The transformation of tropical storm Agnes into an extra-tropical cyclone. Part I: the observed fields and vertical motion computations.

Synoptic analyses and vertical motion patterns are used to describe the behavior of Agnes over a five-day period subsequent to initial landfall during which time Agnes regained tropical storm strength prior to transforming into an extratropical cyclone and dissipating. Redevelopment of Agnes subsequent to initial landfall is the result of the spread of an area of appreciable cyclonic vorticity advection aloft over the periphery of the low-level circulation devoid of significant baroclinicity. Unlike a corresponding midlatitude storm development, however, the initial presence of a warm, moist, high vorticity environment at low levels greatly aids the development. The unique aspect of the regeneration of Agnes is provided by diabatic (primarily latent heat release) heating in the area of heavy rains extending well N of Agnes to the E of the Appalachian mountains. -from Authors