INVESTIGATION OF UPPER-AIR CONDITIONS OCCURRING WITH WARM-SEASON SEVERE WIND EVENTS IN UTAH

Analyses of proximity soundings and upper-air fields for 37-51 Utah se vere wind cases (WIND), reported in the months of May-September and oc curring within 3 h after upper-air observation time, are presented. In addition, a comparison of sample mean values between the WIND cases a nd a climatological sample (CLIM) is made using a standard t test to d etermine which variables are significantly different between the two s amples. This study seeks to determine if the synoptic-scale-derived fi elds 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: 1) more convergent wind in the lower tropospher e (700-mb moisture and wind convergence), 2) greater moisture at 500 m b (dewpoint, mixing ratio), 3) greater positive vorticity advection (5 00 mb) and differential vorticity advection (700-500 mb), 4) a larger lapse rate based on various stability indices, 5) more southerly compo nent flow at levels from 500 to 200 mb, 6) higher absolute vorticity a t levels from 300 to 200 mb, 7) greater 500-mb wind speeds, and 8) lar ger thermal advection (warm) at 200 mb. Taken together, the statistica l 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 l ift. increased thermal instability, and increased midlevel moisture. T hese changes to the environment, when added to the normally dry, well- mixed, neutrally stratified boundary layer of the afternoon-evening ho urs, likely promotes high-based convection with severe downbursts at t imes. Discriminating effects on the subsynoptic scale cannot be determ ined in this study since only the standard upper-air station network o f observations is employed and no surface data is used. Sample mean di fferences are small and intrasample variability is large, so results m ust be used with considerable caution in forecasting applications.