Karen was a minimal tropical storm
that did not affect land. However, it occurred during a very active period for
tropical cyclones and proved to be
noteworthy for its interaction with nearby Hurricane Iris.
a. Synoptic History
Karen originated from a tropical wave
that moved off the west coast of Africa to the eastern tropical Atlantic on 23 August.
This was a very active day in the tropics with Hurricane Humberto
midway between Africa and the Lesser Antilles, Hurricane Iris about
500 n mi east of the Lesser Antilles, Tropical Storm Jerry near
southeast Florida and Tropical Storm Gil in the eastern North
Pacific. Based on ship and island reports, NHC surface analyses
indicated a broad area of low pressure just off the west coast of
Africa in association with the tropical wave.
were initiated by the NESDIS
Synoptic Analysis Branch
(SAB) and the NHC Tropical Analysis and Forecast Branch (TAFB)
on the 23rd and 24th, respectively. The organization of the cloud pattern fluctuated for a couple of days,
and the system could have become a tropical depression
as early as on the 24th when the center
appeared to be southwest of the Cape Verde Islands.
Some reorganization occurred and satellite analysts from SAB, NHC and
Air Force Global Weather Central (AFGWC) all agreed on a T1.5
classification for the first time near 1200 UTC 26 August, when
satellite imagery showed a well-defined low-level cloud center
exposed to the east of a cluster of deep convection. The post-
analysis "best track" begins Tropical
Depression Twelve at that time (Table 1
and Fig. 1 [51K GIF])
about 500 n mi west of the Cape Verde Islands. In relation to Tropical Depression Twelve,
Hurricane Humberto was centered about 900 n mi west-northwest
and Iris, which had weakened to a tropical storm, was centered
over the Lesser Antilles about 1500 n mi west. The depression was moving generally
toward the west-northwest at 10 to 14 knots with the low- to mid-level flow.
Deep convection increased and, based on satellite
classifications, Tropical Depression Twelve strengthened into
Tropical Storm Karen at 0600 UTC 28 August. Hurricane Humberto
had moved northward by this time and was centered about 750 n mi to the
northwest of Karen. Tropical Storm Iris had also moved northward
to a position just north of the Leeward Islands, about 1100 n mi to
the west of Karen.
Humberto continued moving northward and then
northeastward away from Karen. The steering flow weakened somewhat in the wake of
Humberto, and Karen slowed its west-northwestward
motion from 10 to about 4 knots between 28 and 31 August. Karen gradually approached
the even slower moving Iris, which had again strengthened to a
hurricane by late on the 28th. The upper-level outflow from the
stronger Iris resulted in northerly shear over Karen, and the low-
level center of Karen was exposed to the north of the accompanying
convective activity from 28 to 31 August. During this period,
Karen's maximum sustained winds of 45 knots were estimated to have
occurred. Karen was centered about 600 n mi east-southeast of Iris
on the 31st, and began moving more toward the northwest, caught in
Iris' stronger circulation.
Convective activity associated with Karen became disorganized
on 1 September as the tropical storm accelerated and moved
cyclonically around the east side of Iris. Karen weakened to a
tropical depression on 2 September. However, a tightly wrapped
swirl of low- to mid-level clouds could still be seen in satellite
imagery moving to the north of Iris late on the 2nd. The remnant
vortex of Karen was finally absorbed into the stronger circulation
of Hurricane Iris on 3 September when located approximately 175 n
mi to the northwest of the center of Iris.
b. Meteorological Statistics
Figures 2 and 3 (49K GIF)
show the curves of minimum central pressure and maximum one-minute wind speed, respectively,
as a function of time. The observations on which the curves are based are also plotted and
consist primarily of Dvorak-technique estimates using satellite imagery from SAB,
TAFB (formerly TSAF, as in the figures)
and the AFGWC. One tropical storm force wind
measurement reported from a ship at 2100 UTC 31 August is plotted in addition to the satellite estimates.
Karen was not a threat to land, and therefore, did not require aircraft reconnaissance.
However, after flying nearby Hurricane Iris on 2 September,
U.S. Air Force Reserve aircraft
provided one operational center fix on Karen.
c. Casualty and Damage Statistics
There were no casualties or damages reported in association with Karen.
d. Forecast and Warning Critique
During the time when Karen was of tropical storm strength, the
mean official track forecast errors were 55 (19 cases), 100 (17
cases), 113 (15 cases), 147 (13 cases) and 252 (9 cases) n mi at
12, 24, 36, 48 and 72 hours respectively. These errors were about
the same as the long-term averages from the previous ten years at
12 and 24 hours and somewhat lower at the longer time periods.
However, the track was not well forecast late in the period of
interaction with Iris.
The intensity forecasts were generally within 5 to 10 knots of
the verifying best track intensities.