Initiation of Storm A (3 May 99) along a Possible Horizontal Convective Roll

The first supercell of the 3 May 1999 outbreak (Storm A) produced the most devastating tornado of the event; but it formed well away from any fronts, drylines, outflow boundaries or other surface features traditionally associated with genesis of southern Great Plains tornadic supercells. Instead, the storm formed along a pronounced meridional band of concentrated low magnitude (<20 dBZ) reflectivity, as viewed from the lowest two elevation angles of the Frederick, OK, WSR-88D. The reflectivity band was associated with only one notable surface feature: subtle wind shifts at one mesonet site over which it oscillated. Radar-indicated characteristics of this band, and kinematic and thermodynamic properties of the convective boundary layer (CBL) inferred from modified soundings, will be examined to help determine its character, origin and morphology. Preliminary results suggest this feature was the updraft portion of a large horizontal convective roll (HCR). This possible large HCR appears to be superimposed with the eastern periphery of a field of much smaller HCRs of slightly different horizontal orientation.