Forest damage associated with tornadoes in northern Arizona

In recent years there has been an increased effort to conduct damage surveys in the aftermath of tornadoes in northern Arizona. Most of these surveys have been in the forested regions of the Mogollon Rim which ranges in elevation from approximately 6500 to 8000 feet. Conifers and other evergreens are the dominant type of vegetation while the generally dry climate has resulted in a sparseness of deciduous trees.

Damage surveys of the primarily coniferous trees were assessed using the EF-scale damage indicators (DIs) and Degree of Damage (DOD) tables. It is interesting that DODs 1 and 2 were rarely encountered without also having DOD 3 and 4 present. Equally interesting is that DODs 3 and 4 were typically co-located in the same area and even side by side making it challenging to determine the likely wind speed causing the damage. As noted by Peterson (2003), patterns of tornado damage to trees in natural forests suggests that the existing tree damage metrics may be overly simplistic.

Of equal importance was the pattern of tree damage found in many of the surveys. Uprooted trees and snapped tree trunks almost always indicated an asymmetric damage pattern. This asymmetry has been discussed by Knupp (2000) and is most probably caused by weak vortex winds that have a large translational speed such that winds are significantly increased on one side of the vortex.

Here we examine and discuss the DIs and DODs for forest damage as well as the difficulties encountered in obtaining detailed information on tornado damage paths in forested and mountainous terrain.