A cool season severe weather episode in northern Arizona

Severe weather events typically occur in Arizona during the “transition season” between the moist, tropical monsoon environment of summer and the first incursions of baroclinic systems in September. The combination of copious tropical moisture combined with increased instability and deep-layer shear is responsible for the majority of severe weather across the state.

Recently, a late-season severe weather episode occured in the fall in mid October—well after the transition season had ended. During the early morning hours and continuing well into the afternoon of 18 October 2005, long-lived severe supercells were noted across northern Arizona. Damage surveys conducted over the following days indicated that at least two tornadoes, as well as large hail, occurred across northern Arizona.

A deep closed low approached the southwest United States in the days proceeding the severe weather episode. With the approach of the low, deep layer shear increased significantly. Buoyant instability, as measured by the Lifted Index and CAPE, indicated that the atmosphere was only marginally unstable.

In this paper, we examine the background environmental conditions that supported the development of long lived supercells and, additionally, how the local environment may have augmented necessary ingredients for the development of supercells and severe weather.