Hurricane Bonnie (1998) - Meteorological History

Meteorological History

On August 14, 1998, a tropical wave emerged off the west coast of Africa just north of Dakar and moved westward across the Atlantic Ocean. Initially located within cool waters, a strong high pressure area steered the disturbance on a west–southwest track over warmer waters, and convection started to develop. Several small centers of rotation existed within a broad circulation, and at 1200 UTC on August 19, the centers consolidated and the disturbance became sufficiently organized to be declared a tropical depression. Despite being poorly organized, winds slightly to the north of the system's center approached tropical storm strength shortly thereafter. Ship reports revealed a closed circulation, though the center was elongated in a northwest–southeast oriented manner. Upper-level winds were favorable, which suggested that intensification was likely. The cyclone began moving on a northwestward track, and just hours later the center of circulation appeared to reform close to the convection, an indication of a strengthening storm, as good outflow existed over the western side of the storm. Deep convection slowly developed closer to the center, and at 1200 UTC on August 20, the depression was upgraded into Tropical Storm Bonnie as it continued its west–northwest track around the periphery of a high pressure system over the Leeward Islands.

Late on August 20, the first reconnaissance plane entered the storm and found a minimum central barometric pressure of 1001 mb. The storm brushed the Leeward Islands, although the main thunderstorm activity remained to the north of the storm over the open ocean. Bonnie began to organize its broad circulation early on August 21, and within the next day the storm began to intensify. The storm began to look strong on satellite images with banding features over the north and west quadrants. The Hurricane Hunters aircraft found a minimum pressure of 987 mb and a nearly complete eyewall early on August 22, and as a result, the tropical storm was upgraded to hurricane status. Bonnie slowed in forward speed, coinciding with previous forecasts. Later that day, storm was upgraded to a Category 2 on the Saffir-Simpson Hurricane Scale, which occurred with a substantial 15 mb drop in 8 hours. At the same time, steering currents weakened with the dissipation of the high pressure system; this, combined with the effect of a nearby trough, caused the storm to turn in a more north-northwestward direction around the western periphery of an anticyclone to the east. Bonnie became a Category 3 storm, a major hurricane, at 1200 UTC the next day, reaching its peak winds of 115 mph (185 km/h) at the same time.

A mid- to upper-level trough slowed the storm almost to a halt early on August 23, before a drift to the north-northwest began. The next National Hurricane Center (NHC) advisory then reported that the eye was becoming more distinct and well-defined. This strengthening trend abated because the storm had churned up the waters over which it was passing, bringing cooler water to the surface as a result of the slow track. Another inhibiting factor may have been related to the same trough that caused the northward turn, though due to a large anticyclone situated over the hurricane, the weakening effects were not substantial. Despite wind shear, the large and powerful circulation resisted weakening for a time. Early on August 25, the shear and the entrainment of drier air into the hurricane took its toll on Bonnie, giving it a ragged appearance on satellite imagery, and the eye briefly became cloud-filled.

The storm accelerated somewhat by August 26, and early that day, it was moving at about 14 mph (23 km/h). An approaching mid-level trough steered Bonnie north-northeast, and at 2100 UTC on August 26, the eye passed east of Cape Fear, North Carolina. The hurricane once again slowed, and early the next day, it made landfall near Wilmington, North Carolina as a strong Category 2 or weak Category 3 hurricane. Doppler weather radar displays estimated that maximum sustained winds had quickly weakened to below hurricane intensity, and the storm was briefly downgraded to a tropical storm. However, as the storm turned towards the east in response to the approaching trough, the center neared open waters and the winds began to re-intensify. As a result, the cyclone re-attained hurricane status at 0000 on August 28. Offshore, the center began drifting roughly eastward. Entering colder waters, hurricane status was lost at 1800 UTC that day, followed by an acceleration to the northeast. The storm began to lose deep thunderstorm activity, and was forecast to lose tropical characteristics and become an extratropical cyclone within days. By early on August 29, little connection remained over the western semicircle, and only a band of such activity persisted to the southeast of the center. Bonnie became extratropical around 1800 UTC on August 30, to the southeast of Newfoundland.