First WSR-88D Documentation of an Anticyclonic Supercell with Anticyclonic Tornadoes: The Sunnyvale–Los Altos, California, Tornadoes of 4 May 1998

Abstract

On 4 May 1998, a pair of tornadoes occurred in the San Francisco Bay Area in the cities of Sunnyvale (F2 on the Fujita scale) and Los Altos (F1). The parent thunderstorm was anticyclonically rotating and produced tornadoes that were documented photographically to be anticyclonic as well, making for an extremely rare event. The tornadic thunderstorm was one of several ?pulse type? thunderstorms that developed on outflow boundaries on the left flank of an earlier-occurring thunderstorm east of San Jose. Satellite imagery showed that the tomadic storm moved northwestward along a sea-breeze boundary and ahead of the outflow boundary associated with the prior thunderstorms. The shear environment into which the storm propagated was characterized by a straight hodograph with some cyclonic curvature, and by shear and buoyancy profiles that were favorable for anticyclonically rotating updrafts. Mesoanticyclones were detected in the Monterey (KMUX) radar data in association with each tornado by the National Severe Storm Laboratory's (NSSL) new Mesocyclone Detection Algorithm (MDA) making this the only documented case of a tornadic mesoanticyclone in the United States that has been captured with WSR-88D level-II data. Analysis of the radar data indicates that the initial (Sunnyvale) tornado was not associated with a mesoanticyclone. The satellite evidence suggests that this tornado may have occurred as the storm ingested, tilted, and stretched solenoidally induced vorticity associated with a sea-breeze boundary, giving the initial tornado nonsupercellular characteristics, even though the parent thunderstorm itself was an anticyclonic supercell. The radar-depicted evolution of the second (Los Altos) tornado suggests that it was associated with a mesoanticyclone, although the role of the sea-breeze boundary in the tornadogenesis cannot be discounted.