The historical database for the period 19502006 was examined and revealed 75 tornado days over northern Arizona. These events were stratified so that tornadoes during the warm season North American Monsoon were eliminated since the lack of baroclinity and deep-layer shear generally precludes the possibility of supercell-type thunderstorms; instead, non-mesocyclone, non-supercell storms are typical during this regime. The reduced dataset consisted of 36 tornado days.
Analysis of basic synoptic patterns was accomplished using the North American Regional Reanalysis (NARR) data. Large-scale patterns of geopotential height, deep-layer shear, and instability were assessed using this data. Additional analysis of the thermodynamic structure of the environment was accomplished using the North American RAOB database to more fully analyze shear and instability.
The results show that more than half of the tornado days occurred during the approach of a closed low from the eastern Pacific with northern Arizona located in the warm sector of the northeast quadrant of the low. The closed lows produced environments with deep-layer shear and low-level shear comparable to the 3rd and 4th quartiles of tornadic environments discussed by Rasmussen and Blanchard (1998) while instability was small and was comparable to or less than their 1st quartile. These interesting results suggest that shear may be the more important factor and that instability need only be sufficient to initiate and maintain convection long enough for the shear to act upon the updrafts.