Elevated cold-sector severe thunderstorms: a preliminary study

Abstract

A preliminary study of atmospheric conditions in the vicinity of severe thunderstorms that occurred in the cold sector, north of east-west frontal boundaries, is presented. Upper-air soundings, surface data and PCGRIDDS data were collected and analyzed from a total of eleven cases from April 1992 through April 1994. The selection criteria necessitated that a report occur at least fifty statute miles north of a well-defined frontal boundary. A brief climatology showed that the vast majority of reports noted large hail (diameter: 1.00-1.75 in.) and that the first report of severe thunderstorms occurred on an average of 150 miles north of the frontal boundary. Data from 22 proximity soundings from these cases revealed a strong baroclinic environment with strong vertical wind shear and warm air advection from just above the surface through 500 mb. This advection was reasoned to provide a constant source for destabilization from lifting above the frontal inversion. Convective instability was noted in all cases above the boundary layer with stability indices revealing the most unstable parcel located near the 850-mb layer. Despite quite cool and stable surface conditions, CAPE, best lifted index and total totals index values suggested at least a marginal degree of instability was required for cold-sector, severe thunderstorm development. After examining the proximity soundings, PCGRIDDS data were then analyzed to determine which forecast fields from the ETA model best delineated cold-sector, severe thunderstorm development. Reports of severe weather occurred VelY near the ETA model forecast, 850-mb, warm air advection maximum. In addition, a majority of reports occurred along the axis of strongest 850-mb theta-e advection. Constructed cross-sections normal to isotherms or thickness contours showed where areas of elevated or slightly sloped theta-e surfaces were located above the frontal surface. These areas of potential convective instability combined with upward vertical velocity fields correlated well with the location of subsequent severe thunderstorm reports.