Supercell evolution in environments with unusual hodographs

The events that transpired across northern Arizona during the afternoon of 14 August 2003 present an unusual severe weather episode. Although it was in the middle of the summer North American monsoon, typically dominated by high pressure over Mexico and the southwest United States, a continental low pressure system approached from the east and moved westward across Arizona.

The combination of enhanced deep layer shear associated with the cyclone and the ever-present deep moisture and instability associated with the monsoon produced widespread strong thunderstorms and a few supercells. Reports of damaging large hail and funnel clouds were received at the Flagstaff National Weather Service office during the day. Radar data indicated the possibility of at least one tornadic supercell although confirming reports were not available.

An examination of the winds aloft and the shape of the hodograph indicated that the hodograph shape retained the anticyclonic (i.e., clockwise) curvature typically associated with midwestern severe weather hodographs. On the other hand, the hodograph was rotated approximately 180° from the canonical hodographs. The mean wind and deep layer shear associated with this profile resulted in storms moving towards the southwest with right moving supercells attaining a more westerly heading, and left movers headed towards the south. Hook echoes associated with these storms were generally noted to reside on the northwestern flanks.

Our main focus in this study is to examine the vertical structure of the wind field through the use of hodographs and to make some inferences on the development of supercellular convection under these unusual wind regimes.