Preliminary Report
Tropical Storm Iris
16 - 21 September 1989

1989 PRELIMINARY REPORTS

Tropical Storm Allison (TS)
Tropical Storm Barry (TS)
Hurricane Chantal (1)
Hurricane Dean (2)
Hurricane Erin (2)
Hurricane Felix (1)
Hurricane Gabrielle (4)
Hurricane Hugo (5)
Tropical Storm Iris (TS)
Hurricane Jerry (1)
Tropical Storm Karen (TS)

a. Synoptic History

The tropical wave that spawned Tropical Storm Iris moved off the northwest coast of Africa on 12 September immediately behind the wave that spawned Hurricane Hugo. The tropical wave continued westward at rather low latitudes across the tropical Atlantic with signs of organization being evident on the 15th. By 1800 UTC, 16 September, the convective banding led NHC satellite meteorologists to classify the tropical system with a current intensity of 1.5 using the Dvorak technique. Thus, it is estimated that Tropical Depression Twelve formed at that time in the wave of Hurricane Hugo which was then approaching the Lesser Antilles.

Tropical Depression Twelve continued developing at the normal rate of one T number per day and satellite classifications from NHC (KMIA), SAB and KGWC all were in agreement that the T number at 0000 UTC , 18 September , was 2.5 (40 mph). Thus, it is estimated that the depression reached tropical storm strength at that time 390 nautical miles east of Barbados.

Ten hours later (1000 UTC) tropical storm warnings were issued for the islands of Barbados, St. Vincent, and St. Lucia anticipating that Tropical Storm Iris would track into the Caribbean due to its relatively low latitude. However, by 1200 UTC , 18 September, it was evident that the outflow from Hugo shearing Iris and that the circulation center was well to the north of the previous estimate. Moreover, 1200 UTC synoptic reports and satellite imagery indicated that the steering of the more northerly center would be to the northwest and thus the warnings were discontinued at 1600 UTC.

For the next 24 hours, Iris persisted on a general northwesterly track under strong shearing. On the 19th, animated satellite loops indicated that the tropical storm developed its own outflow pattern beyond the outer fringe of the outflow from Hugo. Air Force reconnaissance reports at 1809 UTC on the 19th, when the center was 235 nautical miles northeast of of Antigua, indicated that the maximum flight-level winds at 1500 feet were 83 mph and the sea level pressure was 1001 mb. The reconnaissance estimate of surface winds was 75 mph. The estimate by both NHC and SAB satellite analysts was 52 mph which was in better agreement with the pressure/wind relationship. An 80% reduction in flight-level winds would have given a surface estimate of 63 mph. Thus, it is concluded that Iris probably was just below hurricane strength at that time. This is another example where a system east of the Lesser Antilles is found to be stronger that the satellite estimates. (e.g , Tropical Storm Earl 1986).

As Hugo moved away from the influence of Puerto Rico and Hispaniola, it strengthened increasing the shear over Iris. Thus, the strengthening of Iris was short-lived and it weakened rapidly thereafter while following in the wake of Hugo. Iris's center was completely exposed the next day as a result of Hugo's outflow shearing away the convection. It is estimated that Iris weakened to a tropical depression 235 nautical miles northeast of San Juan, Puerto Rico near 0000 UTC, 21 September, and dissipated about 200 nautical miles east northeast of Turks Island by 0000 UTC, 22 September 1989.

b. Meteorological Statistics

Since Iris followed closely on the heels of powerful Hurricane Hugo, there was no surface reports in the vicinity of Iris because the ships vacated the region.

Although Iris did not strike any land area, a trailing band of showers and thunderstorms extending south from the storm center moved over portions of the Leeward Islands on the 20th and 21st adding locally heavy rains to areas previously damaged by Hugo.

1. Rainfall Data

Many of the rain gages were damaged by Hugo but St. Johns Island in the U.S. Virgin Islands reported 7.53 inches on the 20th after receiving 7.35 inches on the 19th. There were unofficial reports of heavy rains and flooding throughout the Virgin Islands

c. Casualty and Damage Statistics

There were no reports of casualties or damage related to Iris.