| An investigation of the 24 September 1986 "cold sector" tornado outbreak in Northern California - :14461 | National Weather Service (NWS)
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An investigation of the 24 September 1986 "cold sector" tornado outbreak in Northern California
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    Several cold-sector tornadoes and funnel clouds were observed in the Sacramento and San Joaquin Valleys of California on 24 September 1986. The synoptic pattern which occurred that day was one long-recognized by operational meteorologists as being associated with severe weather in the state. Mesoanalyses of hourly aviation observations and examination of radar analyses, infrared satellite images, isodrosotherm and surface streamline analyses showed that the tornadoes were associated with a classical right-moving supercell thunderstorm. This cell developed near Redding where warm, moist upvalley flow was experiencing surface convergence ahead of a post-frontal trough. The tornadoes, which formed on a wall-cloud on the southwest portion of the cell, were associated with the occurrence of other features observed with such events in the Great Plains including collapsing radar top, rear flank downdraft and mesolow-mesohigh couplet. The vertical shear profile was also similar to that found with typical supercells including a curved hodograph indicating wind veer with height and vertical speed shear comparable to that observed with supercells in the Great Plains. The air mass over the Central Valley underwent profound destabilization during the day due to differential temperature and moisture advection, low-level diurnal beating and layer-lifting such that stability indices attained values also comparable to those found in regions experiencing approaching risk of severe thunderstorms. As in the Great Plains, dynamic forcing played a significant role in the thunderstorm development. Quasi-geostrophic diagnostics showed that strong upwards vertical motions passed across northern and central California on this day. The vertical motion field was partially associated with a short-wave trough embedded in west-northwest flow which moved across the state and the ageostrophic accelerations produced by a jet-streak which was also advancing across the state. Some simple calculations showed that the vertical motions associated with the jet streak were of the same order of magnitude as those found with the short-wave trough further east. The results of this study underscored the need for forecasters on the West Coast to be familiar with the techniques of operational mesoanalysis of severe weather.

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