a. Synoptic History
Hilda may be traced back to a tropical wave
that departed western Africa on 26 July. Although the wave did spawn a cluster of
convection northeast of Puerto Rico on 31 July, this system generally remained rather
nondescript during its passage over the Atlantic basin during the ensuing week or so,
and then across Central America on 3-4 August. Convection associated with the wave
increased to the southwest of the Gulf of Tehuantepec on 5 July. Meteorologists at the
Tropical Analysis and Forecast Branch (TAFB) began to track the
center of the weather system on satellite images
on 7 August. The cloud pattern became decidedly better-organized on the 9th, as
convection became more consolidated and curved cloud bands were readily
discernible. Qualitative evaluation of satellite imagery suggested that the system
became Tropical Depression Ten around 0000 UTC 10 August,
about 920 n mi south-southwest of the southern tip of Baja California. The six-hourly
best track positions
and intensities for this cyclone are listed in Table 1.
Figure 1 (15K GIF) shows the overall track of the
tropical cyclone. Initially the depression
moved northward, but it soon turned toward the northwest and west, before eventually
settling on a west-northwestward heading at 7 to 14 knots. Throughout much of its
existence, this tropical cyclone was under the influence of a mid- to upper-tropospheric
cyclone located 5 to 10 degrees to its northwest to west.
From the 10th to the 12th of August, the tropical cyclone was apparently steered around this feature.
Strong vertical shear, produced by southwesterly flow associated with the mid- to upper-level
cyclone, prevented the system from intensifying until around 1800 UTC 11 August,
when the shearing relaxed enough to allow the depression to strengthen into a
tropical storm. Hilda reached its peak intensity of
45 knots about 12 hours later, and maintained more or less the same
strength for 24 hours. Thereafter, southerly shearing increased as the abovementioned upper
cyclone's influence again became dominant, and Hilda began to weaken gradually.
Because the upper cyclone moved west-northwestward essentially in tandem with
Hilda, winds over the tropical cyclone remained unfavorable for re-intensification.
The system decreased below tropical storm strength by early on the 14th, and it
dissipated around 0600 on the 15th.
b. Meteorological Statistics
As is so often the case for eastern Pacific storms,
in situ observations were absent for Hilda, and satellite images were the sole data source.
Figures 2 (11K GIF) and 3 (13K 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
Dvorak-technique estimates from the TAFB, the
Synoptic Analysis Branch (SAB),
and the U.S. Air Force Global Weather Center
(AFGWC) using geostationary and polar-orbiting satellite imagery.
c. Casualty and Damage Statistics
No reports of casualties or damage associated with Hilda have been received at the
National Hurricane Center.
d. Forecast and Warning Critique
Since Hilda was a tropical storm for less than 72 hours, the number of forecasts to
verify was small and the average errors are probably not meaningful. Nonetheless, the
mean official forecast errors were 30 n mi at 12 hr (8 cases), 33 n mi at 24 hr (6 cases),
60 n mi at 36 hr (4 cases), and 59 n mi at 48 hr (2 cases). These are considerably
smaller errors than the long-term averages. The average official forecast errors were,
in general, lower than those for the various track prediction models.
In general, the intensity of Hilda was overforecast. This was primarily because the
official forecasts allowed for the possibility that the shearing effect from the nearby
upper-level low would lessen.
Watches and/or warnings for land areas were not required for Hilda.