Environmental Influences on the Rapid Intensification of Hurricane Opal (1995) over the Gulf of Mexico

Abstract

Hurricane Opal intensified rapidly and unexpectedly over the Gulf of Mexico between 1800 UTC 3 October and 1000 UTC 4 October 1995. During this period the storm central pressure decreased from 963 to 916 hPa and sustained winds reached 68 m s Ϫ1. Analyses that include high-resolution GOES-8 water vapor winds and European Centre for Medium-Range Weather Forecasts (ECMWF) and National Centers for Environmental Prediction (NCEP) gridded datasets are employed to examine the rapid intensification phase of Opal. Opal first reached tropical storm strength on 29-30 September 1995 as it interacted with a trough while situated over the Yucatan Peninsula. Opal deepened moderately (ϳ20 hPa) in the 24 h ending 1200 UTC 2 October as it achieved minimal hurricane strength and as it turned northeastward. The deepening occurred in conjunction with an environmental flow interaction as determined by an Eliassen balanced vortex outflow calculation. As Opal accelerated toward the Gulf coast by 1200 UTC 3 October, it approached the equatorward jet-entrance region of a progressive synoptic-scale trough. The trough tail extended southwestward toward the lower Texas coast. As the poleward portion of the trough moved eastward, the equatorward end of the trough lagged behind, stretched meridionally, and partially fractured as it encountered a deformation region over the northwest Gulf. Enhanced outflow and increased divergence in the upper troposphere poleward of Opal was associated with the deformation zone and the partially fractured trough tail. An analysis of the 300-200-hPa layer-averaged divergence and 6-h divergence change based on an analysis of the water vapor winds shows a significant increase in the magnitude and equatorward extension of the divergence core toward Opal that begins at 1200 UTC 3 October and is most apparent by 1800 UTC 3 October and 0000 UTC 4 October. This divergence increase is shown to precede convective growth in the eyewall and the onset of rapid intensification and is attributed to a jet-trough-hurricane interaction in a low-shear environment. Calculations of balanced vortex outflow based on the ECMWF and NCEP gridded datasets confirms this interpretation. A crucial finding of this work is that the jet-trough-hurricane interaction and explosive intensification of Opal begins near 0000 UTC 4 October when the storm is far from its maximum potential intensity (MPI), and the 850-200-hPa shear within 500 km of the center is weak (2-3 m s Ϫ1). In this first stage of rapid intensification the winds increase by almost 15 m s Ϫ1 to 52 m s Ϫ1 prior to the storm reaching an oceanic warm-core eddy. The second stage of rapid intensification occurs between 0600 and 1000 UTC 4 October when Opal is over the warm-core eddy and sustained winds increase to 68 m s Ϫ1. During this second stage conditions are still favorable for a jet-trough-hurricane interaction as demonstrated by the balanced vortex outflow calculation. Opal weakens rapidly after 1200 UTC 4 October when the storm is near its MPI, the shear is increasing, and t...

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