a. Synoptic History
This tropical cyclone can be
traced to a cloud cluster that moved across Central America on 10 and 11 July.
Moving westward, the cluster showed signs of a low-level circulation by the 15th and
it became a depression
on the 17th, about 350 n mi south-southeast of the southern tip of Baja California.
The track of Cosme begins at this time and is plotted in
Fig. 1 (38K GIF).
Table 1 is a listing, every
six hours, of position, maximum one-minute surface wind speed,
and minimum central sea level pressure.
Movement was slow and toward the northwest from the 17th to
the 20th, while the depression gradually strengthened to a
Cosme then turned and moved a little faster toward the
west-southwest and weakened. By the 22nd, only a swirl of low
clouds remained, about 850 n mi west-southwest of the southern tip
of Baja California.
b. Meteorological Statistics
Figures 2 (24K GIF) and
3 (26K GIF) show the curves
of minimum sea-level pressure and maximum one-minute wind speed as a function
of time, along with the data on which they are based. All of the data in
both figures are estimates from satellite imagery using the
A ship was located about 60 n mi east of the center of Cosme at 1800 UTC
on the 18th and reported a sustained wind speed of 15 knots and 1006.1-mb
pressure. At this time, satellite-based wind speed estimates from both the
Tropical Analysis and Forecast Branch and from the
Satellite Analysis Branch
were 45 kts.
One might normally expect 35-knot winds to extend out to 60 n mi from the
center of a tropical storm, unless it tiny. Cosme was not tiny.
This suggests considerable uncertainty in the determination of the
maximum sustained wind in a tropical cyclone.
c. Casualty and Damage Statistics
Cosme did not affect land and there are no casualties or damage.
d. Forecast and Warning Critique
Average track forecast errors for Cosme range from 84 n mi at
24 hours to 188 n mi at 48 hours to 319 n mi at 72 hours. These
errors are somewhat higher than the long-term errors for the
eastern Pacific basin and were influenced by the turn on the 20th
from northwestward to west-southwestward. Even after the turn
occurred, the track guidance based on the steering from the
Aviation global/spectral model insisted on a northwestward motion.
The simple climatology-persistence model had smaller track errors
than all of the guidance models and the official forecast.