The WEATHER of 1991Eastern Pacific Hurricanes: Long Season, but Normal NumbersWeatherwise Magazine - February-March 1992By Max Mayfield and Edward N. Rappaport
[Max Mayfield and Edward N. Rappaport are meteorologists at the
National Hurricane Center in Coral Gables, Florida.]
The number of tropical cyclones in the eastern Pacific
returned to near normal in 1991. There were fourteen named
tropical cyclones, including ten hurricanes. This was a marked
decrease from the record-shattering sixteen hurricanes in 1990.
The long-tern averages for tropical storms and hurricanes are
sixteen and nine, respectively. These averages, and the
statistics presented below, are based on data from the 1966-91
period of routine weather-satellite surveillance.
Although the 1991 hurricane season seems about average when
viewed as a whole, several records were established or
challenged. The season had the second earliest start, beginning
on May 16 with the formation of Tropical Storm Andres. It also
finished relatively late. Nora was the last system of the year
and was the first eastern Pacific hurricane during the
satellite era to form in November.
Between Andres and Nora, tropical cyclone activity was
sporadic. As often occurs in the eastern Pacific, there were
several periods in which two or more tropical cyclones
developed simultaneously. On the other hand, only two tropical
cyclones formed in all of July. That was four below normal and
the fewest for a July since at least 1973. Hurricane Kevin
provided two highlights. Not only was it the season's strongest
hurricane (with sustained winds estimated at 145 m.p.h.), it
became the most enduring eastern Pacific hurricane on record by
remaining east of 140 degrees (W) for twelve days while at
hurricane strength.
The only landfall on the mainland was by a tropical
depression that moved ashore near Salina Cruz, Mexico, on June
30. This depression, and a few of the named storms that
skirted the coast, produced locally heavy rain over several
states in western Mexico. Tropical Storm Hilda made the closest
approach to the mainland United States, dissipating just
offshore from Southern California after moving on a
northwesterly track (a little west of the Baja peninsula) that
was unusual for early or mid-August.
There were no reports of casualties or significant damage
related to tropical cyclones in the eastern Pacific this year.
Once again satellite imagery provided the bulk of the data
used to estimate the intensity and location of eastern Pacific
tropical cyclones. However, for the first time in five years,
information from instrumented aircraft supplemented the
satellite observations. The data came from NOAA and National
Center for Atmospheric Research (NCAR) aircraft participating
in the Tropical Experiment in Mexico (TEXMEX), a research
program on tropical cyclone development.
Most, if not all, of the 1991 eastern Pacific tropical
cyclones developed from African waves. The five tropical
cyclones that developed into major hurricanes (maximum
sustained winds greater than 110 m.p.h.) are discussed below.
Hurricane Carlos
A tropical wave left the African coast on June showing some
cyclonic curvature in low clouds. It passed Barbados six days
later, when a low- to mid-level wind shift was detected. The
wave was followed by a low-level wind surge, which on June 12
included 40-m.p.h. east-southeast winds at 850 mb over Curacao.
Most of the showers associated with the wave moved westward
over South America and then Panama on June 14. The convection
became organized and developed into a tropical depression about
350 miles south of Salina Cruz, Mexico, near midday on June 16.
Carlos became a tropical storm one day later and reached
minimal hurricane strength on the 18th.
Late on the 20th, Carlos weakened to a tropical storm.
However, a strong high-pressure system became established north
of the tropical storm, forcing it to move west-southwestward
over warmer waters, which allowed the tropical cyclone to
reintensify.
Carlos regained hurricane status, developing a well-defined
eye and strong upper-level outflow. Estimated maximum winds
and minimum pressure in Carlos were 120 m.p.h and 955 mb on June
24.
The hurricane weakened when it moved west-northwestward over
cooler waters and encountered strong upper-level shearing.
Carlos dissipated late on June 27.
Hurricane Fefa
Fefa formed from a westward-moving tropical wave that
emerged from the northwest coast of Africa on July 17. The wave
moved across the Atlantic and Caribbean without development, but
its passage across Barbados and Trinidad was clearly indicated
in wind shifts in upper-air data. Upper-air data from Balboa,
in the Panama Canal Zone, indicated the wave emerged over the
eastern Pacific on July 25.
