damage piled on each other or maybe thrown back upstream along the path.”

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Lenning focused on some of the worst damage in Naperville where the storm left almost nothing but a foundation at one house. This type of damage can quickly raise the rating of a tornado to a more destructive level. Sometimes the damage is very sporadic.

“Like intense damage right next to something that maybe is nothing at all. So you just try understand how the atmosphere can produce something like that,” he said. “Sometimes you don’t know exactly. Maybe debris caused the garage to fail and the wind got inside and was able to destroy the structure while another house the garage was fine and maybe wind never got inside.”

Doppler radar Sunday night, eyewitness accounts and other clues collected from Monday’s survey of damage has lead Lenning to conclude this was a EF-3 tornado on the Enhanced Fujita Scale, named after the late Dr. Theodore Fujita from the University of Chicago. That’s a level 3 out of possible 5 levels. Winds were somewhere between 136 and 165 miles per hour.

Sunday night’s storm reminds us how quickly a quiet severe weather season can turn dangerous.

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This was the first significant tornado — that ‘s a tornado with an EF2 rating or higher — to impact this area since 2015 when an EF-3 tornado touched down in Coal City.

CBS 2 Chicago Staff