Preliminary evaluation of a parameter to forecast environments conducive to non-mesocyclone tornadogenesis

Tornadoes that occur prior to the formation of radar-detected mesocyclones present many problems to the operational forecast community. This type of tornadogenesis usually occurs within minutes of, or prior to, the first detection of radar reflectivity echo (Burgess and Donaldson 1979; Roberts and Wilson 1995). Often, radar signatures of the larger tornado cyclone are quite weak during tornado time, yielding little or negative lead time for warnings issued to the public. Because of this difficult sampling issue, higher situational awareness of the miso- to mesoscale environment conducive to this non-mesocyclone tornadogenesis (NMT) process can provide positive impacts to the warning mission by increasing information flow to the public on possible threats.

Research continues to increase on radar and environmental aspects of the NMT process which is mainly achieved through the stretching of ambient vertical vorticity. Modeling and observational studies suggest NMT typically occurs with convective updrafts in weak wind shear environments characterized by steep low-level lapse rates and strong low-level instability. Further, these updrafts generate along slow-moving or stationary surface boundaries possessing strong horizontal shears with misoscale vorticies (Wakimoto and Wilson 1989; Brady and Szoke 1989; Lee and Wilhelmson 1997, 2000; and Davies 2003).

NMT environmental diagnosis attempts have been made by Davies (2003) by parameterizing the higher low-level lapse rate and higher low-level convective instability (e.g., enhanced stretching potential) along such boundaries. In an effort to further increase situational awareness in the operational forecast environment, a parameter was designed in spring 2005 to build on work by Davies (2003) and others described above by incorporating a measure of deep shear (0-6km bulk shear), low-level convective instability (0-3km ML CAPE), low-level lapse rate (0-1km), convective inhibition (ML CIN), and surface relative vorticity. Surface relative vorticity was included as a measure of the ambient vertical vorticity available to convective updrafts along a surface boundary (e.g., as a proxy to misoscale vorticity presence).

A review of the parameter construct, its application through case examples, and behaviors of the parameter will be presented. Limitations of the parameter, such as false alarms, will also be clearly addressed. The preliminary datasets collected suggest the parameter can provide a positive contribution toward increased forecaster situational awareness in environments favorable for non-mesocyclone tornadogenesis.