Satellite imagery showed that the cloud pattern associated
with the tropical wave became better organized by July 28.
Data from a NOAA aircraft participating in the TEXMEX indicated
a 700- mb cyclonic center within the wave near 10(N), 106(W)
early on July 29 but no clear evidence of a low-level center.
A tropical depression formed a little later that day about
1,000 miles south-southeast of Cabo San Lucas, Mexico, probably
in association with the 700-mb center identified earlier.
The depression intensified quickly and became Tropical Storm
Fefa on the 29th. NCAR aircraft data indicated 60-m.p.h. winds
at the 970-mb flight level that day.
On July 30, NOAA aircraft data for 950 mb indicated a
circulation center with a broad areas of 45-50 m.p.h. winds.
Interestingly, the center of this low-level circulation was
displaced about 35 miles northwest of the center of a 700 mb
vortex detected about 45 minutes earlier. The 700-mb vortex
was presumably the same one detected early on the 29th.
Fefa continued to intensify over the warm Pacific water,
reaching hurricane strength on July 31. Satellite imagery
showed improved upper-level outflow and the appearance of an eye
on August 1. The estimated lowest minimum pressure of 959 mb
and maximum sustained winds of 120 m.p.h. occurred early on
August 2.
The tropical cyclone moved generally west-northwestward from
its inception until August 2, when Fefa turned more toward the
west under the influence of a building ridge to the north.
Operational responsibility for Fefa was passed to the Central
Pacific Hurricane Center in Hawaii when the cyclone moved west
of 140(W) early on August 5. Thereafter, Fefa weakened as it
moved into a more strongly sheared environment near the
Hawaiian Islands. Fefa dissipated near the island of Hawaii
early on August 8. The primary land effects from Fefa included
locally gusty winds, heavy rains, and high surf on the island
of Hawaii.
Hurricane Jimena
The tropical wave from which Jimena likely originated moved
off the African coast on September 5. was a fairly
active-looking system that spawned Tropical Storm Danny over the
eastern Atlantic. The southern portion of the system proceeded
westward over the Caribbean, causing little significant weather
there. As it passed over Panama and Costa Rica on September
14-15, however, convective activity increased. The system moved
westward as an area of disturbed weather in the Pacific
intertropical convergence zone for the next few days. By
September 20 the convection was organized well enough to warrant
tropical depression advisories.
It took about one day for the depression to strengthen into a
tropical storm. The first satellite images on September 21
showed enough banding features to upgrade the system to
Tropical Storm Jimena. Meanwhile, the cyclone was tracking
generally
After becoming a tropical storm, Jimena developed rather
rapidly -- particularly after midday on September 22, when it
took a westward course. Strong upper-level outflow developed
over the storm on the 22nd. Jimena became a hurricane late that
day with a newly formed, banding-type eye. Relatively rapid
development continued over the next 24 hours; satellite
intensity estimates showed maximum sustained winds near 130
m.p.h. on September 23. A NOAA research aircraft mission into
Jimena late on the 23rd confirmed satellite estimates that
reported an extrapolated central pressure of 948 mb, with
maximum winds of 138 m.p.h. at a flight level of 700 mb.
Jimena remained a powerful hurricane for the next three days,
moving almost due westward until it turned toward the west-
northwest on September 26. It is estimated that the minimum
central pressure, 945 mb, occurred late on September 24.
Although it clearly weakened on September 27, Jimena retained
a well-defined eye and maximum winds near 105 m.p.h. on
September 28. The hurricane had turned back to the west but
moved north-westward again on September 29 in response to a
large middle-level trough to the northwest. The influence of
cooler sea- surface temperatures and upper-level southwesterly
winds brought Jimena below hurricane strength by midday on
September 30. The storm deteriorated rapidly thereafter, and
deep convection was no longer present near the center by late
on the 30th. The low-cloud swirl spun down to depression
status on October 1, and the system dissipated on October 2 when
the center of the cloud pattern lost definition.
