a. Synoptic History
Alma, the first named tropical cyclone
of the 1996 eastern north Pacific hurricane season developed about 240 n mi
south of Acapulco Mexico. Although it is difficult to identify
the origin of this disturbance,
it appears to be related to the southern extension of the same
tropical wave which triggered
Tropical Storm Arthur
in the Atlantic. The incipient disturbance
crossed from the southwestern Caribbean to the eastern pacific
between the 17th and the 18th of June as indicated by upper-air
observations from Central America and satellite images. This
disturbance was accompanied by an anomalous upper-level anticyclone (200 mb) as shown in
Fig. 1a (100K GIF).
The disturbance moved into the eastern Pacific over warmer
than normal waters. Initially, stronger than normal 200 mb
northeasterly winds and 850 mb southwesterly winds prevailed as
indicated in Figs. 1b and 1c (100K GIF).
This pattern resulted in a shearing environment and a low-level
center located to the
northeast of the convection. However, the shear was not strong
enough to prevent strengthening and the deep convective activity
gradually became aligned with the low-level center. Then, a
tropical depression
formed at 0000 UTC June 20 and reached tropical storm
intensity by 1800 UTC as indicated by
satellite intensity estimates.
When the shear relaxed, Alma intensified and became a
hurricane at 0000 UTC 22 June while moving on a general
northwest track. A mid-level trough located in the vicinity of
Baja California in combination with a mid-to upper- level low
over the southwestern Gulf of Mexico steered Alma slowly
northward toward the southwest coast of Mexico. On that heading,
Alma reached its maximum intensity of 90 knots and minimum
pressure of 969 mb at 1200 UTC 23 June. At that time, objective
and subjective T-numbers were oscillating near 5.0 on the Dvorak
scale. Figure 2 (494K GIF)
is a visible satellite image of Hurricane Alma just prior to landfall.
The steering flow collapsed and Alma began to drift near
the coast. Alma made landfall near the town of Lazaro Cardenas
but it did not move farther inland. The center moved back over
water but a portion of the circulation was involved with land.
Alma meandered for another 36 hours near the coast and never
reintensified. Apparently, the inner core circulation was
severely disrupted by the steep topography of Mexico. It
gradually weakened until dissipation while moving slowly on a
track parallel and not far from the coast.
Alma's track is shown in Fig. 3 (36K GIF).
Table 1 is a listing, at
six-hour intervals, of the best-track
position, estimated minimum central pressure and maximum 1-minute surface wind speed.
b. Meteorological Statistics
The best track pressure and wind curves as a function of time shown in
Figures 4 (22K GIF) and
5 (24K GIF)
are based on satellite intensity estimates from the
Tropical Analysis and Forecast Branch
of the Tropical Prediction Center (TAFB), and denoted as TSAF in the
figures. It was also used estimates from the
Satellite Analysis Branch (SAB) and the
Air Force Global Weather Central (AFGWC).
Alma was an intense hurricane with a very small diameter. The
strongest winds were concentrated within a small area surrounding
the eye.
This was suggested by sparse surface observations within
the area of influence of Alma, as well as satellite images.
Images from Acapulco and Cuyutlan radars, provided by the
"Servicio Meteorologico Nacional de Mexico"
confirmed that Alma was a small diameter tropical cyclone. These radar images,
received in near real-time, were extremely useful to track Alma.
There are no reports of measured strong winds received at this
time. Manzanillo reported 68.8 mm of rainfall in 24 hours.
c. Casualty and Damage Statistics
Newspapers reports from Mexico stated that three people
were killed by Alma in a small town near Lazaro Cardenas when
their house collapsed. Numerous houses were damaged and power
failed in various coastal towns where roads were covered by
debris and water. In Zihuatanejo, several houses and trees were
also damaged. There are unconfirmed reports (Miami Herald, June
25, 1996) that at least 17 people were killed by flooding in
Puebla, about 300 n mi to the east of the landfall point. These
rains were probably related to Alma.
d. Forecast and Warning Critique
Table 2 shows the preliminary forecast errors
for Hurricane Alma. The official forecast errors were by far much smaller than
the 1985-1994 average, and generally better than the track
models. The purely dynamical models performed poorly, in particular the AVN and
GFDL models. One attributes
such large errors produced by dynamical models to the lack of upper-air data
over the ocean south and west of the hurricane. Mexican upper-air data were
available at all times during Alma. Numerical
track forecast by the "Universidad Autonoma de Mexico" were of
comparable accuracy to U.S. dynamical models. However, model
output from Mexico was received at the NHC only twice during
Alma.
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