Hurricane Iris Preliminary Report

Excerpts courtesy of the
National Hurricane Center (NHC)
--Written by Edward N. Rappaport

Synoptic History

Iris formed from the first of four consecutive tropical waves 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. Dvorak technique T-numbers of 1.0 were assigned by the NHC Tropical Analysis and Forecast Branch (TAFB) on the 21st and both the TAFB and NESDIS 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 nmi to the east of the Lesser Antilles. It became Tropical Storm Iris six hours later.

The cyclone 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. 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.

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 Fujiwhara interaction between Iris and Humberto located about 750 nmi 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.

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 winds near 65 knots.

Meteorological Statistics

The ship Pallas Athena reported 34-40 knot winds at 1200 UTC on September 1 while located about 100 nmi 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 were 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 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.

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.


Contributions to this report were made by the meteorological services of Antigua and Martinique.