Observations from VORTEX2: The pretornadic phase of the Goshen County, Wyoming, supercell (5 June 2009)

We present an analysis of the pretornadic phase (2110-2148 UTC; tornadogenesis occurred at 2152 UTC) of the Goshen County, Wyoming, supercell of 5 June 2009 intercepted by the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). The storm resembles tornadic supercells that have been observed in the past. For example, it develops a couplet of counterrotating vortices that straddle the rear-flank downdraft and are joined by arching vortex lines, with the cyclonic vortex becoming increasingly dominant in the time leading up to tornadogenesis. The outflow in the hook echo region, where sampled, has relatively small negative buoyancy during this stage of evolution. In addition to discussing the evolution of the kinematic fields derived from dual-Doppler mobile radar observations, we will present a Lagrangian analysis of the 3D vorticity vector along trajectories that approach the mesocyclone region. Changes in the orientation and magnitude of the vorticity will be related to barotropic and baroclinic processes along a trajectory, with the latter being much more difficult to diagnose given the challenges of assessing horizontal buoyancy gradients.