An unusual combination of mid-tropospheric wind flow and temperature patterns surmounting an atypically buoyant sea breeze contributed to a shear profile in the south Bay Area on 4 May 1998 favorable for left-moving anticyclonic supercells. One such storm interacted with the sea breeze front to produce the initial anticyclonic vortex with a combination of landspout and supercellular characteristics. The second clockwise vortex apparently was formed from the traditional "supercell cascade." Meso-anticyclones identified by the MDA on KMUX represent the first WSR-88D documentation of anticyclonic mesocirculations in combination with anticyclonic tornadoes.

This case underscores the utility of the latest suite of WSR-88D algorithms to detect rotation in thunderstorms, even in the low-topped supercells common in the Pacific Coast states. This provides forecasters in the western United States with the ability of utilizing the WSR-88D radar with confidence in strong-to-severe weather conditions. Heretofore, the algorithms were not particularly successful in identifying tornadic circulations in the relatively weak low-topped convection which is typical in California. Also, until the present MDA upgrade, these algorithms were completely unable to identify clockwise tornadic circulations.

Both Blier and Batten (1994) and Monteverdi and Quadros (1994) have cautioned meteorologists in the West that all forecasters need to be adept at interpreting buoyancy and shear profiles with regard to the development of severe convection. Both studies pointed out that such convection is much more frequent in the state than previously thought, particularly in the Los Angeles area and the Central Valley. In the present case, despite the fact that the event occurred in densely-populated residential neighborhoods, the mid-afternoon timing ensured minimal injuries because residents were largely indoors. Obviously, such fortuitous timing cannot be relied on in the future. Thus, the Sunnyvale/Los Altos tornadoes have served as an additional warning to California meteorologists that anticipation of severe weather is an important part of the forecasting problem, even in the San Francisco Bay Region.


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