Shear parameter thresholds for forecasting California tornadic thunderstorms

A study of 70 non-tornadic and tornadic thunderstorms in California disclosed that stratification of the stronger tornadic events (associated with F1/F2 tornadoes) on the basis of 0-1 km positive shear and 0-6 km positive shear magnitudes was justified statistically. Shear values for the weaker F0 events could not be distinguished statistically from the "background" values calculated for the non-tornadic (null) thunderstorm events observed during the period. An important ancillary conclusion was that buoyancy was low for each of the bins and that there was no statistically significant difference between the buoyancy observed for each of the bins; thus, buoyancy is not of operational significance in forecasting California tornadic thunderstorms. Shear magnitudes indicated that most (if not all) of the F1/F2 events were probably supercellular, while most (if not all) of the F0 events were non-supercellular. Finally, POD and FAR values calculated for various threshold values of shear magnitudes indicates that there may be justification for optimism in assessing tornado risk in California based upon a consideration of shear values obtained from the morning raob.