a. Synoptic History
Tropical Storm Ignacio
formed from a large area of disturbed weather that persisted well west of mainland Mexico from 14-16
August. Dvorak-technique satellite analyses were made on at least
three different centers of activity within the weather system during that period.
The disturbance that became Ignacio was first
analyzed early on the 16th, several hundred miles to the southwest
of the southern tip of Baja California. By 1800 UTC on the 16th,
convective bands had developed there and they were turning
cyclonically around a mid-level cloud-system center.
Animation of visible satellite pictures indicated that low clouds were not as
well organized at that time, and their motion did not appear to
describe a closed circulation. The system continued to become
better organized, however, and it is estimated that it became a
tropical depression at 0000 UTC on the 17th, about 450 n mi to the
southwest of Cabo San Lucas (Table 1 and
Fig. 1 [20K GIF]).
The genesis occurred to the northwest of the area where most
eastern North Pacific tropical cyclones form,
just south of that basin's sharp gradient in sea-surface temperatures. It was also to
the south of a mid-level to upper-level low centered offshore of
southern California. A trough trailed southwestward from the low
and the steering currents associated with the low and trough moved
the tropical cyclone toward the northwest and then the north at 10
to 15 knots from the 17th through the 19th.
The most intense burst of deep convection near the cyclone's center
occurred early on the 17th and intensity estimates from the NOAA
Tropical Analysis and Forecast Branch (TAFB)
reached 35 knots at
1200 UTC that day. This is the time that the system is analyzed as
having become Tropical Storm Ignacio.
The strengthening trend was short-lived. Ignacio moved over much
cooler waters and encountered southerly shear. In that
environment, deep convection was sustained only intermittently in
the northwest quadrant, where low-level convergence was strongest,
and Ignacio weakened back to a tropical depression early on the 18th.
Satellite pictures indicated a rejuvenation of cold cloud tops,
mostly in the cyclone's northern semicircle on the 19th. This
development was likely induced by baroclinic processes, occurring
over relatively cool waters where a mid-latitude trough was
approaching from the northwest. These observations form the basis
for designating the system as extratropical at 1200 UTC on the
19th. The extratropical low dissipated near the south-central
California coast about 24 hours later.
Clouds and precipitation associated with the remnant circulation
aloft moved northward on the 19th and 20th, through northern
California, Oregon, Washington and southern British Columbia. On
the 20th, this moisture was incorporated into the eastern part of
a large offshore extratropical cyclone associated with the remnant
of Hurricane Guillermo.
b. Meteorological Statistics
Table 1 lists the post-storm
"best track" data for Ignacio.
Figures 2 (19K GIF) and
3 (19K GIF) show the tropical storm'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 Global Weather Center
(AFGWC), TAFB and NOAA
Synoptic Analysis Branch (SAB).
ERS-2 data was helpful in the analysis of the cyclone's surface circulation.
There were no observations of tropical storm force winds at surface sites.
Rainfall totals were generally 0.5-1.25 inches over coastal areas
of central California, with a maximum of 2.17 inches recorded at
Three Peaks, in the coast range about 100 n mi south of San
Francisco. Such rains are rare events in the summer months in
California. About 1 inch of rain fell in San Francisco and that
was more than had previously occurred there for the entire month of
August since records began in 1850.
c. Casualty and Damage Statistics
There were no reports of casualties or damages. Thunderstorms were
apparently responsible for some power outages in central California.
d. Forecast and Warning Critique
While the few NHC track and intensity forecasts for Ignacio were
good, the system was a not a tropical storm long enough to allow
for a meaningful quantitative evaluation of forecast accuracy.
Coastal watches and warnings were neither issued nor necessary.
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