A Rare South Texas Tornado produced during the January 9, 2011 Quasi-Linear Convective System

In the early morning hours of 9 January 2011, a Quasi-Linear Convective System (QLCS) developed and moved across South Texas and the adjacent coastal waters. The QLCS produced widespread straight-line wind gusts between 50 and 80 mph which resulted in widespread wind damage across South Texas. Besides the severe wind gusts, a very rare EF1 tornado developed from a mesovortex along the leading edge of the QLCS and stayed on the ground for 21 miles. The tornado developed near Alice, TX and then tracked through Robstown, TX before lifting in the Calallen area of Corpus Christi, TX. Given that this was the first ever recorded January tornado to affect South Texas, a thorough examination of the synoptic, mesoscale and storm-scale factors that contributed to this rare January tornado was completed.

On the synoptic level, a very strong and progressive southern stream shortwave trough axis approached South Texas from northern Mexico and the Big Bend region of Texas. Meanwhile, a 999 hPa surface low pressure area with an accompanying warm front moved eastward across South Texas, aiding in the influx of 20 degree Celsius temperatures and 17 degree Celsius dew points from the Gulf of Mexico. Several hours in advance of the QLCS, the 0-6 km effective bulk shear values were as high as 41 m/s across South Texas. Also within several hours in advance of the QLCS, the Rapid Update Cycle (RUC) Model BUFKIT soundings indicated large curved hodographs in the 0-3km, indicative of a supercell environment. The very strong wind shear and dynamical forcing along with the presence of a warm front and surface low pressure area all proved to be strong contributing factors for tornadic mesovortex development to occur along the QLCS. CAPE values of only around 500 J/kg proved to be a non-factor due to the strong vertical motion and strong wind shear already present within the storm environment.

During the mature stage of the QLCS, a rear-inflow jet as high as 36 m/s developed a bowing segment near the surface low and warm front intersection. As this occurred, a tornadic mesovortex began to develop. A cyclonically-curved appendage observed in the radar reflectivity was observed along with a tightening velocity couplet. A hook-echo appendage gradually developed along the northern portion of the bowing segment. Mesovortex formation was maximized in the area just north of the rear-inflow jet. Peak rotational velocities of nearly 31 m/s were observed while the tornado was on the ground. The tornadic mesovortex then began to weaken as the hook-echo occluded back into the main line of thunderstorms due to the rapid movement of the cold outflow. The rotational velocities dropped to less than 26 m/s during this weakening non-tornadic phase. The mesovortex re-strengthened as it moved over the Middle Texas coastal waters. A second cyclonically-curved appendage in the radar reflectivity was again observed as it moved east over the water. However, due to the lack of spotters over the water and limited visibilities at night, a confirmation of a second tornado could not be determined.