Influence of a Predecessor Rain Event on the Track of Tropical Cyclone Isaac (2012)

Analysis of a predecessor rain event (PRE) over the Straits of Florida ahead of Tropical Cyclone (TC) Isaac on 25 August 2012 is presented. 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. Forecasts from the National Centers for Environmental Prediction-Global Forecast System (NCEP-GFS) initialized at 0000 UTC 21-24 August 2012 failed to predict the PRE and as a result recurved TC Isaac over Florida and the eastern Gulf of Mexico rather than continued Isaac on a northwest course toward southeast Louisiana. Vorticity inversion and detailed diagnosis of the NCEP-GFS TC track forecast initialized at 0000 UTC 22 August is also presented to assess the relationship between the PRE and TC Isaac's track.