Two tornadic thunderstorms in ostensibly weak deep layer shear environments in southeastern Colorado:  cyclic supercells of  May 25 (Kiowa County) and May 31 (Baca County) 2010

 

           

I.  Introduction

 

            On May 25 and May 31, 2010, outbreaks of cyclic tornadic supercells occurred in Kiowa and Baca Counties, respectively, in southeastern Colorado. While the occurrence of tornadic supercells in this portion of the country is not unusual climatologically, what ultimately led to our interest in documenting these events centered on the unusual kinematics in the mesoscale environment near initiation time.

            Neither storm developed in a shear environment that conventional wisdom suggests could support long lived cyclic supercells.  While the formation of rotating storms was anticipated,  the weak deep layer shear environments (exemplified by winds initially 10 m s-1 or less at 500 mb) suggested that the rotating phase of such storms would be brief because precipitation would overwhelm the updraft areas before storms could take advantage of the low-level shear environment. 

            Strongest flow in the middle and upper troposphere extended, generally, across the western portions of Colorado, into eastern Wyoming and then northeastward, with only a fringe of 12 to 15 m s-1 winds impinging on the western High Plains of Colorado,  extreme western Kansas and the northwestern Panhandles in both cases. In both cases, initial hodographs showed that ground-level winds at the top of the 0-3 km layer were nearly calm or negligible, despite enormous clockwise loops in the hodographs.  Initial estimates of the ground level shear environment were anemic.  Because of this, the Storms Prediction Center (SPC) had indicated both areas in a 2% risk zone for tornadoes, the lowest probabilistic category for tornadoes that SPC indicates.

            Yet the Kiowa County storm of 25 May produced at least three supercell tornadoes as it moved into western Kansas northwest of Tribune.  The Baca County storm of 31 May produced two supercell tornadoes near Pritchett and eventually moved into the Oklahoma Panhandle and produced at least two additional major long-lived and long track tornadoes.

            The authors noted separately some other unusual aspects of the storm development on these two days.   The primary author, Monteverdi, documented the Kiowa County storms and noted that initial storm motions deviated almost 100 degrees from the anticipated storm motions computed using classical hodograph analysis techniques (i.e., Bunkers technique).  Umscheid and Bookbinder documented the entire life cycle of the Baca County storms and also noted that storm motions were nearly 90 degrees from the anticipated storm motions.  It was these observations that led us (a) to consider the hypothesis that the unusual storm motions created storm-relative wind and shear profiles favorable for long-lived tornadic supercells; and (b) to realize that these cases were united not only by similar kinematic environments, but also by the probable fact that the unusual storm motions transformed the risk from minimal to significant that tornadic storms would occur, despite conventional wisdom.

             Initially, the authors were separately considering researching the storm they observed, but eventually realized that both events shared some very unusual, but common, characteristics.  So, we decided to combine our studies into one, stressing the common aspects (but noting the differences).

            The authors view this conference presentation as a pilot study.  In this manuscript and poster, we seek merely to examine the evidence that our hypothesis has basis, and that there is enough justification to seek a formal publication in the future.  Hence, the purpose of this study is to provide a summary of the synoptic and mesoscale environment within which these storms formed.  In particular, we seek to examine the thermodynamic and wind shear environment that helped foster initially unusual storm motions that transformed this event to one supportive of briefly rotating, nearly pulse thunderstorms to one that should have been anticipated (in hindsight) to support tornadic supercells.