A tropical cyclone tornado parameter for use in situational awareness forecasting

This study introduces a new sounding-based composite parameter, called the Tropical Cyclone Tornado Parameter (TCTP), which identifies regions within the tropical cyclone environment most conducive to supercell formation and tornadogenesis. The parameter is analogous to the mid-latitude Supercell Composite Parameter (SCP) and the Significant Tornado Parameter (STP) often used by forecasters to enhance their situational awareness of severe weather potential and threats. The study uses a database of over 5000 rawinsondes launched within 800 km of all U.S. landfalling hurricanes between 1997 and 2008. First, a storm-relative climatology of sounding parameters traditionally used to forecast mid-latitude supercells and tornadoes was completed for the onshore TC environments, and compared to previous studies. Next, a subset of “proximity soundings” released within 185 km and 3 h of a reported tornado were examined to identify significant differences in instability and vertical shear from the non-tornadic soundings assuming that most tornadoes are spawned by miniature supercells. The hurricane tornado environment is most often characterized by non-zero instability, large boundary layer moisture, strong low-level vertical shear, and large near-surface cell-relative helicity. The most significant parameters are combined into a new TCTP, which is shown to outperform the mid-latitude SCP and STP in the hurricane environment when employing the developmental database. Advantages of using the TCTP during landfalling hurricane events as well as our ongoing efforts for independent testing will be discussed at the conference.