First WSR-88D documentation of an anticyclonic supercell with anticyclonic tornadoes: the Sunnyvale/Los Altos tornadoes of 4 May 1998

On May 4 1998, a pair of tornadoes occurred in the San Francisco Bay area in the cities of Sunnyvale (F2) and Los Altos (F1). There is anecdotal information that a third tornado occurred in the city of east Palo Alto. The near-storm environment of the day was characterized by a cyclonically curved hodograph, and moderate instability. Although deep-layer shear was weak, 0-1 km negative shear was great and consistent with the cyclonically-curved hodograph. Storm motion was off and to the left of the hodograph which is behavior consistent with previous modeling results of storm behavior associated with such hodographs. The tornadoes occurred in association with a left-moving anticyclonically-rotating supercell and were themselves documented photographically and visually to be rotating anticyclonically as well, making for an extremely rare event. Also presented here are WSR-88D radar images from the Monterey, California (KMUX) radar which were output from NSSL's new Mesocyclone Detection Algorithm (MDA). Meso-anticyclones were detected in association with each tornado by the MDA. Between 0425 PDT and 0432 PDT, 5/4/98, an outflow boundary from neighboring storms triggered the thunderstorm in the Sunnyvale area. We postulate that the tornadoes occurred as the storm intercepted, tilted and stretched the vorticity associated with the boundary, giving the tornadoes both landspout and supercell characteristics. To our knowledge, this is the only documented case of a tornadic anti-mesocyclone in the United States that has been captured with WSR-88D Level-II data.