Summary

A detailed analysis of the 27 May 1997 central Texas tornadic storm complex is undertaken in an attempt to document the pre-storm environment and identify the roles played by preexisting boundaries on storm maintenance/propagation and rotation. Analysis is carried out using a broad suite of synoptic and sub-synoptic data but focuses on the level II and level III WSR-88D data from three Texas radars.

The 27 May 1997 central Texas event was characterized by a back-building storm complex that produced at least 12 tornadoes including the F5 Jarrell, Texas tornado. Furthermore, 5 of the 8 longest-lived cells during the analysis period are shown to contain mid-level mesocyclones. However, one-dimensional metrics calculated using representative vertical profiles of state variables reveal that, despite the extreme values of CAPE in place (>6500 J kg^-1), the (1D) environment associated with this is best classified as only marginally favorable for supercells and unfavorable for significant, supercellular tornadoes. Furthermore, the observed wind shear values are shown to be more in line with the vertical shear values typically associated with non-severe back-building storms.

Two preexisting boundaries in place during this event are found to have been essential to the maintenance/propagation of the storm complex: a cold front and a dryline. The storm-generated gust front is also shown to have been important for storm maintenance/propagation, but it was only the distorted gust front (that portion of the gust front whose orientation was distorted largely by the horizontal shear across the cold front) that is shown to have played a role in cell redevelopment.

The back-building of the "Lake Belton" storm (the first "envelope" of cells during this event) is shown to have been controlled by the propagation of a single cell that redeveloped quasi-continuously to the south through forced ascent along the southwestward moving frontal segment (the small segment of the cold front that had overtaken the dryline) and the distorted gust front. In contrast the "Jarrell" and "Pedernales Valley" storms (the second and third storms) are shown to have back-built through successive discrete cell redevelopment primarily along the southwestward moving frontal segment and distorted gust front. It is argued that the preexisting boundaries were not just essential but necessary for the maintenance and propagation of the storm complex.

It is argued that the southwestward motion of the frontal segment ultimately governed the back-building propagation/maintenance of all three storms. The frontal segment represented the location where the cold front and dryline were "zippered" together (i.e., where the cold front first overtook the dryline and penetrated into the most unstable air to the east) and its motion was a consequence of the southwestward motion of the intersection between the cold front and dryline. It is argued that this "zippering" of the cold front and dryline was essential for the back-building of the storm complex in this event.

While mid-level mesocyclones were present in five of the eight longest-lived and well-sampled cells, analysis of the relationship between observed cell motion, expected cell motion, expected supercellular deviation, and boundary motion for the longest-lived and well-sampled cells reveals no evidence that deviate motions generated through supercellular dynamics governed cell motions. Instead, it is shown that boundary motions largely controlled the propagation of individual cells.

Images and Animations
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Soundings representative of the 27 May 1997 event.

Boundaries, surface observations, and reflectivity field at 20:00Z.

Animation of base reflectivity, boundaries, and longest-lived cells

Tracks of convective cells during the analysis period.

Zippering of the cold front and dryline that led to back-building

Participants

Adam L. Houston and Robert Wilhelmson

Publications

Houston, A.L., and R. B. Wilhelmson, 2006: Observational analysis of the 27 May 1997 central Texas tornadic event. Part I: Pre-storm environment and storm maintenance/propagation. Mon. Wea. Rev., Accepted.

Houston, A.L., 2004: The role of preexisting airmass boundaries in the maintenance and rotation of deep convection in a high-CAPE, low-shear environment. Ph.D. thesis, Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, 275 pp.

Houston, A.L and R.B. Wilhelmson, 2002: The role of the preexisting boundary on tornadogenesis in the 27 May 1997 Central Texas event Preprints, 21st Conf. Severe Local Storms, San Antonio, TX, Amer. Meteor. Soc., 469-472. Extended abstract, Slides from talk