a. Synoptic History
Hurricane Isidore formed from a tropical wave
that had a well-defined cyclonic circulation of clouds and was clearly marked at
mid-levels in the Dakar sounding data when it crossed the west
coast of Africa on 22 September 1996. Deep convection increased
and satellite analysts provided the first Dvorak scale
T-numbers on the 23rd, while the circulation passed to the south of the Cape
Verde Islands. Thunderstorms became more concentrated, T-numbers from the NOAA
Tropical Analysis and Forecast Branch (TAFB)
increased to 2.0, and ship reports suggested the formation of a surface
circulation by 1200 UTC on the 24th. The "best track"
begins at that time (Fig. 1 [54K GIF],
Table 1), indicating the start of the
tropical depression stage of Isidore.
The tropical cyclone
was initially located to the south of a deep-layer anticyclone.
It moved toward the west-northwest at 15-20 knots and intensified.
An intense convective band wrapped around the
center and the system became a
tropical storm on the 25th.
Further intensification ensued, an eye
began to appear intermittently, and Isidore reached
hurricane intensity on the following day.
The mature and dissipating stages of Isidore were influenced by a
well-defined mid- through upper-level low that was quasi-stationary
near 25°N 45-50°W through the 25th. The low then weakened and
lifted northward to near 35°N, but was reinvigorated there by its
interaction with a mid-latitude short-wave trough which passed by
to the north on the 27-28th. The steering currents on the east
side of the low gradually turned Isidore in a general northward
direction. The forward speed slowed to about 10 knots during the
turn on the 28th, but then increased to 20 knots on 1 October.
During this period, Isidore reached its estimated maximum intensity
of 100 knot winds. The eye disappeared on the 29th and upper-level
westerly to southwesterly winds of around 60 knots contributed to
a shearing and weakening of Isidore, down to a tropical storm with an exposed low-level cloud
center on the 29th, and then to a tropical depression on the 1st. Deep convection dispersed and
Isidore transformed to extratropical
status on the 2nd.
b. Meteorological Statistics
The "best track" (Table 1) was obtained from the
data presented in Figs. 2 (31K GIF) and
3 (27K GIF). Those figures show
Isidore's estimated central pressure and maximum one-minute wind speed, respectively,
versus time. Position and intensity estimates from satellite pictures were provided by the
NOAA
Synoptic Analysis Branch
(SAB) and TAFB, and by the
Air Force Global Weather Central (AFGWC).
They are the basis for showing Isidore's maximum wind speed at 100
knots and minimum pressure at 960 mb. Isidore passed through the eastern part of the
NOAA drifting buoy network. Observations from
those platforms helped define the western part of the cyclone's low-level wind field.
The ship Magnific reported southeast (140°) winds of
58 knots at
1200 UTC on the 30th of September, while located at 25.1°N 37.2°W,
about 175 nm from the center of Isidore. The reliability of that
measurement is in doubt because the estimated maximum surface wind
near the center was 50 knots at that time. This was the only
surface sustained wind report of 34 knots or higher to be possibly
associated with Isidore.
c. Casualty and Damage Statistics
No reports of casualties or damages were received.
d. Forecast and Warning Critique
Table 2 lists track forecast error statistics.
The official forecasts and many of the numerical models had track prediction
errors that, on average, were at least 33% smaller than normal.
The GFDL and GFDI performed best.
CLIPER errors were very large at 72 hours.
NHC intensity forecasts were good, in general, but did not show
fast enough weakening on the 27th and 28th.
Hurricane and tropical storm watches and warnings were neither
issued nor necessary.
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