First WSR-88D documentation of an anticyclonic supercell with anticyclonictornadoes: The Sunnyvale-Los Altos, California, tornadoes of 4 May 1998

On 4 May 1998, a pair of tornadoes occurred in the San Francisco Bay Area i n the cities of Sunnyvale (F2 on the Fujita scale) and Los Altos (F1). The parent thunderstorm was anticyclonically rotating and produced tornadoes th at were documented photographically to be anticyclonic as well, making for an extremely rare event. The tornadic thunderstorm was one of several "puls e type'' thunderstorms that developed on outflow boundaries on the left fla nk of an earlier-occurring thunderstorm east of San Jose. Satellite imagery showed that the tomadic storm moved northwestward along a sea-breeze bound ary and ahead of the outflow boundary associated with the prior thunderstor ms. The shear environment into which the storm propagated was characterized by a straight hodograph with some cyclonic curvature, and by shear and buo yancy profiles that were favorable for anticyclonically rotating updrafts. Mesoanticyclones were detected in the Monterey (KMUX) radar data in associa tion 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 i nitial (Sunnyvale) tornado was not associated with a mesoanticyclone. The s atellite 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 cha racteristics, even though the parent thunderstorm itself was an anticycloni c 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.