Hurricane Kevin
Kevin formed from a westward-moving tropical wave that
emerged from the northwest coast of Africa on September 11.
Satellite imagery showed that the wave crossed the Atlantic and
Caribbean without development and moved into the eastern Pacific
on September 21. Cloudiness and convection became better
organized on September 23-24, and by the next day convective
banding was sufficient to indicate the wave had developed into
a tropical depression while centered about 400 miles southeast
of Acapulco, Mexico.
The banding pattern became more pronounced, and the
depression developed into Tropical Storm Kevin on the 25th.
During the next 24 hours, the convective banding increased and
the upper-level outflow gained definition. Kevin was upgraded
to hurricane status on September 26 while centered about 225
miles south- southwest of Acapulco.
Kevin continued to strengthen, and satellite images showed
intermittent glimpses of an "eye" on the 27th and 28th. By
September 29, a well-defined eye had developed, and this
feature persisted for several days. The hurricane appeared to
reach its peak intensity on October 1 while centered about 400
miles south-southwest of the southern tip of Baja California;
the estimated maximum winds were 145 m.p.h. and the minimum
pressure was 935 mb. Kevin was moving toward the west near 10
m.p.h. at this time but turned toward the west-southwest on the
2nd in response to a strong high-pressure system center
northwest of the hurricane. Gradual weakening then occurred,
with the eye becoming poorly defined by October 4, and Kevin's
maximum winds decreased to an estimated 85 m.p.h. by October 6.
However, as the hurricane turned toward the northwest, a
well-defined eye reappeared in a satellite imagery. Based on
satellite intensity estimates, Kevin's strength increased to
maximum winds of 115 m.p.h. and a minimum pressure of 960 mb on
October 7.
Thereafter, Kevin began weakening as it moved over cooler
water. Operational responsibility for Kevin was passed to the
Central Pacific Hurricane Center when the tropical cyclone
moved west of 140(W) on the 9th. Kevin lost its tropical
characteristics three days later about 600 miles northeast of
Hawaii.
Hurricane Linda
The tropical wave that ultimately produced Hurricane Linda
emerged from the northwest coast of Africa on September 16. It
could be tracked reasonably well across the tropical Atlantic
and Caribbean using satellite imagery. The southern portion of
the wave, identified as a few disorganized cloud clusters,
crossed to the eastern Pacific on September 25. Convection
flared up on the 30th, but during the next few days the system
generally was poorly organized. The organization gradually
improved enough for the system to be a tropical depression by
October 3.
The tropical depression moved northwestward while
strengthening and soon became Tropical Storm Linda. Steering
currents weakened about that time, and Linda drifted toward the
north, then north-northeast, and reached hurricane strength by
October 5. Based upon satellite imagery, the hurricane reached
its estimated peak intensity of 120-m.p.h. winds and a 957-mb
minimum central pressure late on the 5th. At that time, the
hurricane was near where Hurricane Kevin had been five days
earlier.
Hurricane Linda began weakening after crossing Kevin's trail,
even though satellite water-vapor images showed the upper-level
environment to be favorable for strengthening. However,
satellite-derived sea-surface temperatures revealed a cool
tongue of water oriented east-southeast to west-northwest
immediately north of Kevin's track. It was concluded that cold
water upwelled by Kevin's passage contributed to Linda;s
weakening.
Linda turned toward the west-northwest and whipped Socorro
Island with 80-m.p.h. sustained winds early on October 7, but
later that day it weakened to a tropical storm. For the next
two days, Linda gradually turned toward the west on a track
about 100 miles north of and parallel to Kevin's track. Late
on October 9, satellite images revealed that strong upper-level
winds had sheared the deep convection near Linda's center; by
the 10th the storm weakened to a tropical depression.
Deep convection appeared periodically during the next several
days while some middle-level clouds and a swirl of stratocumulus
clouds moved westward, southwestward, and then turned
northwestward. The system dissipated by October 14.
