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Abstract
On 31 May 1998, an F3 tornado struck Mechanicville, New York, injuring 68 people and causing $71 million in damage. The tornado was part of a widespread, severe weather outbreak across the northeast United States. The synoptic conditions that caused the outbreak and the mesoscale and storm-scale environments that produced the tornado are discussed.
The coupling of two strong upper-level jets and a very strong low-level jet, in association with an unseasonably strong surface cyclone, created a synoptic-scale environment favorable for severe weather. As the result of these jet interactions, a very warm, moist air mass was transported into the Northeast with an associated increase in the wind shear in the lower troposphere. A terrain-channeled low-level southerly flow up the Hudson Valley may have created a mesoscale environment that was especially favorable for tornadic supercell development by increasing storm-relative helicity in the low levels of the atmosphere and by transporting warm, moist air northward up the valley, leading to increased instability.
A broken line of locally severe thunderstorms moved eastward across New York several hours prior to the tornado. The storm that produced the Mechanicville tornado developed over central New York ahead of this line of storms. As the line of storms moved east, it intensified into a solid line and bowed forward down the Mohawk Valley of New York. These storms were moving faster than the isolated supercell to the east and overtook the supercell where the eastern end of the Mohawk Valley opens into the Hudson Valley. Based on limited observational evidence and the results of simulations of idealized quasi-linear convective systems reported elsewhere in the literature, it is hypothesized that backed low-level flow ahead of a bookend vortex at the northern end of the bowing line of storms over the Mohawk Valley may have contributed to the tornadogenesis process as the squall line overtook and interacted with the intensifying supercell.
Abstract
On 31 May 1998, an F3 tornado struck Mechanicville, New York, injuring 68 people and causing $71 million in damage. The tornado was part of a widespread, severe weather outbreak across the northeast United States. The synoptic conditions that caused the outbreak and the mesoscale and storm-scale environments that produced the tornado are discussed.
The coupling of two strong upper-level jets and a very strong low-level jet, in association with an unseasonably strong surface cyclone, created a synoptic-scale environment favorable for severe weather. As the result of these jet interactions, a very warm, moist air mass was transported into the Northeast with an associated increase in the wind shear in the lower troposphere. A terrain-channeled low-level southerly flow up the Hudson Valley may have created a mesoscale environment that was especially favorable for tornadic supercell development by increasing storm-relative helicity in the low levels of the atmosphere and by transporting warm, moist air northward up the valley, leading to increased instability.
A broken line of locally severe thunderstorms moved eastward across New York several hours prior to the tornado. The storm that produced the Mechanicville tornado developed over central New York ahead of this line of storms. As the line of storms moved east, it intensified into a solid line and bowed forward down the Mohawk Valley of New York. These storms were moving faster than the isolated supercell to the east and overtook the supercell where the eastern end of the Mohawk Valley opens into the Hudson Valley. Based on limited observational evidence and the results of simulations of idealized quasi-linear convective systems reported elsewhere in the literature, it is hypothesized that backed low-level flow ahead of a bookend vortex at the northern end of the bowing line of storms over the Mohawk Valley may have contributed to the tornadogenesis process as the squall line overtook and interacted with the intensifying supercell.
Abstract
The process of tornadogenesis in complex terrain environments has received relatively little research attention to date. Here, an analysis is presented of a long-lived supercell that became tornadic over complex terrain in association with the Great Barrington, Massachusetts (GBR), F3 tornado of 29 May 1995. The GBR tornado left an almost continuous 50–1000-m-wide damage path that stretched for ∼50 km. The apparent rarity of significant tornadogenesis in rough terrain from a supercell well documented in operational Doppler radar motivated this case study. Doppler radar observations showed that the GBR supercell possessed a midlevel mesocyclone well prior to tornadogenesis and that the mesocyclone intensified as it crossed the eastern edge of New York’s Catskill Mountains and entered the Hudson Valley. Tornadogenesis occurred as the GBR mesocyclone crossed the Hudson Valley and ascended the highlands to the east. Subsequently, the mesocyclone weakened as it approached the Taconic Range in western Massachusetts before it intensified again as it moved downslope into the Housatonic Valley where it was associated with the GBR tornado. Because of a dearth of significant mesoscale surface and upper-air observations, the conclusions and inferences presented in this paper must be necessarily limited and speculative. What data were available suggested that on a day when the mesoscale environment was supportive of supercell thunderstorm development, according to conventional indicators of wind shear and atmospheric stability, topographic configurations and the associated channeling of ambient low-level flows conspired to create local orographic enhancements to tornadogenesis potential. Numerical experimentation is needed to address these inferences, speculative points, and related issues raised by the GBR case study.
Abstract
The process of tornadogenesis in complex terrain environments has received relatively little research attention to date. Here, an analysis is presented of a long-lived supercell that became tornadic over complex terrain in association with the Great Barrington, Massachusetts (GBR), F3 tornado of 29 May 1995. The GBR tornado left an almost continuous 50–1000-m-wide damage path that stretched for ∼50 km. The apparent rarity of significant tornadogenesis in rough terrain from a supercell well documented in operational Doppler radar motivated this case study. Doppler radar observations showed that the GBR supercell possessed a midlevel mesocyclone well prior to tornadogenesis and that the mesocyclone intensified as it crossed the eastern edge of New York’s Catskill Mountains and entered the Hudson Valley. Tornadogenesis occurred as the GBR mesocyclone crossed the Hudson Valley and ascended the highlands to the east. Subsequently, the mesocyclone weakened as it approached the Taconic Range in western Massachusetts before it intensified again as it moved downslope into the Housatonic Valley where it was associated with the GBR tornado. Because of a dearth of significant mesoscale surface and upper-air observations, the conclusions and inferences presented in this paper must be necessarily limited and speculative. What data were available suggested that on a day when the mesoscale environment was supportive of supercell thunderstorm development, according to conventional indicators of wind shear and atmospheric stability, topographic configurations and the associated channeling of ambient low-level flows conspired to create local orographic enhancements to tornadogenesis potential. Numerical experimentation is needed to address these inferences, speculative points, and related issues raised by the GBR case study.
