P4.1 A case study of a long-lived warm-core circulation in the southern plains during the summer of 2007

Tuesday, 28 October 2008
Madison Ballroom (Hilton DeSoto)
Chad M. Shafer, University of Oklahoma, Norman, OK ; and K. H. Goebbert, A. D. Schenkman, and N. Snook

From the period 19 June to 7 July 2007, an excessive rainfall event transpired in much of the Southern Great Plains (SGP). On 19 June, several mesoscale convective systems developed in the region. By 1200 UTC 20 June, a mesoscale convective vortex (MCV) was clearly evident in radar imagery. After a period of about two days, the MCV was no longer discernible from a shortwave trough that moved through the region from 22-23 June. However, another MCV developed from a small thunderstorm cluster in southwest Oklahoma and northwest Texas on 24 June. This second MCV rapidly grew upscale into a nearly tropospheric warm-core circulation (WCC) by 27 June.

This WCC remained in the SGP through 7 July and was responsible for several extreme rainfall events, including the 27-28 June central Texas flood and the 28 June to 1 July southeast Kansas flood. This long-duration WCC was analyzed using surface data, upper-air analyses and sounding data, radar and satellite imagery, and model analyses to determine the causes of the development, evolution, and decay of this unusual event. Results from the analyses indicate that there were several contributing factors leading to the development and strengthening of the WCC. These include a persistently unstable and nearly saturated environment in much of the region as a result of an anomalously wet spring in the SGP and the long-duration transport of very moist and unstable air from the Gulf of Mexico, very weak vertical shear throughout the period, the rapid intensification of a shortwave trough in southern Canada and the northern US from 25-27 June (leading to large-scale ascent near the WCC), repeated periods of convection and subsequent latent heat release near the center of the circulation, and upper-level divergence provided by a strong ridge developing in the southeastern US.

Furthermore, the WCC remained in the SGP for several days as a result of a blocking pattern in much of North America. A deep trough developed in southeastern Canada and the northeastern US, while upper-level anticyclones developed in the northern plains and the southeastern US. The resulting col in the middle Mississippi Valley and a strong western US ridge prevented the WCC from moving away from the SGP. A weakening southeastern ridge and the entrance of a strong upper-level jet streak allowed the WCC to merge with stronger steering flow and exit the region on 7 July.

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