A synoptic climatology of tornadoes in Virginia

Although tornadoes are microscale meteorological features, their occurrence is linked to the larger synoptic-scale environment. Most conceptual models of synoptic situations that give rise to tornadoes are based on conditions prevalent in the Great Plains of the United States. However, tornadoes develop in many different synoptic environments, particularly when they form beyond the boundaries of "Tornado Alley." Tornado occurrences in the state of Virginia are identified from 1951 through 1992. Using upper air data for the nearest site and observation time, a synoptic classification is produced via application of principal components analysis and cluster analysis. The five resulting synoptic classes each exhibit distinctive thermodynamic and flow conditions that serve as air-mass paradigms for tornado occurrence in Virginia. The groups include thermodynamic/northwest flow air masses, a mixed thermodynamic/dynamic category, a polar front jet situation, cool season midlatitude cyclone events, and Great Plains type tornadic air masses. A sixth type, tornadoes spawned by tropical systems, is defined separately. A variety of indices have been developed by forecasters to provide information about the likelihood of severe weather and tornadic activity. The lifted index, K-index, Showalter index, SWEAT, Totals index, and a wind shear index are calculated for each tornado event. Analysis of variance indicates that most of these indices are statistically different between synoptic classes. These results suggest that the synoptic-scale mechanisms responsible for tornado occurrence in the mid-Atlantic states are quite variable. Enhanced predictive capability could be gained by considering the prevailing synoptic conditions associated with individual storms.