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The National Weather Service estimated that the storm that struck Moore, Okla., on Monday had wind speeds of up to 200 mph, and was at least a half-mile wide. The 1999 storm had winds clocked at 300 mph, according to the weather service website, and it destroyed or damaged more than 8,000 homes, killing at least two people.
Kelsey Angle, a weather service meteorologist in Kansas City, Mo., said it's unusual for two such powerful tornadoes to track roughly the same path. The 1999 twister was part of a two-day outbreak sweeping mostly across central Oklahoma - similar to the past two days.
The weather service has tentatively classified the Moore twister's wind speeds as an EF4 on a 5-point scale. Angle said less than 1 percent of all tornadoes reach EF4 or EF5.
The thunderstorm developed in an area where warm moist air rose into cooler air. Winds in the area caused the storm to rotate, and that rotation promoted the development of a tornado. The most destructive and deadly tornadoes develop from rotating thunderstorms.
The biggest known tornado was nearly 2 1/2 miles wide at its peak width, which the weather service describes as near the maximum size for a tornado. It struck Hallam, Neb., in May 2004.
The deadliest tornado, which struck March 18, 1925, killed 695 people in Missouri, Illinois and Indiana.
Deaths from twisters have been declining in recent years because of improved forecasts and increased awareness by people living in tornado-prone areas, especially in smaller and rural communities.
Those trying to contact loved ones in the disaster zone may want to visit the Red Cross' "Safe and Well" database online.
You can access that on RedCross.org where you can also make a donation for the relief effort or by calling 1-800-RED-CROSS.