a. Synoptic History
Blas may be traced back to a tropical wave that emerged
from the west coast of Africa on 8 June. Although the wave did spawn intermittent clusters of convection,
this system generally remained rather nondescript during its passage over the Atlantic Ocean and Caribbean Sea.
Cloudiness and convection increased off the Pacific coast of Central America on 19 June. The first satellite
classifications were received on 20 June. Convective banding increased as a broad cyclonic circulation
became established. The "best track" indicates that a
tropical depression formed from the disturbance
near 0000 UTC 22 June while centered about 500 n mi south of the Gulf of Tehuantepec
(Fig. 1 [30K GIF] and Table 1).
During the first few days of its existence, the tropical cyclone moved generally
northwestward to west-northwestward near 10 knots in response to deep-layer-mean steering. This
track roughly paralleled the southwest coast of Mexico, with the center
remaining 250 to 300 n mi offshore.
Deep convection became concentrated near the circulation center and the depression
strengthened into Tropical Storm Blas at 1200 UTC 22 June
while centered about 350 n mi south of Puerto Angel, Mexico. Convective banding increased and Blas became a
hurricane at 1800 UTC 23
June while centered about 300 n mi south-southwest of Acapulco, Mexico. As the upper-level outflow
became well established, Blas intensified with an eye appearing in
satellite imagery on 24 June. Strengthening continued and it is estimated that Blas reached its peak intensity of
120 knot one-minute surface winds and 943 mb minimum central pressure
near 0600 UTC 25 June while centered about 500 n mi south-southeast of the southern tip of Baja
California. The eye remained visible for a few more days, although cloud-top temperatures
surrounding the eye gradually warmed.
A deep-layer-mean ridge to the north turned Blas toward a more westward track by 26
June. This movement persisted through the remainder of the tropical cyclone's life. Blas moved over
cooler water and weakened to a tropical storm by 1200 UTC 28 June while centered about 700 n mi
west-southwest of the southern tip of Baja California. The cyclone
weakened to a tropical depression at 0000 UTC 30 June and was considered dissipated by 1 July although a low-level cloud swirl
continued westward for several more days, passing a few hundred miles south of the Hawaiian Islands on 5 July.
b. Meteorological Statistics
Figures 2 (19K GIF) and
3 (18K GIF) show the curves of minimum central pressure and
maximum one-minute wind speed, respectively, versus time, along with the observations on which
they are based. As usual for an eastern Pacific tropical cyclone, satellites provided the primary source
of observational data. Dvorak technique location and intensity
estimates from the satellite data were produced by the
Air Force Weather Agency (AFGWC),
the NOAA Synoptic Analysis Branch (SAB)
and the NOAA Tropical Analysis and Forecast Branch (TAFB). Objective Dvorak T numbers
ranged from 6.0 (115 knots) to 7.0 (140 knots)
between 0000 and 0600 UTC 25 June. The highest 3-hour average objective Dvorak
T numbers were also at these times and is the basis for estimating the peak intensity near 0600 UTC on
this day. A new objective technique developed by the University of Wisconsin consistently estimated
the intensity at approximately 125 knots (Dvorak Current Intensity
number 6.4) from 0315 to 1900 UTC 25 June. Although this new scheme shows promise, it will not be
used to replace the more subjective technique used operationally until further tests are done with
ground truth.
c. Casualty and Damage Statistics
The Associated Press attributed four deaths in the Mexican state of Michoacan to
Blas. Three boys and their grandmother living in a wood and cardboard home in the village of El
Chaparro were killed in a hillslide. At the time the deaths occurred, late on 23 June, the center of Blas
was passing more than 250 n mi offshore and satellite pictures showed that the main cloud shield of
the hurricane was offshore as well. Therefore, the National Hurricane Center (NHC) is not considering
these deaths to be a direct effect of the hurricane. There were no other reports of casualties or damage
from Blas received at the NHC.
d. Forecast and Warning Critique
The NHC average official track forecast errors for Blas (excluding the tropical depression stage) were
29 (28 cases), 50 (26 cases), 72 (24 cases), 93 (22 cases) and 131 n mi (18 cases), respectively, for the
12-, 24-, 36-, 48- and 72-hour forecast periods. These were all lower than the 1988-1997 average
errors. The NHC average official track forecast errors were lower than the averages from all of the
operationally available track prediction models.
The NHC official intensity forecasts showed a distinct negative bias (i.e., intensity was
underestimated). The largest intensity forecast error occurred on the initial advisory. The 72-hour
forecast from this advisory that verified near the time of maximum intensity was 40 knots too low.
Watches and warnings were neither issued nor necessary for Blas.
![[NCEP Logo]](graphics/ncep.gif)
![[1998 East Pacific Hurricane Season]](graphics/smalllogo.gif){kind=small}
