Environmental and Synoptic Conditions Associated with Cool Season Strong and Violent Tornadoes in the North Central United States

This study is an examination of synoptic conditions associated with significant tornadoes (F2-F5) over the northern and central plains, the middle and upper Mississippi River Valley, the lower Ohio River valley, and the western Great Lakes during the cool season. A climatology of 61 tornadoes that occurred on 22 days from November 15th through February 28th during the 27 year period covering 1979-2005 is presented. A previous analysis indicated significant tornadoes encompassed a larger percentage of the cool season data set compared to the year as a whole. A closer examination of the cool season subset shows the transition from autumn to winter (the last half of November and December) is the most active time for cool season tornadoes with no favored time of day.

Surface and upper air analyses of these days are compared in an effort to find common synoptic patterns associated with such tornadoes. These tornadoes tend to occur in strongly dynamic patterns with nearby upper level jet streaks associated with progressive shortwave troughs. Each case also possessed a sharply baroclinic boundary moving into anomalously high low level moisture being transported northward by a low level jet. Data from observed proximity soundings and the North American Regional Reanalysis (NARR) are stratified to determine how background atmospheric conditions compare to accepted kinematic and thermodynamic parameters. Our study determined these soundings tend to possess weak thermodynamic profiles (Most Unstable CAPEs < 1,000 J/kg). However, the wind fields associated with these systems lead to significant kinematic parameters (0-6 km shear >20 m s-1, 0-3km Storm Relative Environmental Helicity > 250 m2 s-2).