a. Synoptic History
A broad area of strong cyclonic low-level westerly flow became
established over the eastern Pacific east of 100W while Tropical
Storm Andres was near the coast of El Salvador. A tropical
disturbance grew within this perturbed environment and developed
into Tropical Depression Two-E in the Gulf of Tehuantepec.
Blanca's track begins at 1800 UTC 9 June. Under an upper-level ridge and
over a warm ocean, the depression quickly became Tropical Storm
Blanca. Initially, Blanca moved toward the west-northwest toward
the coast of Mexico. However, a strong mid-level ridge over
northern Mexico later steered the storm westward away from the coast.
Blanca reached its maximum intensity of 40 knots
and a minimum pressure of 1002 mb at 0000 UTC 11 June. At that time, there was
a good outflow and some banding features while the T-numbers were
2.5 and 3.0 on the Dvorak scale. On the contrary,
surface observations indicated that Blanca's circulation was not well
defined and the SSM/I 85 GHz images revealed that the
tropical cyclone was quite small.
Soon thereafter, a weakening process began. Satellite and
surface data suggested that the system no longer had a closed
circulation at 1200 UTC 12 June. Blanca was then declared dissipated.
Blanca's track is shown in Fig. 1 (13K 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 2 (13K GIF) and
3 (12K GIF) are based on satellite intensity estimates
from the Tropical Analysis and Forecast Branch (TAFB), estimates from the
Satellite Analysis Branch (SAB)
and from the Air Force Global Weather Center
c. Casualty and Damage Statistics
There are no reports of damages or casualties associated with Blanca.
d. Forecast and Warning Critique
Initially, most of the guidance and the official forecast,
indicated a west-northwest track toward the coast of Mexico. This
prompted a tropical storm warning
for a portion of the coast from Puerto Angel to Acapulco at 0300 UTC 10 June.
Warnings were discontinued the next day when Blanca turned westward away from the
coast. Since Blanca was a short-lived storm, the evaluation of the
average forecast track errors is not very meaningful.
Blanca never reached hurricane
strength as anticipated in the official forecast and suggested by SHIPS97 (intensity
forecast model). The official forecast was based on the fact that Blanca
was moving over surface temperatures that were 2.5 C warmer than
normal and under a high pressure ridge. It is not clear why the
intensification process stopped. On the other hand, global and the
models consistently weakened the tropical cyclone.