The Westminster, TX Tornado of May 9th, 2006

Meteorological Analysis

The Westminster tornado was triggered by a classic Colorado Surface Low. Several surface lows that have resulted in famous tornado events in the past have formed from lift created by the Colorado Rockies. Over the next day, the low slowly moved southeast and deepened (intensified) at a rather slow rate. Late on May 8th, a warm front pushed through the Dallas Fort Worth Metropolitan area, beginning destabilization ahead of the surface low. As the day progressed, the air continued to become more and more unstable, and more and more conducive for severe weather. However, a trigger was needed to result in supercell initiation, or else the environment would go unused. Lift is always triggered by a surface low, no matter the intensity of it. The surface low became the trigger for severe weather. The surface low also influenced favorable wind profiles for tornadoes, with surface winds out of the southeast.

Every day, usually at 1200 UTC and 0000 UTC, the National Weather Service releases a balloon into the atmosphere. This balloon is not your regular, average balloon, however. It is specifically engineered to collect data from the atmosphere, to help forecasters understand what the atmosphere is capable of. At 1200 UTC on May 9th, 2006 (approximately 7:00 AM CDT), the National Weather Service released a balloon to collect data from the atmosphere above Dallas. The environment had already destabilized quickly by this point, with the warm front having lifted through more than 12 hours earlier. Overall, the environment was quite favorable for tornadoes with the exception of one mesoscale aspect, the capping inversion. The capping inversion acts like a lid on the environment. It prevents hot air from rising up, preventing supercell development from occurring until it either weakens or is overcome by extreme levels of forcing along the cold front or by an unusually intense surface low. In this particular instance, the surface low was not that strong, which eliminated the possibility of overcoming the capping inversion using lift from the surface low alone. The trigger would have to come from a weakening of the capping inversion, or the cold front.

By 0000 UTC, the capping inversion had vanished into thin air. However, the wind shear values and hodograph had declined. The Critical Angle increased significantly increased. Those three mesoscale aspects had declined significantly for tornadoes, but there were also other mesoscale aspects of the environment that had significantly increased favorability for tornadoes. As the day continued, the environment continued to destabilize more and more. Surface heating caused the capping inversion to fade completely, favoring supercell development. The updraft profile had become so thick, vertical, and strong that if any supercell were to form, the updraft would be extremely efficient at turning the wind shear available into a tornado. Remember, the more vertical the updraft profile, the more vertical the mesocyclone, and the more efficient the mesocyclone becomes at using wind shear. Finally, the surface low was closing in on Dallas. The trigger for supercell thunderstorm developments had arrived. Everything was in place for a day of intense severe weather with some tornado potential.

Analysis of the Parent Supercell Thunderstorm from NEXRAD Radar Data

The supercell thunderstorm that produced the tornado first began to show a hook echo on NEXRAD radar at 03:15 UTC. The circulation passed near Anna at 03:25 UTC, which is the likely time of tornado genesis. It is officially listed as 03:37 UTC, but this is likely incorrect. The hook echo gradually occluded as the supercell thunderstorm moved east, and the tornado eventually dissipated as a result of the mesocyclone occlusion. NEXRAD radar also allows us to look inside of the parent supercell, and to allow us to identify the nature of its updraft, where the strongest winds are, and where the strongest precipitation is located. A volumetric scan of the supercell, while the tornado was in progress, revealed the mesocyclone to be vertical. This means that the mesocyclone was extremely efficient at using the wind shear available in the environment, and this is in accord with the picture painted by the observed soundings. The supercell's anvil reached a maximum height of 59,400 feet according to the echo tops product.

One unusual aspect of the radar data available was how weak the mesocyclone appeared to be, for the intensity of the damage that was produced. The mesocyclone's maximum gate-to-gate shear value (a measurement of how intense the strongest winds in a mesocyclone are) only reached 92.1 mph, much weaker than other violent tornadoes in the past. However, the answer to this lies in the wind profiles of the environment. The parent supercell was located quite far from the radar. The radar beam was scanning the tornadic mesocyclone at a height of 5,997 feet. We'll just round to 6,000 in this case. 6,000 feet is equal to approximately 1.8 kilometers high. The wind direction at 1.8 kilometers high in the atmosphere, according to observed sounding data, was almost due west. This is identical, in fact almost exactly the same as the wind direction at 15 kilometers high. This means that there was very little directional shear in the upper levels of the atmosphere. The change in wind direction only became significant at the lowest levels of the atmosphere. Due to this fact, the mesocyclone was spinning much slower at a height of 6,000 feet than it was at 1,000 feet. It's very likely that had the Westminster Supercell occurred much closer to the radar, the rotational signature would have been much more impressive.