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.