Uncovering the Physical Mechanisms Responsible for the "Mystery" Boundary on May 3

The historical tornado outbreak in Oklahoma and Kansas on 3 May 1999 has been well-documented and publicized. The supercell thunderstorm that eventually spawned the Oklahoma City tornado formed well ahead of the dryline in an area of weak surface convergence, as detected by Oklahoma Mesonet data. Examination of radar data revealed that initiation occurred along a north-south “fine line” just east of the Frederick, OK radar. This boundary had a different orientation than numerous NNE-SSW oriented horizontal convective rolls (HCRs) that were also apparent in the radar data just to the west. The possibility exists that this boundary is also an HCR; if so, the change in orientation still implies a shift in the low-level winds with associated convergence.

Evidence will be provided in support of the argument that this convergence developed due to differential heating. A persistent pressure-fall maximum in southwest Oklahoma, which correlates well with the temperature-rise maximum, may have helped perturb local surface winds to produce the convergence. Possible reasons for the local maximum in temperatures will be provided. The possible role of the cirrus “hole” in storm initiation will also be discussed.

For the purposes of this study, we have assumed that the existence of the boundary was a necessary but not sufficient condition for convective initiation. The goal of this work is simply to attempt to uncover the physical mechanisms responsible for the existence of the boundary, and not to speculate on the ultimate cause of storm development. This case illustrates the importance of closely monitoring surface and radar observations when nowcasting thunderstorm development, and emphasizes the value of the Oklahoma Mesonet.