On 10 June 2010, the second Verification of the Origins of Tornadoes Experiment (VORTEX2) armada observed a supercell thunderstorm near Last Chance, Colorado. Tempest unmanned aircraft system (UAS) data collected in the rear-flank outflow revealed what appeared to be an elevated outflow head, turbulent wake, and a cold rear-flank internal surge (RFIS). Surface thermodynamic and kinematic data collected by the StickNet and mobile mesonet indicated that the outflow wake may have extended to or very near the surface, perhaps modifying or outright replacing the leading edge of the outflow at times. Single-Doppler data collected by the NOAA X-Pol Mobile Polarimetric Doppler Radar (NOXP) were supportive of the possibility of a downdraft in the outflow wake associated with low-level divergence. A conceptual model of the hypothesized rear-flank outflow structure in the nontornadic phase of the Last Chance supercell is presented. The observed turbulent wake is consistent with mixing associated with the release of Kelvin-Helmholtz instability rearward of a density current head. Observations also support the hypothesis that the RFIS would not have existed without the turbulent wake.
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