Severe storms cannot be resolved well by climate models because they occur at too fine of a scale and because of limitations in climate models. We therefore examine the changes in frequency of large-scale conditions conducive to severe storms with respect to time. First, using previously published results from Craven and Brooks (2004), the relationships between moderately high levels of convective available potential energy (CAPE J/kg) and vertical shear (m/s) and the occurrence of observed convective storms are reviewed. This review supports the use of high values of the product of CAPE and shear as a proxy for severe convective events.

Second, to gather information over a greater domain of space and time, CAPE and shear are calculated from the NCEP/ NCAR reanalysis data set, which provides data on an approximately 2-degree square grid, 4 times daily from 1958 through 1999. Focusing on the indicator, CAPE*shear, we examine the behavior of statistically modeled extreme values of the data using the spatial extension of the extRemes toolkit. Initial results are consistent with empirical extreme aluves, and show promise for analyzing trends in extremes of this large-scale indicator of smaller-scale severe weather activity. An extension of these analyses to the output of global and regional climate models may allow inferences to be made regarding the frequency of convective storms under future climate change scenarios.