A synoptic-scale environment associated with significant nocturnal tornado events in the Great Plains

A synoptic pattern favoring the development of significant nighttime tornadoes in the Great Plains is identified from an examination of 16 springtime cases from 1999-2009. This pattern is characterized by a well-defined midlevel trough west of the Rocky Mountains with prevailing southwesterly winds downstream over the central U.S. In the low levels, the underlying topography of the region contributes to the development or deepening of a lee cyclone over the High Plains and the establishment of a nocturnal low-level jet (LLJ). A critical component of this pattern is the early development of the LLJ in the 0100-0300 UTC timeframe, which is sooner than that documented by previous climatological studies of the plains LLJ. It is believed that the early onset of LLJ formation can be attributed to the augmentation of the diurnal oscillation by dynamical forcing associated with the large-scale pattern. In addition to locally destabilizing the environment through moisture and heat fluxes, the LLJ serves to enhance vertical wind shear, especially in the lowest kilometer above ground level. These processes can lead to an increased potential for significant tornadoes during the time of day when the threat is typically decreasing. In all of the documented cases, intensities of the nocturnal tornadoes rival or exceed that of any antecedent, daytime occurrence.

In order to determine the possible failure modes of this particular synoptic pattern, a similarly-sized set of non-tornadic cases was compiled and compared to the tornadic cases. Kinematic analyses derived from the Wind Profiler Network (WPN) and WSR-88D VWP data indicate that the LLJ response and vertical shear are quite similar in both subsets. However, thermodynamic properties derived from Rapid Update Cycle proximity soundings indicate notable differences, particularly in the lower troposphere thermal profile. Statistical analysis of several thermodynamic parameters indicates that the assessment of convective inhibition is of critical importance when attempting to discriminate between the tornadic and non-tornadic subsets. The 100 mb mean mixed-layer (ML) convective inhibition and the ML level of free convection were two thermodynamic fields which showed the strongest discrimination.