A Rare Tornadic Thunderstorm in Northeast Utah

On 3 September 1999 an isolated severe thunderstorm tracked across the Uinta Basin of northeast Utah. The storm produced hail in excess of one inch in diameter and spawned at least one tornado that traveled four miles. One injury was reported and property damage was consistent with that described as F1 scale. The thunderstorm formed in a mesoscale environment characterized by moderate instability and speed shear of 30 ms-1. Synoptic scale lift was provided by an approaching shortwave and the left exit region of a 250 hPa jet. The WSR-88D radar tasked with surveying this region resides on Grand Mesa in western Colorado at an elevation of 3050 m MSL. The severe storm was over 185 km from the radar site, and consequently only two beam slices intersected the storm; the 0.5 and 1.5 slices. The lack of coverage, in both the x and z planes, is a major disadvantage facing operational forecasters in the western United States. The highly variable topography makes it difficult to view storms at increased distances from the radar, because of beam blockage and overshooting. A brief overview of the storm environment will precede a detailed discussion of radar performance and coverage, the latter of which is inherently problematic over complex terrain.