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At least 2 tornadoes were observed on 29 April 2009 by myself along with TWISTEX in Floyd County Texas near the town of Cedar Hill. Cedar Hill is a small town east of Plainview on the edge of the Cap Rock. This was my first chase of the 2009 season and I was testing out the new mobile mesonet data acquisition system that I had redesigned during the previous months. The instruments were not yet calibrated and we only had one mesonet system out on this event so I don’t believe any meaningful data was collected. |
| The atmosphere on this day was prime for severe thunderstorm and supercell development. There was a dryline running north to south through the Texas panhandle with an outflow boundary triple point just north and east of Plainview by mid afternoon. The storm of interest developed on the dryline west of Plainview. As it moved off the dryline it interacted with the outflow boundary rapidly causing the storm to go from high-based outflow dominant to low-based tornadic. A NAM sounding from 0000 UTC near Cedar Hill, TX is shown here (click on the image for a closer view). It showed a decent amount of CAPE with 2347 j/kg, a lifted index of -6.7, and a nice looking hodograph with 220 m2/s2 of 0-3 km storm relative helicity, which resulted in an EHI of 2.9. The mid level flow was a bit lacking with only 35 kt at 500 mb, which might have contributed to this day not producing too many tornadoes. If it wasn’t for the storm/boundary interaction there likely would have been no tornadoes in the panhandle on April 29th. |  |
Shown here is the GOES-12 visible satellite from 2315 UTC, which was the time of the tornado reports. The tornado location is represented by the red square. A defined overshooting top can be seen suggesting that a very strong updraft was present. The atmosphere was convection free to the south and east of the storm allowing for high theta-e air to be ingested as inflow. An interesting observation is that the storm has sort of a hybrid multicell-supercell appearance on satellite with muliple updrafts apparent within the large anvil cloud.
Shown here is the 2315 UTC NEXRAD image with the tornado location labeled with a “T”. On radar, this storm does not look too organized. It has the appearance of a supercell embedded within a multicellular cluster. The tornado was located within sort of a “notch” with maybe a “fat” hook off to the west. It’s difficult to determine whether this an actual hook or a new cell developing without examining a radar loop.
In the field, tornadogenesis did appear to unfold in a rather non-classic fasion with an RFD dry slot punching into a very large low rain-free base from the southeast (given the storm structure, typically this would come from the west or southwest). Plus it appeared that the tornadic region developed on the southern portion of strong RFD push from a previous cycle. We were tracking a strong anticyclonic area of rotation in this region about 15 minutes or so prior to tornadogenesis. TWISTEX moved well north and east of this area to where the cyclonic member would have been to find that tornadogenesis was occurring to our southwest. Maybe the anticyclonic region spun down, changed sign, and became an area of cyclonic rotation very rapidly? It’s tough to say just from visible observations, but that’s what appeared to have happened. Overall, this was a very interesting day and one of the highlights of my 2009 chase season. This event will be featured on “Storm Chasers” season 3 episode 2.