Developing a Parameter for Forecasting Tornadoes in Landfalling Tropical Cyclones

Tornado reports are combined with hurricane data to generate a dataset of tornadoes associated with tropical cyclones affecting the coastal Gulf of Mexico and Florida's East Coast between 2000 – 2008. A pool of 28 potential predictors of tornado occurrence in tropical cyclone environments is narrowed to 7 using classic stepwise regression. The result is a composite parameter for forecasting the number of tornadoes per cyclone quadrant during a 12 h period. The equation is applied to five cases from the past nine years to qualitatively assess the parameter and compare its performance with two composite indices developed for classical severe storms (the Significant Tornado Parameter and the Energy Helicity Index). Results show that our Tropical Cyclone Tornado Parameter (TCTP) has skill at identifying regions of tornadic potential although its ability to quantify this potential remains uncertain. Tornadoes in some tropical systems appear to be over predicted, but under predicted in other systems. Comparisons with the classic severe parameters show that TCTP forecasts well, particularly for tropical cyclones making landfall as hurricanes. The shear and instability terms in our guidance product indicate that shear is larger in the northeast quadrant of tropical cyclones when tornadoes occur, compared to when they do not occur. Similar to the results of previous studies, a small negative correlation between instability and tornado occurrence is noted. TCTP is developed with the goal of aiding forecasters in a time-limited forecast environment. In this way, TCTP can provide a "quick look" at regions where a forecaster could then conduct detailed analyses.