UW-Madison AOS 453 - A Mesoscale Analysis of Southern Plains Severe Convection on May 9, 2006

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A Mesoscale Analysis of Southern Plains Severe Convection on May 9, 2006Michael HileyAOS453 - Professor Tripoli and Dan HenzMay 5, 2009ABSTRACTTwo groups of severe convection in Oklahoma and Texas on May 9, 2006 are analyzed from the perspective of various data sources. The environment surrounding a group of discrete, supercellular convection in the Texas panhandle is contrasted with the one surrounding more linear convection in eastern Oklahoma. In both cases, forcing for convective initiation is found to be dominated by mesoscale boundaries in a region of relatively weak synoptic ascent; however, significant mesoscale differences between eastern Oklahoma and the Texas panhandle explain the majority of the observed differences in storm behavior in each area. Radar data demonstrates the importance of convective mode in determining type of severe weather produced, with discrete supercells favoring hail and tornadoes while the linear convection produces primarily high wind. Finally, a high resolution simulation of the event is performed with the Weather Research and Forecasting (WRF) model. The results of this model run are first compared to observations before being used to aid in diagnosing the event.I. Introduction On May 9, 2006 a round of intense storms impacted the Texas panhandle and eastern Oklahoma, leading to several tornado reports, including one tornado ranked as a three on the Fujita scale, which caused three fatalities in Westminster, Texas (NWS Ft. Worth), as well as about four significant (greater than 2”) hail reports. As the storms in eastern Oklahoma organized and moved into Arkansas overnight on May 10, scattered damaging wind gusts (greater than 50 knots) were reported across much of the western half of the state (Fig. 1). In terms of the areal coverage, quantity, and power of the severe weather produced, this event barely compares to many of the more infamous Great Plains severe weather outbreaks of early May. As just one example, the outbreak of May 4-5, 2007 produced over 80 tornado reports in states from Oklahoma all the way to Iowa and South Dakota, including the first tornado to receive a rating of five on the Enhanced Fujita scale in Greensburg, Kansas on May 4, 2007, which killed at least 11 people. However, the three fatalities in Westminster on May 9, 2006 demonstrate that even events which may seem minor when compared to classic outbreak type events can and do produce powerful and destructive weather. As a result, it is important to understand and study more subtle events like the subject of this paper. While massive outbreaks like May 4-5, 2007 tend to be synoptically evident with many clues to warn of the coming significant weather, events like this one are far more subtle; the process of finding the mechanisms that will eventually result in convective initiation can actually be more complex, and the clues leading to a successful forecast can require a far more mesoscale-minded analysis of the data. Consequently, this paper will attempt to characterize and analyze the mesoscale environments that led to the two primary areas of severe weather on May 9, 2006: the Texas panhandle and the eastern half of Oklahoma. While the two areas are only separated by about 200 km, the character of the resulting storms was quite distinct. In the Texas panhandle, initiation occurred along a north-south line around 23Z, resulting in a line of discrete supercells. The cells exhibited splitting behavior and tended to remain quite discrete. These storms produced primarily very large hail, with about three reports of 2-2.5” hail and a single report of 4.5” hail in Samnorwood, Texas. Later in the evening, one storm became tornadic, producing an F2 tornado on the east side of Childress, Texas (NWS Lubbock). In eastern Oklahoma, initiation also occurred around 23Z, however, the evolution of these storms was quite different from those in the Texas panhandle. The initial Hiley 2 Fig. 1: SPC storm reports for May 9, 2006.convection was indeed cellular and produced two brief tornadoes, but by 00:30Z the storms had already begun congealing into a single line. By 2Z on May 10, a single line of convection had become established in extreme southeast Oklahoma, producing primarily severe winds. However, a single discrete cell roughly 75 km south of the southern end of the line produced the F3 tornado in Westminster, Texas. Gallus et al. (2008) statistically demonstrates the importance of convective mode in determining the type and intensity of severe weather produced: lines of discrete, cellular storms tend to produce primarily hail and tornadoes, while squall lines and bow echoes have severe wind as the greatest threat. The two groups of storms in the current case are generally consistent with these findings. The line of discrete cells in Texas produced large hail and one significant tornado. The more congealed, linear convection in Oklahoma produced primarily wind, with a single discrete cell at the south end of the line producing a significant tornado. Despite the importance of storm mode in determining the type of severe weather produced, short-term prediction of storm mode remains a difficult problem in the operational forecast environment (Dial and Racy 2004). This provides the primary motivation for this paper: to investigate the reasons for such distinct storm modes by comparing and contrasting the mesoscale environments in which each group of storms formed. Focus will be placed the evolution and character of initiating boundaries as well as vertical shear profiles. It will be shown how extreme mesoscale variability led to the two distinct storm environments, and that thr o u g h c a r e f ul d i a gno s i s o f th e s e environments, many of the characteristics of the two groups of storms can be explained. In addition, a high resolution run of the Weather Research and Forecasting (WRF) model is used. While the


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UW-Madison AOS 453 - A Mesoscale Analysis of Southern Plains Severe Convection on May 9, 2006

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