Environmental Comparison of Supercell and Squall Line Verified Tornado Warnings and False Alarms

The National Weather Service's (NWS) core mission is to protect life and property through the issuance of critical weather warnings. Assessments from recent high-impact events, such as the EF-5 Joplin, Missouri tornado suggested that the general public does not always respond to these potentially life-saving warnings due to complacency developed in part by a high false alarm rate (FAR). FAR can be especially high at times across the Ohio and Tennessee River Valleys where multi-mode, complex tornadic convection is common. As a result, this study examines tornadic near-storm environments in an effort to reduce NWS tornado warning FAR across the Ohio River Valley.

A large sample of tornado warnings dating back to 2010 was collected from the NWS offices in Louisville, Paducah, Indianapolis, Lincoln (IL), and Nashville, along with corresponding sectorized Storm Prediction Center (SPC) mesoanalysis data. If a tornado occurred within 50 miles and 2 hours of an issued warning, the environment was classified as tornado-producing. Using mosaicked Doppler radar base reflectivity data, each tornado-warned storm was then classified as either part of a quasi-linear convective system (QLCS) or as a discrete cell. Statistical analyses were performed on over a dozen thermodynamic and kinematic environmental parameters. The results of these tornadic versus non-tornadic environments, including detailed comparisons of cool versus warm season QLCS and supercell environments within the Ohio River Valley, are discussed. Preliminary findings reveal that low-level shear parameters, including 0-1km storm-relative helicity, 0-3km bulk shear and storm-relative helicity calculated over an effective inflow layer tend to be the best discriminators between tornadic and non-tornadic convection in the Ohio Valley. Instability parameters such as surface-based convective available potential energy (CAPE) and mean-layer CAPE were shown to be the worst discriminators. Newly created parameters, developed from unique combinations of basic kinematic and thermodynamic parameters, were statistically proven to have significant and at times superior abilities to discriminate tornadic events from non-tornadic as well.