The Synoptic and Subsynoptic Structure of a Long-Lived Severe Convective System

Abstract

A long-lived severe convective system in the southern United States from 2?4 May 1978 is documented. The distinguishing feature of the convection was its origin in a region of deep synoptic scale ascent and its subsequent steady motion away from that ascent toward increasingly warmer and more moist, unstable boundary layer air. On 2 May 1978 a north-south oriented squall line originated above and within a shallow cold air mass in west central Texas north of a quasi-stationary east-west oriented frontal boundary. Potential instability was generated by a warm, moist airmass from the Gulf of Mexico that advected westward beneath a warm, dry plume of air moving northward ahead of the trough aloft. Convection first appeared along an inverted cough in western Texas that separated southward-flowing cool air, dammed up against the New Mexico mountains, from slightly warmer northeasterly and easterly flow to the east. Frontogenetical processes associated with the inverted trough played a crucial role in triggering and focusing the convection. The squall line, once formed, intensified eastward across Texas as it ingested increasingly unstable air. By 1200 UTC 3 May 1978 the squall line had propagated away from the synoptic scale ascent center that spawned it and had reached the Alabama-Mississippi border beneath the downstream ridge line aloft. Over the next 12 h the decaying squall line and other isolated patches of thunderstorms amalgamated into a large mesoscale convective system that moved steadily toward warmer air. The main precipitation area was continuous throughout, progressing regularly, shrinking somewhat to the north and broadening and strengthening to the south. A wake trough trailing the squall line exhibited gravity-wave-like characteristics. Vigorous new convective elements erupted in a region of maximum surface frontogenesis along the intersection of the decaying squall line boundary with an old east-west frontal boundary that had been locally strengthened by previous convection. During the mature phase of the organized mesoscale convective system the synoptic scale forcing was distinguished by its lack of vertical coherence. The convection remained fastened to the convergence of Q-vector forcing at 850 mb while the synoptic scale ascent in the middle and upper troposphere remained well to the west. By 1200 UTC 4 May 1978 the mesoscale convective system had become identified with an active squall line within a moist, unstable warm airmass across northern Florida and the extreme eastern Gulf of Mexico. New convective cells grew continuously southward from the original focal point along the east-west frontal boundary into the Gulf of Mexico ahead of an advancing cold front.