The Cool-Season Tornadoes of California and Southern Australia

Abstract

Examples of cool-season tornadic thunderstorms in California and southern Australia are examined. Almost one-half of the reported Australian tornadoes and the majority of those in California occur in the cool season. It is shown that in both areas the typical synoptic pattern shows an active midlatitude trough just upstream, with a strong jet streak aloft. In both areas the tornadic thunderstorms occur with weak to moderate levels of thermodynamic instability in the lower troposphere but with extremely high values of low-level positive and bulk shear. Statistical tests on null cases (nontornadic thunderstorms) in the Central Valley of California indicate that large values of 0?1-km shear provide a discriminator for more damaging (F1?F3) tornadoes, whereas bulk measures of buoyancy, such as CAPE, do not. Australian case studies and tornado proximity soundings show similar characteristics. A ?cool-season tornadic thunderstorm potential? diagnostic for Australian conditions, based on regional NWP analyses and forecasts, is described. It identifies those locations at which negative 700-hPa surface lifted index, near-surface convergence, and surface?850 hPa shear >11 m s?1 are forecast to occur simultaneously, and it shows considerable potential as an objective alert for forecasters. During the winter of 1996, all nine occasions on which tornadoes were reported were successfully identified in 24-h forecasts. After a variety of assessments suggested the value of this diagnostic, and following positive forecaster feedback during preoperational trials, it became an operational forecast product in May of 2000.