Investigation of Upper-Air Conditions Occurring with Warm Season Severe Wind Events in Utah

Abstract

Analyses of proximity soundings and upper-air fields for 37?51 Utah severe wind cases (WIND), reported in the months of May?September and occurring within 3 h after upper-air observation time, are presented. In addition, a comparison of sample mean values between the WIND cases and a climatological sample (CLIM) is made using a standard t test to determine which variables are significantly different between the two samples. This study seeks to determine if the synoptic-scale-derived fields play a significant role in producing severe wind for a region in which subsynoptic effects, attributed to uneven terrain, are important. The WIND sample environment had the following important differences when compared to CLIM: more convergent wind in the lower troposphere (700-mb moisture and wind convergence), greater moisture at 500 mb (dewpoint, mixing ratio), greater positive vorticity advection (500 mb) and differential vorticity advection (700?500 mb), a larger lapse rate based on various stability indices, more southerly component flow at levels from 500 to 200 mb, higher absolute vorticity at levels from 300 to 200 mb, greater 500-mb wind speeds, and larger thermal advection (warm) at 200 mb. Taken together, the statistical results combined with examination of individual cases and composite maps, suggest that severe wind events in Utah are commonly associated with an approaching upper-level trough system that provides enhanced lift, increased thermal instability, and increased midlevel moisture. These changes to the environment, when added to the normally dry, well-mixed, neutrally stratified boundary layer of the afternoon?evening hours, likely promotes high-based convection with severe downbursts at times. Discriminating effects on the subsynoptic scale cannot be determined in this study since only the standard upper-air station network of observations is employed and no surface data is used. Sample mean differences are small and intrasample variability is large, so results must be used with considerable caution in forecasting applications.