Tropical-midlatitude Interactions during the “Perfect Storms” of 1991

The now-famous “Perfect Storm” (PS1) of late October and early November 1991 over the northwest Atlantic Ocean was a high-impact weather event that produced unexpectedly high winds and coastal flooding along the northeastern coast of the United States (US). PS1 formed as a baroclinic system as the result of converging air streams representing arctic, midlatitude, and tropical source regions. The intensification of PS1 was assisted by the interaction of a significant upper-level transient potential vorticity (PV) anomaly with the lower-level vorticity and tropical moisture remnants of Hurricane Grace (HG). Interestingly, HG initially formed through the tropical transition (TT) pathway to tropical cyclogenesis in the subtropical North Atlantic. The resulting strong oceanic cyclone (PS1) later became an unnamed hurricane as a core region of higher winds and deep moist convection developed.

The development and track of PS1 was strongly influenced by the existence of an anomalous high-amplitude large-scale flow pattern over North America and the western Atlantic Ocean. Strong tropospheric deformation associated with this anomalously high-amplitude flow pattern resulted in a meridional contraction and zonal elongation of the surface baroclinic zones associated with PS1. In response, PS1 underwent a warm seclusion and retrograded westward toward the coast of the northeastern US. Concurrently, a second intense surface cyclone (PS2) formed downstream of PS1 in a region of deep moist convection along the surface warm front extending eastward form PS1. Following the retrogression, PS1 underwent TT, becoming the “Unnamed Hurricane” of 1991, and performed a cyclonic loop in the North Atlantic towards the Canadian Maritimes. Meanwhile, PS2 rapidly propagated eastward across the North Atlantic where it interacted with an upper-level PV anomaly of arctic origin and deepened 50 hPa in 24 hours to the west of Great Britain.

The dynamical and thermodynamical processes associated with the development of PS1 and PS2 were strongly dependent upon the role of Hurricane Grace. Additionally, the resulting TTs of HG and PS1 revealed a unique opportunity to understand the dynamical interactions between transitioning tropical and extratropical cyclones occurring in both lower and middle latitudes. This presentation will focus on (1) the initial TT of Hurricane Grace in the subtropical North Atlantic, (2) the formation of PS1 and PS2 associated with the remnant low-level vorticity and moisture from extratropical Hurricane Grace, and (3) the subsequent TT of PS1 into the Unnamed Hurricane of 1991.