Isis made two landfalls in Mexico, one in southern Baja California as a
tropical storm, and another near Los Mochis as a category one
hurricane on theSaffir/Simpson Hurricane Scale.
a. Synoptic History
A tropical wave, which on 19 August produced the tropical depression
that became Atlantic Hurricane Bonnie, moved westward across the Caribbean Sea from 21-24 August
and crossed Central America on 25 August. As the wave moved into an area covered by a large lower-tropospheric
cyclonic circulation (or, at least, cyclonic turning) covering southern Mexico and the adjacent waters, its
forward speed slowed. A broad area of disturbed weather, associated with these atmospheric features,
persisted over the tropical eastern north Pacific between 90W-110W from 26-29 August. A more
localized area of low-level cyclonic rotation became evident on 29 August a little over 500 n mi
south-southeast of Cabo San Lucas. There was little change in organization for the next day or two. On 31
August, the system was characterized by two main areas of dense cloudiness located a few degrees
northeast and southwest of the center of circulation. By 0000 UTC 1 September, even though deep
convection was still not well-organized, the low-cloud circulation was sufficiently well-defined so that
the system could be designated as a tropical depression, centered slightly less than 300 n mi south of
Cabo San Lucas (Table 1). The overall track of this
tropical cyclone is displayed in
Figure 1 (16K GIF).
As the cyclone moved slowly northward to north-northwestward, it gradually
intensified. Ship observations indicate that the cyclone strengthened into Tropical Storm Isis by 1800
UTC 1 September, located about 170 n mi south of Cabo San Lucas. With a 500 mb trough, extending
south-southwestward from the California/Arizona border, providing a southerly steering flow, Isis
moved northward at about 10 knots. Its center made a first landfall,
over extreme southeastern Baja California, at 1200 UTC 2 September. Isis then moved slightly east of north,
over the Gulf of California while strengthening to a 65-knot hurricane,
an eye becoming apparent
on visible satellite imagery. The hurricane more or less maintained this strength until the center made its
final landfall in Mexico at Topolobampo, close to Los Mochis, around 0300 UTC 3 September. Isis
weakened to a tropical storm a few hours after landfall, and to a depression by 1800 UTC 3 September.
The system dissipated over the mountains of Mexico soon thereafter.
b. Meteorological Statistics
Figures 2 (11K GIF) and 3
(11K 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 TAFB, the
Synoptic Analysis Branch, SAB, and the U.S.
Air Force Global Weather Agency,
AFGWC in the figures) using satellite imagery.
Islas Marias reported south-southwesterly winds at a ten-minute average speed of
40 knots at 0000 UTC 2 September. San Jose del Cabo reported
270/20 knots (ten-minute average) with gusts to 40 knots at 1445 UTC
The following 24-hour rainfall totals were reported: 12.99 inches at Los Cabos, Baja
California Sur; 8.86 inches in Sinaloa; and 4.72 inches in Sonora.
Table 2 lists ship reports of 34-knot or greater winds associated with Isis.
c. Casualty and Damage Statistics
According to information provided by the government of Mexico to the
World Meteorological Organization,
Isis caused 14 deaths in Mexico, and destroyed 769 houses.
d. Forecast and Warning Critique
Mean official track forecast errors for Isis at 12, 24, and 36 hours were 36, 79, and 87 n
mi, respectively. Although these errors are low, there were too few cases six, four and two respectively)
for them to be considered statistically meaningful. In any case, the cyclone moved fairly close to the
track predicted in the NHC advisories.
The average official absolute wind speed forecast errors were also rather low;
nonetheless, the slight strengthening of Isis to a hurricane was not forecast.
Table 3 lists the watches and warnings that the government of Mexico issued for their
country. The center of Isis reached Baja California 15 hours after the issuance of a tropical storm
warning. There was only about six hours between the time of issuance of a hurricane warning on the
coast of mainland Mexico and the time of arrival of the center.
Table 3. Watch and warning summary, Hurricane Isis, September, 1998.
|1/2100||tropical storm warning issued
||Baja California south of a Dolores to Puerto Cortez line|
|2/0300||tropical storm warning extended
||Baja California south of a line from Santa Rosalia to Punta Abreojos|
|tropical storm warning issued||coast of Mexico from El Dorado to Guaymas|
|2/2100||hurricane warning issued
||coast of Mexico from El Dorado to Bahia Kino and east coast of Baja California from Dolores to Punta San Gabriel|
|tropical storm warning shifted northward
||west coast of Baja California from Puerto Cortez to Punta Eugenia |
|3/0900||all warnings discontinued
|hurricane warning replaced by tropical storm warning
||coast of Mexico from Los Mochis to Guaymas|
|3/1800||tropical storm warning discontinued
||coast of Mexico from Los Mochis to Guaymas|