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About Hurricane Opal

Overview

According to the preliminary report on Hurricane Opal by the NCDC (Graumann, 1995), Opal made landfall in the Florida panhandle near Pensacola Beach around 2200 UTC on 4 October as a marginal Category 3 hurricane. Opal achieved maximum intensity around 1200 UTC as a Category 4 hurricane with sustained winds near 130 knots and a minimum central pressure of 916 mb. Despite significant weakening prior to landfall, Opal still caused extensive damages, ranking currently as the fourth costliest hurricane to make landfall in the United States from 1900-1996. A comprehensive summary was given by the National Hurricane Center on Opal, if additional information is desired. A more detailed description of the mesoscale to storm scale environment is given on the Severe Storms page, so the focus below will be on the synoptic scale environment.

There were several distinguishing features about Opal which made it an interesting case to study. First was the impressive rate of deepening which occurred with Opal, at rates up to 3 mb/hr during the 12 hours prior to reaching maximum intensity. Hennings (1997) attributed part of this rapid intensification to a convective burst phenomena at the hurricane core. Hennings also stated that only around 5% of all hurricanes achieve this rate of deepening, and Opal was the only one known to have occurred during the month of October.

Another unique feature of Opal was the highly asymmetric structure (from Henning) within the wind field for such an intense storm. This asymmetry was attributed by Hennings (1997) to an MCC in the vortex center, and to synoptic scale flow interaction as southerly winds intensified with a front approaching the Gulf of Mexico by Powell and Houston(1996). The largest pressure gradient within the storm was very near the core, within 28 km of the center the pressure was 958 mb, with 917 mb observed in the eye. The eyewall was highly contracted at maximum intensity, with a radius of only 18 km (Mayfield 1995). Generally, Opal was not a significantly destructive hurricane owing to high winds, with only limited damage over a fairly small area well east of the storm center at landfall.

Opal generated high waves on top of an impressive storm surge, which was responsible for a majority of the recorded storm damage (Mayfield, 1995). A joint university survey on the Impact of Hurricane Opal on the Florida/Alabama Coast showed Opal as the 9th costliest storm considering only wave and storm surge damage. Fortunately, Opal struck at low tide, which greatly lessoned the potential impacts from storm surge.

Finally, Opal was a prolific generator of tornadoes. Hurricane Opal produced 22 documented tornadoes, which was over twice the mean number for a landfalling hurricane in the U.S. (the mean is 10 from Novlan, 1974). Tropical cyclone (TC) mini-supercells generated about 40% of the observed tornadoes with Opal, the remainder were attributed to spin-ups within the eyewall region of the storm (core tornadoes). Shear profiles were favorable for supercells and instability was above average for a landfalling hurricane (concentrated in the lower troposphere). Additional information on the severe weather aspects are given on the Severe Storms page.

Opal, after reaching maximum intensity during the morning hours of October 4, began to rapidly fill at a rate near 2 mb/hr prior to landfall. The cause of this weakening was likely due to an increasingly sheared environment, cooler SST's underlying the eye, and a collapse of the inner eyewall (Mayfield 1995). The forward speed of Opal near landfall of around 20 knots helped reduce the potential flooding impacts, with most rainfall totals in the 5-10 inch range along the coast. In fact, there were no deaths associated with flooding in the United States attributable to Opal. Nine deaths were caused by Opal within the US, with one from a tornado in Florida and the remainder from falling trees. The rapid filling and an apparent mid-level dry intrusion appeared to weaken the convective intensity around the eye. Using radar imagery near the time of landfall, it is apparent that the eye was largely open along the southern periphery, with a definite rainfree zone between the eyewall convection and the outer rainbands to the east. These merged into a region of mainly stratiform type precipitation well north of the circulation center near a synoptic cold front (connecting band).

Synoptic Brief

Hurricane Opal originated from a tropical wave off the west coast of Africa on September 11, 1995. Opal was classified as a tropical depression near Cozumel, Mexico on September 27, 1995. The tropical cyclone then slowly crossed the Yucatan Peninsula, and rapidly strengthened to Category 4 intensity early on October 4 while moving northward toward the Florida Peninsula. Opal then weakened to a moderate Category 3 strength just prior to landfall, which occurred near Navarre Beach, FL at 2200 Z on 04 October, 1995 with a central pressure of 942 mb. Opal was significantly influenced by a synoptic scale trough in the Gulf of Mexico, causing substantial asymmetry in the precipitation and wind fields. This resulted in nearly all convection lying north and east of the hurricane track as the tropical cyclone approached land.


I am a member of the Convective Modeling Group at the Department of Atmospheric Sciences within the University of Illinois.