Factors Affecting the Posttransition Intensification of Hurricane Irene (1999)

Hurricane Irene (1999) is examined as a case study of extratropical transition. Irene began its life in the Gulf of Mexico and made quick landfalls over both Cuba and Florida before entering the Atlantic Ocean at about 0900 UTC 16 October; the storm then paralleled the East Coast, tracking along the Gulf Stream for 2 days. Extratropical transition took place from 1800 UTC 17 October to 0000 UTC 19 October; during this time, the interaction of Irene with an upper-level jet streak contributed to intensification during and after transition. The details of the interaction are examined here using simulations of storm interactions with surface and upper-level features as well as quasigeostrophic omega and potential vorticity diagnostics. These analyses reveal that the extratropical transition of Irene was facilitated by the presence of an upper-level trough and jet streak, which contributed to the cyclogenesis ahead of the transitioning storm as well as to the posttransition intensification of the storm.