Tornadoes associated with cold-core, closed 500mb lows: The 20 March 2006, northwestern Oklahoma tornadoes

On the afternoon of 20 March 2006, a tornadic supercell occurred under unusually cool and dry conditions in western Oklahoma. As a closed, cold-core 500mb low moved across the northern Texas panhandle and into southwestern Kansas, a surface low developed and moved towards the east across western Oklahoma through the afternoon and evening. Despite surface dewpoints in the 5 °C to 10 °C range, cold mid-level temperatures augmented the cool and dry surface conditions to yield convective potential instability. As convection developed through the afternoon, a storm located near Putnam, Oklahoma, to the immediate northeast of a surface low pressure center, appeared to contain features and exhibited evolution typical of a supercell. These features included a well-defined rear-flank downdraft, wall-cloud, vault region, and updraft occlusion. Around 2215 UTC, the supercell, located very near a low-level baroclinic and significant wind-shift line (sources for low-level vertical vorticity), produced a tornado east of the town of Putnam. While areas with temperatures near and below 0 °C were located less than 150 km from the storm, and snow was reported less than 100 km to the northwest, the supercell developed another mesocylone and rear-flank downdraft to the west of Oakwood, Oklahoma, upon the occlussion of the updraft that produced the earlier tornado. This supercell also contained unusually low-density hail, the consistency of which was more like that of balls of snow than hail.

We will present evidence that the ambient convective potential instability, enhanced by cold mid-level temperatures, allowed for the tilting of low-level horizontal vorticity along the surface boundary into the vertical, with low-level stretching associated with the updraft of the thunderstorm enhancing said vorticity leading to the development of a low-level mesocyclone and tornado. As has been noted in regard to other tornadic supercells in general, the tornado did not develop until the rear-flank downdraft made significant progress around the south and southeast side of the updraft. High-resolution analyses and convection-permitting numerical simulations of the event using the ARPS model were performed in an attempt to illuminate the effects of the surface boundary on the development and subsequent evolution of the tornadic supercell.