Preliminary Report
Hurricane Danielle
24 August - 08 September 1998

1998 PRELIMINARY REPORTS

Tropical Storm Alex (TS)
Hurricane Bonnie (3)
Tropical Storm Charley (TS)
Hurricane Danielle (2)
Hurricane Earl (2)
Tropical Storm Frances (TS)
Hurricane Georges (4)
Tropical Storm Hermine (TS)
Hurricane Ivan (1)
Hurricane Jeanne (2)
Hurricane Karl (2)
Hurricane Lisa (1)
Hurricane Mitch (5)
Hurricane Nicole (1)

Danielle had a long track across the Atlantic. Although it did not seriously impact land as a tropical cyclone, it battered portions of Great Britain as an extratropical system.

 

a. Synoptic History

A tropical wave moved off the west coast of Africa on 21 August accompanied by disorganized cloudiness and showers. Within 24 hours, deep convection became somewhat more consolidated in clusters near an ill-defined center of cyclonic cloud rotation. Initial Dvorak technique classifications were assigned at 1100 UTC 22 August. Thereafter, the organization of the disturbance continued to gradually improve as cloudiness and showers became concentrated in a circular area. By 0600 UTC 24 August, the Dvorak T-number was analyzed at 2.0 and it is estimated that Tropical Depression Four formed around this time, centered a little less than 600 nautical miles west-southwest of the Cape Verde Islands. Strengthening continued, as satellite images showed convection becoming more tightly wrapped around the center, and the tropical cyclone is estimated to have become Tropical Storm Danielle by 1800 UTC 24 August. Upper-tropospheric outflow was well-defined over the area, and Danielle intensified further. The first visible satellite pictures on 25 August revealed a "pinhole" eye, indicating that the system had become a hurricane. Danielle was a quite compact system, with tropical storm force winds covering an area estimated to be only a little more than 100 nautical miles in diameter. Based on satellite data, this rapidly strengthening hurricane reached a peak intensity of near 104 mph around 0600 UTC 26 August, while centered about 900 nautical miles east of the Leeward Islands.

After reaching its first peak in strength, some southeasterly vertical shear appeared to disrupt Danielle's organization. By the time the first reconnaissance aircraft reached the hurricane around 0000 UTC 27 August, the system was not as well-organized on satellite imagery as it had been. This first aircraft mission found a maximum wind of 104 mph at the 850 mb flight level, but a remarkably high central pressure of 993 mb. Such values of wind and pressure show how much deviation from the typical wind vs. pressure relationship can occur in compact hurricanes. In contrast to this, the much larger Hurricane Bonnie, which was in progress over the western Atlantic around the same time with a comparable maximum wind speed, had a minimum central pressure that was 25 mb lower than the value noted in Danielle.

Moderate vertical shear continued to preclude much strengthening of Danielle. However, aircraft data indicate that a second 104-mph intensity peak occurred around 1200 UTC on the 27th. For the next few days, some weakening took place even though the eyewall structure was generally maintained and atmospheric conditions seemed to be favorable for intensification. By the 30th, Danielle was barely of hurricane strength. Movement over waters that were cooled by the earlier passage of Hurricane Bonnie may have been the main cause of weakening, but this is uncertain.

From the time the cyclone formed, and for about six more days, the motion was toward the west-northwest, with the forward speed gradually slowing from 21-23 mph over the eastern Atlantic to 10 or 12 mph on 30 August. By the latter time, Danielle was nearing the western periphery of the subtropical anticyclone which had steered it across much of the Atlantic. Continuing to decelerate, the hurricane turned toward the northwest and north, reaching its westernmost longitude, about 74°W, early on the 31st. The hurricane began to restrengthen, and a third 104-mph intensity peak was noted around 1200 UTC that day.

In response to increasing southwesterly mid-tropospheric steering flow, ahead of a trough near the United States east coast, Danielle completed its recurvature and began to move northeastward on 1 September. It also regained 104-mph maximum winds for the fourth, and final, time. The center of the accelerating hurricane passed slightly less than 200 nautical miles northwest of Bermuda early on the 2nd. Danielle began to lose its tropical characteristics on 3 September, as its center passed about 200 nautical miles south of Cape Race, Newfoundland. It is estimated that Danielle became an extratropical cyclone by 0000 UTC 4 September, although it was still a strong storm with hurricane force winds. The storm moved eastward to east-northeastward across the north Atlantic for the next couple of days, with only slow weakening. It turned northeastward several hundred miles to the west of the British Isles on the 6th, its forward speed slowing to 7-12 mph. The cyclone became indistinct when it merged with another extratropical low a couple hundred miles north of Ireland on 8 September.

b. Meteorological Statistics

The highest wind reported in Danielle was 112 mph, at 700 mb from the Hurricane Hunters at 1141 UTC 27 August. Additional Hurricane Hunter wind observations of 110 mph at 850 mb and 106 mph at 700 mb were taken at 1235 UTC 31 August and 1656 UTC 1 September, respectively. These data, along with satellite-based intensity estimates on 26 August, are the main bases for the four intensity peaks of 104 mph in the best track. The minimum pressure estimate, 960 mb, was derived from a lowest pressure observation of 962.6 mb from the (Canadian) Laurentian Fan buoy, 44141 at 0900 UTC 3 September. This observation, as well as other ship and buoy reports of tropical storm force or greater windspeeds associated with Danielle, are listed in Table 2. It is also noteworthy that the aforementioned buoy measured a significant-wave height of 52 feet with a maximum wave height of 88 feet at the time of its lowest pressure.

Sustained winds of 39 mph with gusts to 54 mph were observed at Bermuda at 1100 UTC 2 September.

c. Casualty and Damage Statistics

No reports of casualties due to Danielle have been received at the National Hurricane Center. The western part of Great Britain was lashed by Danielle as an extratropical storm on 6 September. Several persons were rescued from treacherous sea conditions in the area. On the coast of Cornwall, beach areas had to be evacuated after waves became so high that they were breaking over some houses. A police all-terrain vehicle on the Isles of Scilly was swept into the sea by a rogue wave as it was being driven down a concrete pier in one of the island's main towns.