Two Cases of Tornadic Wintertime Convection in Central Texas
Patrick Herald and Carl Morgan
NWSFO Austin/San Antonio, TX
Two tornados were generated over Lee County, Texas within a ten-day period in January, 1998. Lee County is located about 35 miles east of Austin and has a relatively sparse population. The first tornado occurred on the evening of January 11 and caused no fatalities or injuries; however, it did damage a school and lifted a truck off a roadway near Giddings. The second "pogoed" across the eastern portion of the county and did some minor damage in the town of Dime Box, with no injuries or fatalities. The storms are of interest because of the time of year, similarities in environments, and because of the differences in the radar signatures. This paper will briefly examine the environments, look at some of the radar data, and suggest some improvements for detection of weak, low-level tornados.
The atmosphere over North America on the morning of January 11 was characterized by an upper low over central Canada with the southern and polar jet streams merging over the Gulf Coast states. A shortwave was approaching western Texas as indicated by analysis of 500 and 700 mb plots. An 850 mb low was centered over South Dakota with a trough extending across the central Plains into Texas. On the surface, a lee-side trough had formed over New Mexico, west Texas, and northern Mexico. An inverted trough extended across south-central Texas. High pressure dominated the surface east of the Mississippi Valley. A "backdoor" cold front had penetrated into the Texas Hill Country the previous day and had stalled. By daybreak on the 11th, the boundary was moving northward as a warm front with moisture surging out of the coastal plain.
The 1200 UTC 850 mb analysis showed a 30 kt low-level jet had driven a moisture axis across the coastal counties northeastward to the Shreveport vicinity. Early afternoon surface analysis (Fig. 1) revealed the warm front over northern Texas with the inverted trough extending from Laredo to College Station. The 1200 UTC Corpus Christi sounding was missing due to maintenance problems, but the models indicated lifted indices at -4 C and CAPEs around 1000 j/kg over the coastal plain between Austin and Corpus Christi. Surface dewpoints to the east of the trough were in the 60s. Satellite imagery (Fig. 2) revealed a cumulus field south and east of the trough axis, with a stratus deck to the north. Temperatures under the stratus were in the 50s, with 70s over the coastal plain. The stratus deck was eroding from south to north, leaving a narrow band of clear skies which rapidly filled in with showers and thunderstorms. The inverted trough remained stationary through the evening, and showers and thunderstorms continued to develop in this corridor just east of the Austin and San Antonio metropolitan areas.
Showers and thunderstorms began developing in the coastal plain between San Antonio and Victoria early in the afternoon and moved north-northeastward near 20 kt. By 2130 UTC, a shower and thunderstorm cluster was approaching the western border of Lee County. Maximum reflectivity as measured by the Austin/San Antonio WSR-88D (KEWX) was 52 dBZ, cell-based VILs were 11 kg/m², and echo tops were less than 30 thousand feet. The storms continued to move northeastward and were through the area by 2345 UTC. The most intense storm remained south of Lee County, with reflectivity of 65 dBZ elevated to 13 thousand feet, a cell-based VIL of 68 kg/m², but echo tops were less than 30 thousand feet. The radar flagged this storm with a mesocyclone and a TVS alarm, and drew attention away from the cluster of storms moving across Lee County.
The Gray Air Force Base WSR-88D (KGRK) is located about 30 miles north of Austin and was 20 nm closer to the Lee County storms than KEWX. Maximum reflectivity was in the 50-60 dBZ range, cell-based VILs were in the 20s kg/m², and echo tops were still less than 30 thousand feet. Neither radar flagged a mesocyclone with these storms. Just before 2300 UTC, a tornado briefly touched down near Giddings and produced some minor damage. The storm-relative velocity map (SRM) from KEWX at 2255 UTC indicated 16 kt of rotational velocity at 62 nm in the lowest scan. This corresponded to a beam elevation near 7000 ft. Stronger shear was shown at 1.5 deg with 31 kt of rotational velocity over a diameter of 1.6 nm. The SRM from KGRK showed shear values in the lowest scan at 0.03s-1 with 41 kt of rotational velocity over 0.7 nm. The beam elevation for KGRK at 42 nm was around 4000 feet. The velocity signatures occurred for one volume scan, then weakened considerably.January 21
The January 21 atmosphere was characterized by a negatively tilted upper trough axis from the north-central plains of the United States into western Canada. A short wave was moving down the east side of a West Coast ridge and approached the western New Mexico border at 1200 UTC. An 850 mb low was centered in eastern Kansas and high pressure was over Florida with southerly wind funneling Gulf moisture between the two. A surface cold front was moving through the Hill Country with an inverted trough from Brownsville to near Shreveport (Fig. 3).
