A Comparison of Tornadic and False Alarm Convective Cells in the Outer Rainbands of Hurricane Harvey

Tropical cyclones routinely impact the United States, often posing numerous threats to society. Hurricane Harvey is infamously known for the catastrophic flooding it caused in southeastern Texas. Aside from the flooding, however, land-falling tropical cyclones such as Harvey are often capable of producing tornadoes. Forecasting and communicating the dangers of tornadoes to the public is often a difficult task for forecasters, given the multiple, often more pressing, hazards associated with landfalling tropical cyclones. Harvey was particularly challenging in this respect, as 89% of tornado warnings issued by the Houston-Galveston National Weather Service Office (HGX) were false alarms, and several tornadoes that did occur within the HGX County Warning Area (CWA) were unwarned.

This preliminary study examines 38 convective cells in the outer rain-bands of Hurricane Harvey in the HGX CWA – 19 false alarm storms and 19 tornadic storms – by comparing their near-storm environments. Relevant thermodynamic and kinematic parameters including Convective Available Potential Energy (CAPE), Storm Relative Helicity (SRH), and other widely used tornadic forecast parameters are compared, with the expectation that tornadic storms will have environments more favorable for tornadoes than false alarms. Case studies are also included to demonstrate the environments and radar presentation of examples of false alarm cells and tornadic cells that confirm the hypothesis and examples that do not.