The role of synoptic patterns and temperature and moisture distribution in determining the locations of strong and violent tornado episodes in the north central United States: a preliminary examination

Guerrero et al. (1998) have determined that most strong and violent tornadoes occurring within and near the Aberdeen SD NWS WFO county warning area (portions of central and eastern SD, southeastern ND, and western MN) occur to the north of the latitude of the associated surface low pressure center. Many of these appear to be associated with either an inverted trough extending to the north of the low center or to the north of a warm or quasistationary front extending eastward from the low center. The purpose of this study is to determine if these findings apply to the north central U.S. generally and to identify those meteorological conditions that lead to significant tornadoes occurring outside of the warm sector of a low pressure system.

For this study the area of interest has been expanded to include all of North Dakota, South Dakota, and Minnesota. STORM DATA has been screened for the period 1953 through 1998 to identify all tornado episodes in which one or more F3 or greater intensity tornadoes occurred. A total of 100 such episodes were found. The synoptic patterns and tornado locations for the North Dakota and South Dakota cases agree well with the findings of Guerrero et al. (1998). For Minnesota, the results were less focused. Examination of individual cases in which tornadoes occur north of the warm sector reveals that high values of low level moisture develop on the north side of the warm front (or outflow boundary). While cooler air exists north of the front, the air is still postively buoyant and the inversion north of the boundary is not strong enough to prevent surface based convection. The easterly component to the low level flow north of the warm sector appears to enhance low level helicity and storm inflow in this region. Typically, in such cases the air mass in the warm sector is either strongly capped and/or there is a large boundary layer temperature/dew point spread resulting in high LFCs. These conditions appear to be a function of the season. The seasonal changes in the character and distribution of temperature and moisture patterns are explored and the influence of these changes on tornado location is discussed.