Environmental Characteristics Associated with Nocturnal Significant-Tornado Events in the Great Plains

Abstract

Nocturnal significant-tornado events are investigated in association with a similar synoptic pattern in the spring, over the central and southern Great Plains, from 1999–2009. This pattern is characterized by a well-defined midlevel trough over the Intermountain West, with prevailing southwesterly winds at 500 hPa over the central United States. The underlying topography of the region contributes to the development or deepening of a lee cyclone over the High Plains with the rapid intensification of a low-level jet stream (LLJS) observed around 0000–0300 UTC. The LLJS development is as much as 3–6 h sooner and driven by different dynamical processes than that documented with the nocturnal boundary-layer wind maximum (NBLWM). In the 15 documented tornado cases, ratings of the nocturnal tornadoes exceed that of any antecedent, daytime occurrence. To determine which aspects of the local environment are critical to nocturnal tornado development within the context of the identified synoptic pattern, a similar sample of 18 nontornadic cases was compiled during the same time period. Rapid Update Cycle-2 (RUC-2) soundings representative of the warm sector environment revealed that the most important differences between the tornadic and nontornadic cases involved low-level thermodynamic profiles. Comparison of several thermodynamic parameters indicates that low-level static stability is a strong discriminator between the tornadic and nontornadic cases, with the tornadic cases characterized by larger mixing ratios, smaller convective inhibition, and a lower level of free convection.