An explanation is offered for a case of violent and prolonged non-frontal thunderstorms in New England. The thunderstorms occurred in the vicinity of an upper level trough. An analysis of the situation points to the development of marked divergence in the upper troposphere and convergence at middle levels. The resulting vertical motions acted to decrease the stability of the air mass. The increased divergence-convergence aloft resulted from an intensification of the ridge to the west and not from any deepening of the trough in the thunderstorm area.

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