The 850 mb moisture axis was analyzed across extreme eastern Texas at 1200 UTC, and surface dewpoints along and to the east of the surface trough were in the 60s. Once again, satellite imagery revealed that the slow-moving, inverted trough axis was the focus of convection, as storms developed in that area of enhanced moisture convergence. The modified 1200 UTC Corpus Christi sounding recorded CAPE near 2300 j/kg and lifted indices near -7 C. The modified energy-helicity index of 3.04 indicated that strong (F3) tornados were possible. Storm-relative helicity of 82 (m/s)² increased as the front sagged southward and turned the storms to a more eastward motion.
Showers and thunderstorms began developing in and east of the Interstate 35 corridor between Waco and San Antonio around 1800 UTC. The storm that pummeled Lee County formed in southern Bastrop County around 1900 UTC and moved northeastward at 20 kt. By 2000 UTC, alphanumeric products from KEWX indicated a 13 thousand feet mesocyclone, cell-based VIL at 42 kg/m², and maximum reflectivity at 63 dBZ. A funnel cloud was reported at this time as the storm approached the Lee County line. The storm turned to the right as it entered Lee County and KEWX continued to define a minimal to moderate mesocyclone. At 2030 UTC, the storm merged with a developing cell near Highway 77, to the west of Dime Box, and produced a brief tornado just before the 2100 UTC. 2054 UTC half-degree SRM showed 41 kt of rotational velocity over a diameter of 3.8 nm and the cell-based VIL was recorded at 68 kg/m². The combined shear product showed numerous pixels of 100 to 150 x10-4s-1 beginning at 2048 UTC. These readings continued as the storm skirted Dime Box and produced a second tornado near the town. Spectrum width indicated 16 kt in the affected area, and these readings also continued until the storm neared the eastern county line just after 2100 UTC.
Identifying a developing tornado is one of the most important and challenging responsibilities of the WSR-88D operator. Many tornados form in storms which lack the classic structure, behavior or environment, and therefore lack the "classic" radar signatures. Although detection of developing tornados becomes more difficult in these situations, the wide array of products available to the radar operator in many cases offers enough information to distinguish a tornadic rotation.
Several factors can influence the ability of a given WSR-88D to detect a tornadic signature within a thunderstorm. Among the most obvious is the location of the storm in relation to the RDA. For low-topped, multicellular convection, such as the case of January 11, almost the entire life cycle of the tornado can occur within the lowest levels of the atmosphere, or over one or two volume scans, and therefore go undetected and unwarned. The January 11 tornado serves to emphasize some radar interpretation rules, such as the use of alternate radars with dial-in user functions at the ready, a healthy spotter network for ground truth, and the fallacy of relying too heavily on the algorithms.
The January 21 storm was a more typical, right-moving supercell. Although the storm formed in a similar environment and location as the January 11 storm (Fig. 4), it was more organized, thus, severe weather was more detectable from the KEWX vantage point. This event demonstrated the operational benefit of two radar operators (one handling alphanumeric products), and provides further evidence that future investigation of the combined shear and spectrum width products may be valuable.
The authors would like to thank NWSFO Lubbock for supplying surface and upper-air plots, Scott Bachmeier of CIMMS, University of Wisconsin-Madison, for contributing satellite imagery for this case study, and Jim Ward, Science and Operations Officer, NWSFO Austin/San Antonio, for review and input to this manuscript.