Winter Tornado Outbreaks

Abstract

Winter tornado outbreaks comprise 99% of all major outbreaks since 1950. In four out of five cases, they are accompanied by widespread blizzard, heavy snow conditions, and/or extensive glazing on the cold side of the responsible weather system. Two mean storm tracks for these outbreaks have been established but one predominates. This track has its origin over the southern plains states and moves northeastward to the upper Great Lakes region. Owing to the location of its origin, it is associated with a stronger baroclinic zone than the typical winter storm of the Plains states, since the 500 mb trough is typically farther south and the southerly low-level (850 mb) air flow out of the Gulf of Mexico is significantly more moist. A comparable number of heavy snow/ice storm occurrences without major tornado outbreaks was investigated and it appears that the intensity of the baroclinic zone and the location of the low-level features with respect to the Gulf moisture are the determining factors in differentiating between outbreak and non-outbreak situations. While the intensity of winter outbreak tornadoes is comparable to spring outbreak tornadoes, long-track winter outbreak tornadoes account for a greater percentage of deaths than the long-track spring outbreak tornadoes. Possible reasons for this are presented.