A case study of a well-documented tornadic thunderstorm in the San Joaquin Valley, California

This study documents a right-moving supercell thunderstorm that occurred on November 22, 1996 in the central San Joaquin Valley of California. The storm spawned two mesocyclone-induced tornadoes (F0 and F1) at the Lemoore Naval Air Station, hail up to 2 1/2 inches in diameter and caused extensive damage to property and agricultural interests in the area. The documentation of this storm is unique because this study is the first of a right-moving supercell in California for which WSR 88D, high-resolution satellite and photographic evidence were available. Previous studies of California tornadic storms often were inferential and conclusions about storm nature were made on the bases of a consideration of shear and buoyancy combinations alone.

The Lemoore storm formed ahead of the intersection of a subsynoptic trough and a lee side trough that had developed in the San Joaquin Valley. The synoptic pattern corresponded well with that observed for other tornadic storms in the Central Valley and was associated with a cold sector, low buoyancy environment. Proximity hodographs of the storm environment just before initiation showed a slight clockwise curved hodograph with accompanying veer of the wind shear vector. This veer of the wind shear vector, also observed with other supercellular-tornadic events in the valley, underscores the fact that the local low level shear environment in the Valley (often dominated by topographic channelling and related effects) is conducive for the development of right-moving supercells. Both low level and deep layer (0-6 km) positive shear, Bulk Richardson Numbers and storm relative helicities were consistent with ranges of these parameters documented for tornadic supercells elsewhere. However, as observed in previous comprehensive studies of stronger California tornadoes (i.e., those associated with F1 or larger tornado damage), both 0-1 km positive shear and 0-1 km SREH were stronger than observed with supercell tornadic storms in the Great Plains and near the upper end of the ranges suggested by previous studies in the literature.

Analyses of radar and high-resolution satellite images showed that the storm was a right moving thunderstorm at the southern end of a convective line. Although the storm resembled the "classic" paradigm for much of its life, it occasionally exhibited High-Precipitation (HP) structure. Doppler radar reflectivity data from Hanford (HNX) clearly showed a hook echo with the core greater than 65-dBZ that persisted for nearly one and a half-hours. Cross-sectional reflectivity analysis through the supercell showed the presence of a BWER while a cross-section analysis of storm relative wind velocities displayed the presence of a velocity couplet just prior the development of the first F0 tornado. Wind velocity data analysis at a half-degree tilt displayed a velocity couplet with gate-to-gate shear exceeding 32 knots within the mesocyclone during the occurrence of the F1 tornado. The development of this tornadic supercell highlighted the significance of the very large topographically induced low-level wind shear (0-1 km) in contributing towards the development of F1 and stronger mesocyclone induced tornadoes in California's Central Valley.