a. Synoptic History
The formation of Hurricane
Darby can be associated with a tropical
wave that generated little cloudiness during its passage across the tropical North
Atlantic Ocean and Caribbean Sea on 4-16 July. Convection near the wave began to
increase on the 19th, when the activity passed a few hundred nautical miles to the
south of Acapulco, Mexico. Two days later, the cloud pattern displayed some
curvature on satellite pictures and Dvorak T-numbers were assigned
to the system for the first time. Convective bands developed within the pattern on the 22nd and
the disturbance became sufficiently well-organized so that it can
be estimated that tropical depression status was reached around
0000 UTC on the 23rd (Table 1).
It was then centered about 625 n mi to the south of the southern tip of Baja
California, several hundred miles west of the area of most frequent formation in
the eastern North Pacific Ocean.
A mid- to upper-tropospheric ridge extending westward from an
anticyclone initially over the U.S. southwest was the dominant large-scale
circulation feature over the eastern Pacific during Darby's development. That
feature helped provide a steering current that drove the tropical cyclone toward
the west or west-northwest at about 10-15 knots for nearly a week (Fig. 1 [32K
GIF]). Early in this period, the cyclone quickly strengthened
(Figs. 2 [20K GIF] and 3 [18K GIF]).
Convection became more concentrated and outflow aloft increased over the cyclone's western semicircle.
The depression became Tropical Storm Darby
at 1800 UTC on the 23rd and, just 36 hours later with a 10-15 n mi wide eye,
reached Category 3 on the Saffir-Simpson Hurricane Scale. On the 25th-26th, the
eye disappeared from geostationary satellite imagery and then reappeared with a
diameter of 20-30 n mi. This occurred in conjunction with a possible eyewall
replacement cycle inferred from the imagery. During this period, Darby's winds
are analyzed to have decreased a little from 100 kt and
then returned to that level.
Darby was likely its strongest, with 100 kt
winds and a minimum pressure of 958 mb, near 1800 UTC on the 26th when
objectively determined T-numbers reached their peak for this tropical cyclone.
The hurricane stayed nearly that strong until early on the 28th when a steady,
four-day decline in strength began while Darby gradually crossed sea-surface
isotherms toward colder waters and encountered increasing southwesterly vertical
wind shear. On the 29th, Darby was downgraded to a tropical storm by the
Central Pacific Hurricane Center
(CPHC) shortly after its center crossed 140°W and entered
their area of responsibility. The analyses from
(e.g., Fig. 1 and Table 1)
show Darby dissipating a few hundred miles to the north of the Hawaiian Islands
early on 1 August.
b. Meteorological Statistics
Table 1 provides the post-storm analysis of
"best track" location and intensity estimates for Darby.
Most of the data west of 140°W (i.e., after about 0000 UTC on the 29th) were provided by the
Figures 2 and 3 show the
hurricane's estimated central pressure and maximum one-minute wind speed,
respectively, versus time and the associated satellite data. Position and
intensity estimates from satellite pictures were provided by the
Air Force Weather Agency
(AFGWC in figures), NOAA Tropical Analysis and Forecast Branch (TAFB) and NOAA
Synoptic Analysis Branch (SAB).
Satellite estimates from analysts at the
CPHC are not shown in the figures.
There were no surface observations of tropical storm force winds.
c. Casualty and Damage Statistics
Darby did not directly affect land and there were no reports of
casualties or damages.
d. Forecast and Warning Critique
In general, the track forecast errors from the numerical models and
the NHC were much smaller than the long-term NHC averages (Table 2). The AVN,
ETA, NGM, and NOGAPS models provided the exceptions.
The intensity forecast models did not indicate the initial rapid
strengthening. The NHC intensity forecasts were a little better than the guidance
during that phase, but began the forecast of Darby's steady demise about a day too
Watches and warnings were neither issued nor necessary for this