The National Weather Service has received several inquiries about why a tornado warning was issued for Marion County and nearby parts of some surrounding counties on the evening of April 7.

A line of thunderstorms moved into the area ahead of an approaching strong cold front. Other scattered storms formed ahead of the line. As the line of storms moved into the Indianapolis area, the storms interacted with a boundary left over from some of the storms ahead of the main line. This produced rotation in the lowest levels of the atmosphere. Wind shear, the turning of winds with height, was present all evening. When radar indicated rotation in a marginally favorable environment, a tornado warning was issued.

Below are some radar images from near the time the warning was issued. Click any image to enlarge:

 

Reflectivity Image from Terminal Doppler Radar Velocity Image from Terminal Doppler Radar
This is a reflectivity image from the Terminal Doppler Radar, which is located in southern Hendricks county. The area of concern is inside the white box. Blue lines indicate interstates. This is a velocity image from the Terminal Doppler Radar. The area of concern is inside the white box. Blue lines indicate interstates. Green colors show winds blowing toward the radar, while red colors indicate winds blowing away from the radar. The green right next to the red shows rotation in the storm.

 

Storm Relative Velocity from the NWS Doppler Radar
This is a storm relative velocity image from the NWS Doppler Radar, located near the Indianapolis International Airport. The area of concern is inside the white box. Blue lines indicate interstates. Green colors show winds blowing toward the radar, while red colors indicate winds blowing away from the radar. The 4 different windows show the winds in the lowest 4 levels of the atmosphere that the radar looks at. The green right next to the red shows rotation in the storm, extending up through the lowest levels of the atmosphere.


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