An Analysis of a Mesocyclone–Induced Tornado Occurrence in Northern California

Abstract

One documented F2 tornado and several other unconfirmed tornadoes were reported in California's Sacramento Valley on 24 September 1986. The synoptic pattern which occurred that day was one long-recognized by California operational meteorologists as being associated with severe weather in the state. The present study documents this event and shows that the parent thunderstorm had supercellular characteristics and that the tornado was mesocyclone-induced. As is the case elsewhere when severe thunderstorms are observed, the mesoscale environment established a focus for late morning and early afternoon deep convection. A quasi-stationary leeside trough acted in concert with local channeling effects to promote the advection of relatively moist air to the northern portions of the Valley. This channeled flow contributed to low-level shear which was much stronger than that evident in the Oakland hodograph and one which was comparable to that found with supercell and mesocyclone development elsewhere in the United States. The large scale environment acted as a destabilizing agent and provided a vertical motion field which encouraged the convection. Although CAPE estimates based upon an evaluation of Oakland sounding data were low, estimates of bulk Richardson number in the Valley were within the range observed with supercell storms elsewhere in the country. The thunderstorm initially developed when the vertical motion field associated with an advancing subsynoptic scale cloud band interacted with the moist and destabilizing air moving northward in the Valley. Diurnal heating and layer lifting probably contributed to the destabilization. Photographs show that the F2 tornado was characterized by multiple vortices, had formed from a wall-cloud on the south portion of the right-moving supercell, and was accompanied by a rear flank downdraft. Forecasters familiar with severe weather parameters could have anticipated the potential for supercellular development in California with the aid of computerized analysis and diagnostic routines now available in real-time throughout much of the operational community.