The synoptic-scale environment associated with this tornadic event was characterized by weak cyclonic flow through the depth of the troposphere, significantly cooler-than-normal temperatures aloft, and relatively abundant moisture. At 1200 UTC 4 May 1998, a 500 hPa low was centered about 1100 km west-southwest of San Francisco, with a central height of approximately 549 dm; a very weak upper trough extended from the low center through north-central California (Fig. 4). The system had an equivalent barotropic structure, with analyses of sea level pressure and 850 hPa height (not shown) appearing qualitatively similar to the 500 hPa height analysis. Over the ensuing 12 hours, the system moved eastward, but at 0000 UTC 5 May (not shown), still lay well to the west of the coast, with only very minor enhancement of the cyclonic flow over California having occurred. Analyses of sea level pressure and 1000/500 hPa thickness at both times indicated only very weak thermal advection and vorticity advection over central California.

Fig. 4. AVN model initialized 500 hPa geopotential height (m) and temperature (oC) for 1200 UTC 4 May 1998. Selected radiosonde observations are also shown. (Click on image for larger reproduction.)