A preliminary look at deep layer shear and middle level lapse rates associated with major tornado outbreaks

Experienced forecasters at the Storm Prediction Center have sometimes found that large values of deep layer shear and steep middle level lapse rates are present during tornado episodes, even if traditional parameters such as 0-3 km Storm Relative Helicity and/or CAPE are marginal. An attempt to identify favorable ranges and thresholds for surface to 6 km shear and 700-500 mb lapse rates during significant tornado outbreaks was undertaken. Approximately 75 cases were analyzed from the period 1950 to 1998. Cases were included if the Destruction Potential Index (DPI) was greater than 100, usually indicative of multiple long track strong and violent tornadoes. Initial results indicate that the majority of these outbreaks occur with surface to 6 km shear in excess of 22 m s^-1. 700-500 mb lapse rates were mainly greater than 6.8 ^oC km^-1.