a. Synoptic History
Iris formed from the first of four consecutive
to generate tropical cyclones (Iris,
Humberto, Karen, and Luis)
on their generally westward trek across the tropical eastern Atlantic
Ocean. Iris' evolution was greatly influenced by two of those
systems, Humberto and Karen.
The wave associated with the formation of Iris crossed the
coast of Africa and began moving over the Atlantic Ocean on August
16th. Surface analyses showed a closed circulation around a 1009
mb pressure center
located just south of Dakar. A day later, the
circulation was evident in surface observations and satellite
pictures near the Cape Verde Islands. Associated deep convection
diminished on the 18th and 19th, but then gradually redeveloped.
T-numbers of 1.0 were assigned by the NHC
Analysis and Forecast Branch (TAFB; TSAF in figures) on the 21st
and both the TAFB and
Synoptic Analysis Branch
indicated T-2.0 on the 22nd. From the satellite
data it is estimated that the system became the 10th Atlantic
tropical depression of the
season at 1200 UTC on the 22nd, when located about 600 n
mi to the east of the Lesser Antilles (Table 1 and
Fig. 1 [100K GIF]). It became
Tropical Storm Iris six hours later.
took a jog to the northwest on the 23rd and quickly strengthened. The
first reconnaissance flight into Iris took place that evening and found
the system to be stronger than operational estimates based on satellite pictures
(Figs. 2 and 3 [72K GIF]).
The aircraft encountered 92 knot 10-second winds at a flight level
of about 500 m, and a central pressure of 991 mb was reported. From
this data, Iris is analyzed as a hurricane
at 1800 UTC on the 23rd (Table 1).
Iris moved toward the west-southwest at about 10 knots on the
24th and 25th. The change in heading was probably a consequence of a
between Iris and Humberto located about 750
n mi to the east--Humberto had developed from a depression on the
22nd to a 95-knot hurricane by late on the 24th.
On the 25th, Iris neared the Lesser Antilles. An upper-level
cold low was centered then to the north of Puerto Rico. Westerly
vertical wind shear occurred, separating deep convection from the
low-level cloud center, disrupting the circulation, and slowing the
general westward progress of the cyclone. Iris weakened back to
tropical storm strength. Reconnaissance aircraft and radar data
indicate a reformation of the center to the east of the former
position while the system meandered for about a day before moving
into the islands.
Steering currents ahead of a trough to the northwest then
turned Iris generally toward the north-northwest on the 27th. On
this track, Iris moved up the chain of Leeward Islands and
strengthened as the shear decreased. Late on the 28th, Iris
regained hurricane status over the south-central Atlantic.
Iris began a Fujiwhara interaction on the 30th, with Tropical
Storm Karen to its southeast. The interaction swept the weaker
Karen on a spiral path around, and then into, Iris where it was
absorbed on September 3rd. The interaction could have contributed
to Iris' erratic motion during this period (Fig. 1).
An eye appeared intermittently
and the intensity of Iris fluctuated from August 29th through September 2nd.
Iris reached its peak intensity of 95 knots several hundred miles to the
southeast of Bermuda on the 1st. Iris then weakened, temporarily,
in an environment of shear and relatively cool water. It dropped
below hurricane strength and became extratropical
while accelerating northeastward well to the southeast of Newfoundland on
the 4th. It then turned eastward and deepened. The pressure fell
from around 1000 mb to near 957 mb in about 48 hours. On the 7th,
Iris battered western Europe as a powerful extratropical storm with
sustained wind speeds near 65 knots.
b. Meteorological Statistics
The "best track"
intensities were obtained from the data presented in
Figs. 2 and 3. Those figures
show estimated central pressure and maximum one-minute wind speed,
respectively, versus time.
The ship Pallas Athena reported 34-40 knot
winds at 1200 UTC on September 1 while located about 100 n mi to the south-southeast
of the center of Iris. This was the only ship report of wind
speeds 34 knots or greater received by the NHC for the tropical cyclone phase of Iris.
The only available observation of sustained tropical storm
force winds in Caribbean islands came from Desirade (just east of
Guadeloupe) where a 45 knot (2-min) wind and 54 knot gust occurred.
