The outbreak from April 25-28 was very abnormal and not a regular occurrence, the tornadoes were stronger and lasted for much longer than usual. The environment was very unusual, especially on April 27. In a journal by Doswell, Carbin, and Brooks (2012), “There must be at least one other ingredient, in addition to those already listed, and research reveals that tornadic supercells are favored when both wind direction and wind speed change with height – a condition referred to as vertical wind shear”. That is the primary driver for the supercell that caused the tornadoes in the outbreak. Also, from the same article by Doswell et al. (2012) The other ingredients needed for a storm is moisture content, instability, and lift vary the strengths of the storm. These storms were very efficient at producing tornadoes, this comes from a journal by Knupp et al. (2014). “The parent storms, including supercell storms and strong convective elements within QLCSs [Quasi-Linear Convective System], were efficient in producing tornadoes. For example, about 90% of the supercell storms within the outbreak region produced at least one tornado”. There are also other possible elements that could’ve affected the intensity, especially on April 27th, but aren’t fully clear, from Knupp et al. (2014) “External influences, including a thermal boundary, possible gravity waves, and topography, appeared to play a role in tornado genesis and tornado intensity change.” What we do know is that the tornadoes where exponentially stronger than have been in the past, with 199 tornadoes on the 27th alone (Knupp et al., 2014). One may think that after such a strong series of tornadoes on one day that it would be over but on April 28th there was a reported 43 tornadoes (Knupp et al., 2014). This can be explained best from the journal by Doswell et al. (2012) As the morning thunderstorms moved east and weakened, clearing skies across Mississippi and Alabama contributed to the development of strongly unstable conditions, given the abundant moisture and very steep lapse rates already present. As a very strong jet stream and intense vertical wind shear spread eastwards through the unstable air during the day, supercell thunderstorms redeveloped. The conditions that led to the series of tornadoes were a recipe for disaster.
Tornado paths on 27 April 2011, color coded based on what scale rating. Along with the most deaths being in the colored counties. (Knupp et al. 2014)
Schematic showing how ingredients come together: Blue arrows indicate upper jet streams, red arrow is low-level jet stream, green is low level moisture, while orange shading is low region of unstable lapse rates, with the fronts being noted as dotted lines, brown line is dryline, black dashed region is location of outbreak potential. (Doswell et al. 2012)
Time series of tornadoes forming and title of what caused it, along withtheir EF rating per 30-minute time interval on April 27, 2011. (Knupp et al. 2014)