The deadliest and most destructive tornado outbreak in U.S. history occurred 11 years ago today. What were the parameters that lead to such a generational severe weather outbreak?

All of the ingredients for a major severe weather outbreak came together on April 27, 2011. In the upper levels of the atmosphere, there was a negatively tilted trough, jet streak and wind shear. The negatively- tilted trough allowed the jet stream to dig over the Southeast, allowing for fast-moving storms. Wind shear, or wind that changes speed and/or direction with height, allowed for multiple rotating storms.

At the surface, there was ample warm, humid and very unstable air that lead to the development of very strong storms. This setup led to multiple rounds of storms across our area. The first round was early in the morning, another came in the middle of the day and a final round of supercells came through in the afternoon and evening.

Seeing all of these parameters coming together days out, the system was well-forecasted. The Storm Prediction Center (SPC) had forecasted severe weather over Alabama seven days ahead of time. By the day of the event, forecasters at SPC were forecasting a major severe weather event across the South, with a high risk issued over Alabama.