Observations from VORTEX2: The intensification of the Goshen County, Wyoming tornado (05 June 2009)

The evolution the Goshen County, Wyoming, 5 June 2009 tornado during its intensification period is studied using integrated dual-Doppler and Rapid-Scan radar data as well as mobile mesonet thermodynamic observations, and particle size measurements from disdrometers. During the intensification period, 2202-2212 UTC, delta-V of the tornado is relatively constant between 2204 and 2208, then substantially increases between 2208 and 2212. Preliminary dual-Doppler analysis reveals a weakening of the secondary rear-flank downdraft (SRFD) between 2202 and 2208 and a corresponding decrease in convergence north-northeast of the tornado between 2204 and 2210. At 2210, the SRFD increases in strength and convergence north-northeast of the tornado is observed to increase. This relationship between SRFD strength and convergence and tilting is also observed during the tornadogenesis process with increased convergence (lack of convergence) correlated to tornado intensification (weakening). It is hypothesized that between 2204 and 2208, stretching was the primary mechanism by which the circulation maintained tornadic intensity based on the analysis of slightly decreasing circulation and angular momentum outside of the radius of maximum winds while the delta-V of the tornado is increasing and the radius of maximum winds is decreasing. Additional convergence provided by the SRFD at 2210 is hypothesized to lead to the brief increase in tornado strength observed between 2210 and 2212.