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 hPa to 916 hPa and sustained winds reached 68 m/s. Analyses which include GOES-8 water vapor (WV) winds are employed to examine the rapid intensification phase of Opal.
Prior to and during the rapid intensification Opal was influenced by two smaller-scale upper-tropospheric troughs, an upper-tropospheric deformation zone, and a strong upper-tropospheric jet associated with a progressive large-scale trough. The first smaller-scale trough induced the initial tropical storm development on 29-30 September over the Yucatan Peninsula. This trough, which originated as a fracture from an earlier synoptic-scale middle latitude trough, subsequently weakened but remained situated to the west of Opal through 3 October. The second smaller-scale trough originated over southern Texas near 1200 UTC 3 October as the result of a partial fracture from a more progressive middle latitude trough. Both smaller-scale troughs were drawn toward one another in response to a persistent deformation zone situated over the extreme northwest Gulf of Mexico and the strengthening anticyclonically curved outflow circulation from Opal. The net result of these multiple flow interactions was an increase in an environment favorable for ascent and deep convection over and poleward of Opal.
East of the large-scale progressive trough an associated upper-tropospheric jet was situated over the southeastern United States. Opal moved rapidly poleward across the Gulf of Mexico on 3 October toward the jet-entrance region. Analyses of the 300-200 hPa layer-averaged divergence and 6 h divergence change show a significant increase in divergence and cyclonic vorticity advection over and poleward of Opal between 1200 and 1800 UTC 3 October. Strong convective growth in the eyewall and the rapid intensification immediately follow the increase in layer-averaged 300-200 hPa divergence. The collective evidence is used to infer that a trough-jet-hurricane interaction triggered the onset of the rapid intensification of Opal at a time when the storm was well below its maximum potential intensity.
At 0300 UTC 4 October, ~9 h after Opal began intensifying rapidly, the storm's eye crossed over a deep warm core eddy (WCE). Oceanic heat and moisture fluxes associated with this WCE have been shown in other studies to also be an important contributor to the rapid intensification. Opal weakened shortly before landfall as it exited the WCE and encountered increased vertical wind shear aloft.