a. Synoptic History
Rawinsonde data from Dakar indicated that a well-defined
tropical wave emerged
from the west coast of Africa on 16 July. The wave was tracked across the Atlantic
in satellite imagery, although associated deep convection was minimal. Strong
westerly winds aloft made tracking the wave difficult as it crossed the Caribbean.
However, reasonable extrapolation would place the wave in the vicinity of increased
cloudiness and convection off the Pacific coast of Central America on 27 and 28 July.
Evidence of a poorly-defined cloud system center within a somewhat isolated
tropical disturbance appeared in satellite imagery on 29 July.
Convective banding increased as a broad cyclonic circulation became established. The
"best track" indicates that a
tropical depression formed from the disturbance
near 1200 UTC 30 July while centered about 300 n mi south of Salina Cruz, Mexico
(Fig. 1 [57K GIF] and Table 1).
The tropical cyclone moved west-northwestward at
10 to 15 knots in response to a deep-layer-mean ridge to the north.
Deep convection became concentrated near the circulation center and the depression
strengthened into Tropical Storm Guillermo at 0600 UTC 31 July
while centered about 325 n mi south-southeast of Acapulco, Mexico. A well-defined central dense
overcast developed over the circulation center and Guillermo became a
hurricane at 1800 UTC 1 August while centered about
300 n mi southwest of Acapulco. Upper-level outflow became well established, and an
eye appeared in satellite imagery on
2 August. Strengthening continued and it is estimated that Guillermo reached its
peak intensity with 140 knot winds and 919 mb minimum central pressure
near 0000 UTC 5 August while centered about 700 n mi southwest of the southern tip of Baja
California. Shortly thereafter, cloud-top temperatures surrounding the eye gradually
warmed, although the eye remained visible until 7 August.
Guillermo moved over cooler water and weakened to a tropical storm by 0600 UTC
8 August while centered about 1100 n mi east of Hawaii. It passed 140°W longitude
into the Central Pacific
Hurricane Center's (CPHC) area of responsibility just after 1800 UTC 9 August. The low- to
mid-level flow turned the weakening cyclone more toward the north-northwest around the western periphery
of the subtropical ridge. Guillermo weakened to a tropical depression at 1800 UTC 10 August
but regained tropical storm strength about 24 hours later. The storm temporarily turned more
toward the west-northwest and passed about 700 n mi to the northeast of the
Hawaiian Islands on 13 August.
Guillermo again weakened to a tropical depression at 1200 UTC 15 August and
became extratropical near 0000 UTC 16 August.
The extratropical low recurved over the North Pacific, moving to a position about 500 n mi west
of Vancouver Island, British Columbia on 19 August. The low persisted for a few days longer,
slowly moving to within 300 n mi off the coast of northern California before being
absorbed by a larger extratropical cyclone on 24 August.
b. Meteorological Statistics
Figures 2 (23K GIF) and
3 (25K GIF) show the curves of minimum central pressure
and maximum one-minute wind speed, respectively, versus time (while Guillermo was in the NHC area
of responsibility), 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 Global Weather Center (AFGWC),
Synoptic Analysis Branch (SAB)
and the NOAA Tropical Analysis and Forecast Branch (TAFB).
Most objective Dvorak T numbers were near 7.0 (140 knots)
between 1200 UTC 4 August and 1800 UTC 5 August. The highest 3-hour
average objective T number was 7.3 between 0000 UTC and 0300 UTC 5 August and
is the basis for estimating the peak intensity at that time. Although maximum
sustained winds of 140 knots and minimum central pressure of 919 mb
indicated in the best track makes Guillermo one of the strongest hurricanes on record in the
eastern North Pacific, one should remember that intensities are usually estimated
from satellite interpretations for this basin. Best track records show
Hurricane Linda in September 1997 with maximum sustained winds of
160 knots and Hurricane Ava in 1976 with maximum sustained winds of
140 knots. There could well have been other cyclones as strong as,
or stronger than, Linda, Ava and Guillermo.
In addition to the satellite information on Guillermo, aircraft reconnaissance data
were provided by the NOAA Hurricane Research Division
(HRD) from P-3 aircraft conducting a vortex motion and evolution experiment on 2 and 3 August. Some
unique observations were obtained by research scientists on 3 August when several
Global Positioning System (GPS) dropwindsondes were released from the 700 mb
level within the eyewall of Guillermo. For the first time, wind data with relatively
high vertical resolution from flight level to the surface were recorded within the
eyewall of a major hurricane. Profiles of wind speed versus
altitude showed considerable variations among the individual "drops".
Figure 4 (46K GIF) shows a profile from
one of the GPS sondes dropped within the southwest quadrant of the eyewall at 2342
UTC. In this example, it is noted that the winds are strongest in the low levels, and
in fact, are a little stronger at the surface than at flight level. These data will continue
to be studied to better understand the relationship between flight-level winds and
surface winds. In-depth analyses from HRD scientists are in progress, and
publication of these analyses are eagerly awaited.
The best track data west of 140°W as well as during the extratropical stage were
obtained from the CPHC.
c. Casualty and Damage Statistics
There were no reports of casualties or damage from Guillermo received at the
National Hurricane Center.
d. Forecast and Warning Critique
NHC average official track forecast errors (excluding tropical depression stage) were
29 (39 cases), 48 (38 cases), 68 (36 cases), 93 (34 cases) and 149 n. mi. (31 cases),
respectively, for the 12-, 24-, 36-, 48- and 72-hour forecast periods. These were all
lower than the 1988-1996 average errors. The NHC average official track forecast
errors were lower than the averages from all of the operationally available track
prediction models except at the 48 and 72 hour time periods when the UKMI model
had comparable or slightly lower errors.
The NHC official intensity forecasts showed a distinct negative bias (i.e., intensity
was underestimated) while Guillermo was strengthening and a positive bias (i.e.,
intensity was overestimated) when the cyclone was weakening.
Watches and warnings were neither issued nor necessary for Guillermo.
Aircraft data were provided by James Franklin and Mike Black from the NOAA
HRD. Thanks to Ben Hablutzel from CPHC for providing best track data after 1800 UTC 9 August.