A case study of the 29 May 1995 tornado outbreak in southeastern Pennsylvania is presented in the latter part of this paper. A vigorous squall line with isolated supercell thunderstorms developed across eastern Pennsylvania during the late afternoon, fueled by an approaching cold front, jet dynamics aloft, abundant low-level warmth and moisture, and corresponding high thermodynamic instability (CAPE about 3000 J Kg**-1, LI about -9C). Powerful straightline thunderstorm winds toppled hundreds of trees and power lines, baseball-size hail inflicted over $3 million in damage in Chester County alone, and 4 supercell tornadoes, including and F2 in Millersville and an F1 in the author's hometown of Norristown, left many homes and buildings in their paths roofless. An analysis of the synoptic scale and mesoscale environments that fostered supercell tornadogenesis over southeastern Pennsylvania is presented.
1. Introduction
The tornado is arguably the most intense and awe-inspiring atmospheric phenomenon that has impacted the lives of many throughout Earth's history. Although they occur in numerous countries of the world, including Italy, Japan, and South Africa, tornadoes visit the United States most frequently with an annual average of 850 during the years 1980-1990 (Forbes 1992). The vast majority (about 700 per year) are mini tornadoes (F0 - F1 intensity) which are generally short-lived, nonsupercell tornadoes that produce only minor damage. The strongest tornadoes (F4 - F5), although quite infrequent, are long-lived storms fostered in a supercell thunderstorm environment with the ability to inflict significant destruction and loss of life. In an effort to improve forecasters' understanding of the complex mesoscale processes that promote supercell tornadogenesis, a plethora of research has been performed in this area over the past 30 years, including recently the Verification of the Origin of Rotation in Tornadoes Experiment (VORTEX) during 1994 and 1995.
Although VORTEX focused upon the southern plains states of Texas and Oklahoma, it must be emphasized that areas outside of "Tornado Alley" are not immune to supercell tornadoes. The northeastern US and, specifically Pennsylvania, is most prone to tornadoes during May-August (Forbes 1992) when some of the most destructive vortices have occurred. Forbes (1985) documented the 31 May 1985 Pennsylvania tornado outbreak in which an intense squall line fostered 21 tornadoes statewide including 7 F4 tornadoes and an F5 in Wheatland, PA. Damage exceed $300 million and 65 residents perished during the worst tornado outbreak in Pennsylvania history.
Almost 10 years removed from the memorable tornadic event in 1985, another potent squall line spawned late afternoon supercell tornadoes in Pennsylvania on 29 May 1995. In contrast to the 1985 outbreak which predominantly affected the northwestern part of the state, the 29 May 1995 case produced four tornadoes across the southeastern part of the Commonwealth, including an F1 vortex in the author's hometown of Norristown, PA.
This paper presents a detailed case study of the 29 May 1995 tornado outbreak, highlighting both the synoptic scale and mesoscale features that were responsible for supercell tornadogenesis over southeastern Pennsylvania. The author hopes that, through the presentation of this case, operational forecasters will gain a better understanding of the environmental precursors of tornadoes in order to improve the public warning of such events. Section 2 discusses the process of tornadogenesis with an emphasis upon vorticity production. The third and fourth sections describe the life cycle and types of nonsupercell and supercell tornadoes, respectively. The synoptic and mesoscale environments of the 29 May 1995 case are discussed in section 5, including a Doppler radar analysis of the approaching squall line during the late afternoon when severe weather was prevalent. Finally, section 6 summarizes the tornadic event and offers some conclusions.
Regional surface analysis for 2100 UTC 29 May 1995. Temperatures and dewpoints were near 80F across southeastern Pennsylvania with oppressively high dewpoints in the lower 70sF.
KDIX radar base reflectivity from 2200 UTC 29 May 1995. Three supercell storms are labeled "A", "B", and "C". The most striking feature is the well-defined hook echo associated with supercell "B" located over central Montgomery County. An F1 tornado associated with this hook echo touched down in Norristown PA, and 1.75 in. hail was reported in Royersford PA.