Highest reported gusts elsewhere reached 49 knots at Martinique,
40 knots at Antigua, 37 knots at Dominica,
and 36 knots at St. Kitts.
The lowest pressure reported from the northeastern Caribbean area
was 999 mb at Antigua.
The primary meteorological event caused by Iris in the
Caribbean islands was heavy rain. The totals were particularly
large in Martinique where Ducos (La Manzo) had 17.72 inches for the
event, with 16.18 inches falling in 24 hours. Other peak rainfall
rates in Martinique included 1.89 inches in 30 minutes, 3.01 inches
in 1 hour and 4.61 inches in 2 hours at Trois Ilets, Vauclin, and
Ducos, respectively. An average of 6 inches of rain fell on Antigua.
c. Casualty and Damage Statistics
Four deaths occurred on Martinique, two each in two homes
affected by mud slides. An early media summary indicated a death on Guadeloupe.
Few damage reports have been received at the NHC. There was
extensive flooding in low-lying areas and destruction of banana
trees on Antigua. Similar damage likely occurred on neighboring islands.
d. Forecast and Warning Critique
Iris and Humberto were poorly initialized, or not
initialized, in the Aviation model during the period when Iris approached
the Lesser Antilles. Consequently, the Aviation model tracking
algorithm (AVNI) failed to produce a forecast many times. The poor
initialization also led to bad forecasts by some of the track
models. Some of the models showed Iris immediately turning to a
northward track--as much as two or three days too soon. Other
models (like the statistical-dynamical A90E, which strongly weighs
persistence of heading for short-term forecasts) correctly showed
Iris reaching the Caribbean Islands, but then incorrectly indicated
that the cyclone would continue moving generally westward into the
eastern Caribbean Sea.
Table 2 shows average track forecast errors for the NHC
official forecast and for the objective guidance. NHC average
track errors were near to, and slightly smaller than, the most
recent 10-year averages. The average official error at 72 hours
was smaller than the average error associated with any model.
NHC intensity forecast errors were larger than normal for two
period of relatively large intensity change--when Iris weakened
while nearing the Caribbean and again when the cyclone intensified
upon leaving that area.
Table 3 lists the tropical storm and hurricane watches and
warnings issued in association with Iris.
Contributions to this report were made by the meteorological
services of Antigua and Martinique.
Responding to a request and data from the director of Meteo-France
Guadeloupe, Roland Mazurie, the track and intensity of Iris were re-analyzed
for the period when it was centered near the Lesser Antilles. It was
determined that while reconnaissance aircraft tracked the dissipating
original circulation center westward into the eastern Caribbean Sea, radar
and surface data showed a new center developing closer to deep convection
just east of the islands. To account for this, minor changes (in bold
below) have been coordinated between Mazurie, TPC's Jack Beven and the
author to the position and pressure estimates at three best track times. The
first two positions represent the geometric center between two weak
circulation centers (the weakening original center and strengthening new
center), while estimates at the third time reflect the new center as it
becomes dominant. No changes were made to the estimated maximum wind speed.
Hurricane Iris watch and warning summary
|25/0300||Tropical Storm Watch issued
||Dominica, Antigua, Barbuda and St. Kitts
|25/1000||Tropical Storm Watch issued
||Barbados and St. Vincent|
||Tropical Storm Warning issued
||All islands from the Grenadines and Barbados northward through Anguilla and St. Martin|
|Tropical Storm Watch issued
||Remainder British Virgin Islands|
|26/0300||Tropical Storm Warning discontinued
||Barbados, St. Vincent and the Grenadines|
Tropical Storm Warning issued
||Remainder British Virgin Islands|
|Tropical Storm Warning discontinued||St. Lucia|
|Tropical Storm Watch issued||U.S. Virgin Islands|
|27/0900||Tropical Storm Warning discontinued
|27/1800||Tropical Storm Warning discontinued
|28/0600?||Tropical Storm Warning discontinued
||Tropical Storm Watch discontinued
||U.S. Virgin Islands and Guadeloupe|
|Tropical Storm Warning discontinued
||St. Martin and St. Barthelemy|
|28/1500||Tropical Storm Warning discontinued
|01/1930||Tropical Storm Watch issued
|03/0600||Tropical Storm Watch discontinued