Hurricane Danielle (1998) - Meteorological History

Meteorological History

A tropical wave, which was accompanied by disorganized convection, emerged into the Atlantic Ocean from the west coast of Africa on August 21. After deep convection consolidated somewhat, Dvorak technique classifications began on the system at 1100 UTC on the following day. The organization of the system continued to improve, with convection becoming concentrated. It is estimated that Tropical Depression Four developed at 0600 UTC on August 24, while located slightly less than 690 miles (1,110 km) west-southwest of Cape Verde. The National Hurricane Center initiated advisories at 1530 UTC and noted that the depression was already on the threshold of tropical storm intensity and that outflow was fairly symmetrical. After Dvorak T-numbers increased to 2.5 and satellite imagery indicated convection was tightly wrapping around the center, the depression was upgraded to Tropical Storm Danielle later on August 24.

Following a rapid increase in sustained winds and the development of a "pinhole" eye on satellite imagery, Danielle was upgraded to a hurricane at 1200 UTC on August 25. At the time, Danielle was a compact system, with tropical storm force winds spanning only 115 miles (185 km) from the center. Although the National Hurricane Center was fairly certain that the storm would continue west-northwestward, a veer north or south of the forecast track seemed possible due to the influence of an upper-level cold low to the west-northwest of Danielle. Nonetheless, the storm continued its west-northwest track. At 0600 UTC on August 26, Danielle attained its maximum sustained wind speed of 105 mph (165 km/h). Although the eye disappeared on satellite imagery and southeasterly vertical wind shear increased later on August 26, the National Hurricane Center did not indicate weakening and instead predicted that Danielle would reach winds of 120 mph (195 km/h) by late on August 27. The National Hurricane Center noted shortly thereafter that the eye was probably embedded in an asymmetrical area of deep convection. Early on August 27, a United States Air Force reconnaissance aircraft flight reported surface winds of 80 mph (130 km/h), though post-analysis indicates that the intensity of the storm was underestimated.

The storm briefly re-strengthened slightly on August 27, reaching its maximum sustained wind speed of 105 mph (165 km/h) for the second time at 1200 UTC. Vertical wind shear persisted and Danielle soon began to weaken again. Later on August 27, the National Hurricane Center predicted that the storm would travel along the periphery of an anticyclone centered just east of Bermuda, which would keep the storm away from the East Coast of the United States. Despite an apparently favorable environment, Danielle continued to slowly weaken, as the forecast for decreasing wind shear did not quickly materialize. Additionally, upwelling from Hurricane Bonnie about a week earlier likely contributed to a decrease in strength. Although the storm weakened further, reconnaissance aircraft observations on August 29 concluded that the structure of Danielle improved since the previous day. By early on August 30, Danielle had weakened to the extent that it was only a minimal hurricane, with winds being 75 mph (120 km/h) at the time. Later on August 30, the National Hurricane Center noted that forecast track predicted that Danielle would cross seas less influenced by the previous passage of Bonnie. Thus, the possibility of re-strengthening increased, though predictions only gradual intensification.

By early on August 31, an increase in sustained winds, a decrease in barometric pressure, and a 23 miles (37 km) wide eye observed by reconnaissance aircraft suggest that Danielle was beginning an "intensification cycle". The storm briefly drifted while northeast of The Bahamas, as it was moving north-northwestward. On August 31, the anticyclone near Bermuda caused Danielle to begin re-curving away from the United States. Later that day, a significant increase in intensity allowed the storm to again reach its maximum sustained wind speed of 105 mph (165 km/h). Six hours later after peaking on August 31, it weakened slightly, but remained a Category 2 hurricane. On September 1, there was significant uncertainty in the future intensity of Danielle, as a shortwave trough located northeast of the storm would either enhance outflow, causing strengthening, or increase wind shear, resulting in weakening. Shortly thereafter, the intensity predictions were briefly forecasting no change in intensity for 36 hours, followed by weakening over colder sea surface temperatures. At 1500 UTC on September 1, it was predicted that Danielle would strengthen slightly before weakening and becoming extratropical by September 3. Three hours later, Danielle reached its maximum sustained wind speed of 105 mph (165 km/h) for the fourth and final time.

Early on September 2, the storm weakened again and was downgraded to a Category 1 hurricane. At 0600 UTC on September 3, Danielle reached its minimum barometric pressure of 960 mbar (28 inHg), although sustained winds were only 80 mph (130 km/h) at the time. As late as 1500 UTC on September 3, satellite imagery indicated that Danielle was retaining tropical characteristics. The National Hurricane Center did not forecast any further weakening before the extratropical transition, but instead mentioned the possibility of baroclinic strengthening. Late on September 3, the final advisory was issued on the storm as it was becoming extratropical. According to the tropical cyclone report, Danielle "officially" transitioned into an extratropical cyclone at 0000 UTC on September 4, while located east-southeast of Newfoundland. The remnants of Danielle traversed east and eventually northeastward across the Atlantic Ocean. It produced rough seas in the British Isles on September 6, before merging with an extratropical low pressure area north of Ireland on September 8.