Wednesday, 29 October 2008
Cool season severe weather is increasingly being viewed as a unique and significant winter characteristic of the southeastern United States. Winter tornadoes account for roughly half of all Southeast United States tornadic activity, and the majority of severe weather damage and loss of life can be attributed to tornadoes during the November through February. After reviewing the criteria necessary for thunderstorm development, especially severe activity, a comparison was made between the ingredients needed for severe summer convection and similar events in the winter. This study revealed some common characteristics of cool season tornadoes in the south. Cool season tornadoes are more likely to occur overnight than their warm season counterparts, and cool seasons that have strong 200 mb winds at lower latitudes than normal tend to be associated with more tornadoes. Cool season tornadoes tend to occur when high environmental storm-relative helicity combines with relatively modest instability. Traditional predictors of severe weather and tornadic activity, such as strong surface low pressure or a deep 500 mb trough, were not observed in many of the significant cool season tornado outbreaks in the Southeast in recent decades. Also studied was the nature of the convection which produced tornadoes in the cool season. A sample of cool season tornado events from 1994 to 2008 was selected and radar data were used to qualitative determine whether each tornado was produced from a supercell or a quasi-linear convective system. Results on the relative frequency of their intensity and storm type will be presented. Through analysis of case studies since 1980, a series of forecasting guidelines were created specifically for cool season tornadoes.
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