The Significant Tornado Parameter (STP) and Non-Supercell Tornado Composite (NSTC) as Tools to Illustrate Areas of Greatest Tornado Potential Associated with 2017 Hurricanes Harvey, Irma and Nate.

In addition to other types of impact, including wind damage, storm surge, and inland flooding from excessive rainfall, hurricanes Harvey, Irma, and Nate produced numerous tornadoes over northeast Texas, northeast Florida, and portions of Alabama and the Carolinas, respectively, in the northeast quadrant of the tropical cyclone in all cases.

An interesting new parameter to forecast tornadoes, including strong tornadoes, is the Significant Tornado Parameter (STP), a composite index which highlights where sufficient moisture, instability and vertical wind shear overlap to produce the greatest tornado threat, as given by the equation below.

STP = (sbCAPE/1500 J kg^-1) * ((2000-sbLCL)/1000 m) * (SRH1/150 m² s^-2) * (6BWD/20 m s^-1)

The STP incorporates:

• Instability, through Convective Available Potential Energy (CAPE);
• Moisture, through Lifted Condensation Level Height (LCL); and
• Vertical Wind Shear, through storm-relative helicity (SRH) and bulk wind difference (BWD).

An interesting composite field, which includes 0-3km CAPE and near-surface vorticity (referred to here as Non-Supercell Tornado Composite, or NSTC), is a beneficial one for anticipating tornadoes from non-supercell convection, in which the tornado spins up from below, rather than from a pre-existing deep mesocyclone. The NSTC generally works best for predicting waterspouts or landspouts, and potentially tornadoes from quasi-linear convective systems (QLCSs).

This project illustrates how STP, NSTC and related fields could be utilized to highlight areas and time periods of greatest tornado potential for tropical cyclone landfall events.