Environmental influences on the rapid intensification of Hurricane Opal (1995) over the Gulf of Mexico

Hurricane Opal intensified rapidly and unexpectedly over the Gulf of Mexico between 1800 UTC October and 1000 UTC 4 October 1995. During this period t he storm central pressure decreased from 963 to 916 hPa and sustained winds reached 68 m s(-1). Analyses that include high-resolution GOES-R water vap or 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 in teracted with a trough while situated over the Yucatan Peninsula. Opal deep ened moderately (similar to 20 hPa) in the 24 h ending 1200 UTC 2 October a s it achieved minimal hurricane strength and as it turned northeastward. Th e 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 approac hed the equatorward jet-entrance region of a progressive synoptic-scale tro ugh. The trough tail extended southwestward toward the lower Texas coast. A s 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 ou tflow and increased divergence in the upper troposphere poleward of Opal wa s associated with the deformation zone and the pai partially fractured trou gh 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 i ncrease in the magnitude and equatorward extension of the divergence core t oward Opal that begins at 1200 UTC 3 October and is most apparent by 1800 U TC 3 October and 0000 UTC 4 October. This divergence increase is shown to p recede convective growth in the eyewall and the onset of rapid intensificat ion 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 850-20 0-hPa shear within 500 km of the center is weak (2-3 m s(-1)). In this firs t stage of rapid intensification the winds increase by almost 15 m s(-1) to 52 m s(-1) prior to the storm reaching an oceanic warm-core eddy. The seco nd stage of rapid intensification occurs between 0600 and 1000 UTC 4 Octobe r when Opal is over the warm-core eddy and sustained winds increase to 68 m s(-1). During this second stags conditions are still favorable for a jet-t rough-hurricane interaction as demonstrated by the balanced vortex outflow calculation. Opal weakens rapidly after 1200 UTC 1 October when the storm i s near its MPI, the shear is increasing, and the eye is leaving the warm-co re eddy. This weakening occurs as Opal moves closer to the trough. It is suggested that an important factor in determining whether a storm-tro ugh interaction is favorable or unfavorable for intensification is how far a storm is from its MPI. The results suggest that a favorable storm-trough interaction ("good trough") can occur when a storm is far from its MPI. It is suggested that although the ECMWF (and to lesser extent NCEP) analyses r eveal the trough-jet-hurricane interaction through the balanced vortex outf low calculation, that the failure of the same models to predict the rapid i ntensification of Opal can be attributed to the inability of the model to r esolve the eye and internal strorm structure and the associated influence o f the trough-jet-hurricane interaction on the diabatically driven storm sec ondary circulation. The analyses also indicate that the high spatial and te mporal resolution of the GOES-8 water vapor winds reveal important mesoscal e details of the trough-jet-hurricane interaction that would otherwise be h idden.