Leading Stratiform, Trailing Mesocyclone - Implications for QLCS Tornadoes

On 14 April 2012, an EF2 tornado struck Creston, Iowa shortly before a tornado warning was issued, damaging the hospital, apartments and a community college. The tornado occurred at the trailing edge of an intense, small-scale bow echo convective system, and was preceded by and embedded in 15 minutes of torrential rain. This non-standard storm structure, and the seriousness of the tornado, triggered a search to find similar events and provide a summary of their radar characteristics and near-storm environment.

Most tornadoes associated with squall lines, bow echoes, comma heads and quasi-linear convective systems (QLCS) occur near the leading edge of the higher radar reflectivity. Observational evidence and two decades of research have most often focused on leading edge mesovortices. Linear cases exhibiting rotation and tornadoes on the trailing edge of the higher reflectivity appear to be much less common. Despite being rare, these tornadoes can be strong to violent, with some rated EF2 or greater. It is hypothesized that strong tornadoes are possible with trailing mesocyclone/trailing mesovortex linear convective systems due to the strong vertical wind shear with which they are associated. From a National Weather Service perspective it is also difficult to provide advance warnings, since warning meteorologists are not accustomed to seeing heavy rain immediately preceding rotation and most consider heavy rainfall detrimental to tornado development. In addition, storm spotters will have little chance of seeing a tornado if it is preceded by heavy rain.

This presentation will show the results of a developing study to find common characteristics of mesocyclone/mesovortex events where the tornado occurred at the trailing edge of the bow echo or QLCS reflectivity. Six similar cases will be compared to identify common characteristics of radar structure, including tornado location, origin of rotation (whether it be a mesovortex or supercellular mid-level mesocyclone) and whether the rotation “migrates” from the leading edge to the trailing edge as the system evolves. Common characteristics of proximity soundings from RUC model 00-hour analysis will also be presented, focusing on shear parameters such as line-normal 0-3 km shear, effective deep-layer shear, and low-level shear and helicity parameters shown to be relevant to tornado formation by other researchers. The goal of the study and presentation is to allow all meteorologists to become more familiar with the importance of these atypical systems, and to increase the warning meteorologist's ability to forecast and recognize systems configured in this manner. Understanding that more than a short-lived EF0-1 tornado could be in the offing should allow the National Weather Service and its partners to deliver appropriate decision support services and advanced warnings.