RUC Soundings with Cool Season Tornadoes in "Small" CAPE Settings and the 6 November 2005 Evansville, Indiana Tornado

Several studies of tornadic supercell environments have confirmed that significant tornadoes (F2+ intensity) can occur with relatively “small” mixed-layer convective available potential energy (e.g., CAPE < 1000 J kg-1), particularly during the cool season from November through April. Recently, the nighttime tornado at Evansville, Indiana on 6 November 2005 was an example of a deadly cool season event in a relatively “small” CAPE environment. Because smaller amounts of CAPE are often involved, cool season environments supporting significant tornadoes can appear less “ominous” than tornadic warm season environments where CAPE is typically 2 to 4 times larger.

This study will examine parameters and characteristics from Rapid Update Cycle (RUC) analysis soundings collected during 2001-2005 associated with supercells in environments having less than 1000 J kg-1 of mixed-layer CAPE (“small” CAPE cases). Of 1083 RUC supercell profiles not linked to tropical cyclones or cold-core 500 mb lows, 322 had CAPE less than 1000 J kg-1, including 130 tornadic cases. Thirty of these were significant tornadoes, and all but 3 occurred from November through April, including 15 cases that caused deaths.

Comparing tornadic with nontornadic profiles, it was found that storms producing significant tornadoes in these “small” CAPE settings tended to have greater amounts of 0-1 km storm-relative helicity (SRH), similar to results from prior empirical studies. New information gathered suggests that these tornadic storms were associated with larger low-level (0-3 km) CAPE and significantly lower level of free convection (LFC) heights, even though lifting condensation level (LCL) heights were rather uniform and did little to distinguish between tornadic and nontornadic storms. This indicates that a strongly surface-based environment without a stable layer to inhibit rising parcels is important in “small” CAPE settings that produce significant tornadoes. It may also be important for SRH and CAPE to be co-located within low-levels in tornadic "small" CAPE environments.

The above results will be presented and discussed. In addition, the evolution of environment prior to the 6 November 2005 Evansville tornado will be shown using Storm Prediction Center (SPC) mesoanalysis graphics.