Hurricane Karl was one of four hurricanes in existence over the
Atlantic basin at one time. It remained over water without any direct effects to land.
a. Synoptic History
Hurricane Karl developed from a small low of non-tropical origin that was tracked
from the coast of the Carolinas beginning on 21 September. Deep convection became better organized
as the low moved eastward and the "best track" indicates that a
tropical depression formed from the
disturbance near 1200 UTC 23 September while centered about 50 n mi west-northwest of Bermuda
(Fig. 1 [27K GIF] and Table 1).
Convective banding increased and the system became Tropical Storm
Karl that evening. The tropical cyclone
began moving east-southeastward about this time.
Satellite imagery showed the gradual development of a more symmetrical cloud
pattern with the center becoming embedded within the coldest convective tops. Karl became a
hurricane near 1200 UTC 25 September while centered about 550 n mi east-southeast of Bermuda. At
this time, Hurricane Georges was over the Straits of Florida,
Hurricane Ivan was over the North
Atlantic about 500 n mi west-southwest of the Azores, and Hurricane Jeanne was over the tropical
Atlantic about midway between Africa and the Lesser Antilles. Thus, Karl became the fourth hurricane
to co-exist over the Atlantic. According to records at the NHC, the last time four hurricanes were in
existence in the Atlantic at the same time was on August 22, 1893. Records also note that on
September 11, 1961, three hurricanes and possibly a fourth existed.
Karl began to move toward the northeast in response to a large mid- to upper-level
trough to the west of the hurricane. A well-defined eye developed and it is estimated that
Karl first reached a maximum intensity of 90 knots at 0000 UTC 27 September
while centered about 875 n mi east-northeast of Bermuda. The eye remained distinct for at least six
hours, after which time the hurricane started to weaken primarily due to increasing upper-level shear.
The hurricane accelerated toward the northeast and weakened to a tropical storm by
0000 UTC 28 September while centered over 23C water about 175 n mi west-northwest of the
westernmost Azores. Karl continued moving over increasingly cooler waters and became extratropical
later on the 28th as the circulation center became well removed from any deep convection. The
extratropical cyclone was tracked to south of Ireland by late on the 29th.
b. Meteorological Statistics
Figures 2 (12K GIF) and 3 (13K GIF)
show the curves of minimum central pressure and maximum one-minute wind speed, respectively, versus time, along with the observations
on which they are based. As usual for a tropical cyclone not threatening land, satellites provided the primary
source of observational data. Dvorak technique location and intensity estimates from the
satellite data were produced by the Air Force Weather Agency (AFGWC in figures),
the NOAA Synoptic Analysis Branch (SAB) and the NOAA
Tropical Analysis and Forecast Branch (TAFB). The highest official
Dvorak T number was 5.0 (90 knots) from TAFB and
SAB near 0000
and 0600 UTC 27 and is the basis for estimating the peak intensity near these times.
c. Casualty and Damage Statistics
There were no reports of casualties or damage from Karl received at the NHC.
d. Forecast and Warning Critique
The NHC average official track forecast errors for Karl (excluding the tropical
depression and extratropical stages) were 66 (15 cases), 138 (13 cases), 187 (11 cases), 229 (9 cases)
and 472 n mi (5 cases), respectively, for the 12-, 24-, 36-, 48- and 72-hour forecast periods. These
were all larger than the 1988-1997 average errors of 47, 88, 127, 166 and 248 n mi for the same time
periods. The NHC average official track forecast errors were similar to the averages from the
operationally available track prediction models through 48 hours. No meaningful comparisons can be
made at 72 hours given the limited number of cases available. The guidance models properly indicated
the observed increase in forward speed toward the northeast, but varied considerably on the rate of this
acceleration.
The NHC official intensity forecasts showed a distinct negative bias (i.e., intensity was
underestimated). The largest intensity forecast error occurred 72 hours prior to peak intensity and was
45 knots too low. Early official forecasts did not anticipate significant intensification due to a
predicted increase in vertical shear.
Watches and warnings were neither issued nor necessary for Karl.
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