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 n mi west-southwest of the Cape Verde Islands (Fig.
1 (40K GIF) and Table 1 give the overall "best
track" of this system). 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 n mi in diameter. Based
on satellite data, this rapidly strengthening hurricane reached a peak
intensity of near 90 knots around 0600 UTC 26 August, while centered about
900 n mi 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 90
knots 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 90-knot
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 18-20 knots over the eastern
Atlantic to 9 or 10 knots 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 90-knot 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 90-knot maximum winds for the fourth,
and final, time. The center of the accelerating hurricane passed slightly less than 200
n mi northwest of Bermuda early on the 2nd. Danielle began to lose its
tropical characteristics on 3 September, as its center passed about 200
n mi 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 6-10 knots. The cyclone became indistinct when it merged with another
extratropical low a couple hundred miles north of Ireland on 8 September.
b. Meteorological Statistics
Figures 2 (15K GIF)
and 3 (17K GIF) depict the curves
of minimum central sea-level pressure and maximum one-minute average "surface"
(10 meters above ground level) wind speed, respectively, as a function
of time. Also plotted are the observations on which the curves are based,
consisting of aircraft reconnaissance data from the U.S. Air Force Reserves
(the
Hurricane Hunters)
and NOAA, surface synoptic data, as well as Dvorak-technique estimates
from the Tropical Analysis and Forecast Branch,
TAFB, the Satellite Analysis
Branch, SAB, and the U.S. Air
Force Weather Agency (AFGWC in the figures) using satellite imagery.
The highest wind reported in Danielle was 97
knots, at 700 mb from the Hurricane Hunters at 1141 UTC 27 August.
Additional Hurricane Hunter wind observations of 95
knots at 850 mb and 92 knots 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 90
knots 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 34 knots with
gusts to 47 knots 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.
d. Forecast and Warning Critique
Danielle's track was "well-behaved" and, overall, very well-predicted
in the official forecasts. Table 3 shows the average
track forecast errors from the various objective techniques as well as
the average official errors for Danielle. It can be seen that, at all time
periods, the mean official forecast errors are substantially lower than
the most recent ten-year averages. Also, the mean official forecasts were
as good as, or (in most cases) better, than the corresponding objective
track predictions. An inspection of a number of individual NHC forecasts
reveals that the recurvature of the hurricane was forecasted quite accurately.
Most of the official forecasts predicted too much strengthening,
by as much as 30 to 40 knots in the 48- to 72-h time ranges. A similar
bias can be seen in the output from the Statistical Hurricane Intensity
Prediction Scheme (SHIPS), which is perhaps the best available intensity
forecast tool.
A tropical storm warning
was issued for Bermuda at 1500 UTC 1 September since the southern portion
of Danielle's circulation was likely to affect that island. Sustained winds
of tropical storm force occurred at Bermuda about 20 h after the issuance
of this warning as the center passed well to the northwest and north. No
other warnings (or watches) were necessary for this tropical cyclone.
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