Influence of a predecessor rain event on the track of Tropical Cyclone Isaac (2012)

This PRE is unique compared to previously documented PREs in midlatitudes because it occurred over the oceanic subtropics and impacted the track of an approaching TC. The Isaac PRE developed in conjunction with a tropical moisture plume with precipitable water values over 60 mm that intersected a region of mid- and upper-level frontogenesis and warm air advection on the southeast flank of an upper-level trough.

The PRE occurred in an environment with more abundant tropical moisture and weaker synoptic-scale forcing for ascent compared to the environments in which midlatitude PREs developed. The Isaac PRE contributed to the fracture of the upper-level trough through negative potential vorticity advection by convectively driven divergent outflow. Fracture of the upper-level trough and midlevel cyclonic vorticity amplification associated with the PRE acted to steer Isaac south of Florida into the Gulf of Mexico.