Jet stream, wind shear contributed to extreme 2011 tornado

Fishel said that an axis of strong winds in the jet stream shifted right over North Carolina that day and the interaction of storms increased the updraft, which made thunderstorms stronger.

Low-level wind shear showed extreme values on radar images from that day, but Fishel said the instability in the atmosphere was not as extreme as it could have been.

“You don’t have to have both of them at extreme values. If only one of them is and everything else if favorable for thunderstorm and tornado development then it doesn’t have to be off the charts,” he said.