Hurricane Estelle was the third major hurricane
in the eastern North Pacific basin during the 1998 season.
Estelle moved over the open waters of the east Pacific Ocean and crossed 140°W into the
Central Pacific Hurricane Centers'
area of responsibility before dissipating.
a. Synoptic History
Estelle can be traced back to a tropical wave
which moved off the west Africa coast on 18 July. Its trek across the Atlantic was uneventful with
sporadic convective activity but no signs of development. Rawinsonde data from St. Martin showed that
the wave passed through the eastern Caribbean on the 24th. Subsequent upper-air observations
from Grand Cayman and Key West, Florida showed the wave passage through the western Caribbean and the
southeast Gulf of Mexico on 26 July. After moving across Central America, the wave emerged
into the eastern Pacific on the 27th/28th. Early on the 29th,
meteorologists in the Tropical Analysis and Forecast Branch
(TAFB) of the Tropical Prediction Center, and the
Satellite Analysis Branch (SAB) of the
National Environmental Satellite Data Information Service
initiated Dvorak classifications on the system which was located a couple of
hundred miles south-southeast of Acapulco, Mexico. By the afternoon of 29 July, visible satellite imagery showed distinct banding
features with TAFB and SAB
meteorologists assigning a Dvorak T-number of 2.0, i.e. 30 knots, to the disturbance.
On this basis, the system is estimated to have become a tropical depression
at 1800 UTC 29 July about 150 n mi southeast of Manzanillo, Mexico (see Figure 1
(43K GIF) and Table 1). The depression continued to become better organized
through the evening with increasing convection and distinct upper-level outflow. The system
was upgraded to tropical storm Estelle at 0600 UTC 30 July.
Estelle continued to intensify on the 30th, and reached hurricane strength at
0600 UTC 31 July while located about 480 n mi south-southeast of the southern tip of Baja
California. For the next seven days, Estelle moved on a general west-northwestward course
under the influence of a large mid-tropospheric anticyclone. Intensification continued, with a
well-defined eye, 20 to 30 n mi in diameter, becoming evident in both visible
satellite imagery and SSM/I 85 GHz channel data during the afternoon of 1 August. Estelle is estimated to
have reached a peak intensity of 115 knots at 0600 UTC 2
August. Estelle began to weaken and by the afternoon of the 2nd, the eye was no longer
discernable in satellite imagery and the extent of deep convection had diminished. Over the next
several days, the system continued to weaken while moving on a general west-northwest course
over progressively cooler waters. Estelle weakened to a tropical storm early on 4 August. By
1200 UTC 5 August, after being devoid of deep convection for 12 to 18 hours, the system
degenerated to a tropical depression while located about 1350 n mi west-southwest of the
southern tip of Baja California. Some limited deep convection temporarily returned early on the
6th, mainly over the northern semicircle, but the activity waned by mid day. Estelle crossed
140°W by the afternoon of the 6th. The depression moved on a general west-northwestward
course for the next few days while maintaining a well-defined low-level circulation center.
Upper-level southwesterly vertical wind shear, induced by an upper-level trough near the
Hawaiian Islands, hampered deep convective development, and the system dissipated on the
evening of the 8th while located about 350 n mi east-northeast of the Hawaiian Islands.
b. Meteorological Statistics
As is usually the case for eastern Pacific tropical cyclones,
satellite images were the sole data source for position and intensity estimates.
Figures 2 (24K GIF) and
3 (25K GIF) are the curves of
minimum central sea-level pressure and maximum one-minute sustained 1-minute "surface" (10
meters above ground level) wind speed, respectively, as functions of time. Also plotted are the
observations on which the curves are based, including Dvorak-technique
estimates (from the TAFB, SAB,
and the U.S. Air Force Weather Agency (AFGWC in figures).
Subjective-based Dvorak T-numbers peaked at 6.0, 115 knots, at 0600 UTC 2 August while the
objective-based Dvorak estimates of T-6.0 held from late afternoon on the 1st
until early morning on the 2nd.
Besides the standard satellite-based Dvorak intensity estimates derived from
the GOES and DMSP satellites, the ERS polar-orbiting satellite made a partial pass over Estelle
on 4 August. The ERS scatterometer winds were used, on occasion, to adjust the 34-knot wind
radii.
c. Casualty and Damage Statistics
No reports of casualties or damage associated with Estelle have been received
at the National Hurricane Center.
d. Forecast and Warning Critique
The official NHC average track forecast errors (excluding tropical depression
stage) were 33 (23 cases), 83 (21 cases), 71 (19 cases), 70 (17 cases), and 90 n mi (13 cases),
respectively, for the 12-, 24-,36-, 48-, and 72-hour forecast periods. These are considerably
below the most recent 10-year average forecast errors (1988-1997). With the exception of the
24-hour period, the NHC average official track forecast errors for Estelle were lower than the
averages from all of the operationally available track prediction models.
The official average absolute intensity errors were 6 (16 cases), 10 (14 cases),
14 (12 cases), 18 (10 cases), and 17 (6 cases) knots, respectively, for the 12-, 24-, 36-, 48-,
and 72-hour forecast periods. The average absolute intensity forecasts errors from SHIFOR are
comparable while the Statistical Hurricane Prediction Scheme SHIPS errors were lower for all
forecast periods.
No watches or warnings were required for Estelle.
Acknowledgments:
The data west of 140°W in Table 1 and all figures was provided by the
Central Pacific Hurricane Center.
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