Tropical Storm Debby Preliminary Report

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

Synoptic History

Debby formed from a westward-moving tropical wave that entered the eastern tropical Atlantic on the 4th of September. Satellite pictures showed the system to be quite large on the 6th and, a day later, indicated several clusters of strong convection with cloud-top temperatures lower than -80 C. the wave was then located about midway between Africa and the Lesser Antilles islands.

The amount and intensity of deep convection decreased the next day as the system encountered a northwesterly vertical wind shear. The shearing became more westerly and weakened enough for an area of concentrated convection to redevelop when the wave axis was a few hundred miles east of the Lesser Antilles late on the 8th.

At 1200 UTC on the 9th, a ship located about 125 nmi southeast of Barbados reported a light north wind just ahead of the system. About 12 hours later, surface observations and satellite pictures suggested that a surface circulation center was located just west or northwest of Barbados. NOAA was then conducting the first reconnaissance flight into this system. At a flight-level of 1500 feet, 50-60 knot winds were measured in a 20-30 nmi wide band that was centered in the vicinity of thunderstorms about 40 nmi north through east of the center. They also estimated surface winds at 50 knots. A few hours later, this part of the system, passed over Martinique. Surface observations from Martinique and a nearby ship indicate that, despite the disorganized appearance on satellite pictures, the system was then a tropical storm with one-minute wind speeds of about 60 knots. It is estimated that about 18 hours earlier (at 1200 UTC on the 9th) the system had become a tropical depression with a strong circulation aloft. In the vicinity of Martinique, the winds were efficiently conveyed to the surface by thunderstorms.

The tropical cyclone moved toward the west-northwest at about 17 knots. Westerly vertical wind shear affected the growth of the cyclone by limiting the amount and intensity of thunderstorms and by displacing most of the activity 50-150 nmi to the east of the low-level circulation center. The flight-level and estimated surface wind speeds reported by the Air Force Reserve Hurricane Hunter's near 1200 UTC on the 10th were not as strong as noted earlier, but, just six hours later, returned to magnitudes that were comparable to those found on the NOAA flight. Meanwhile, the wind at Martinique only slowly abated as the storm receded. Hence, while some fluctuation in surface wind speed likely occurred in association with the variations in deep convection, it is estimated that the surface wind speeds in Debby remained around 50-60 knots through the 10th.

Even though the system continued to produce locally strong winds on the 11th, its circulation became more disrupted by the strong wind shear. By 0200 UTC, a closed circulation center could not be identified by the crew aboard the reconnaissance aircraft. The cyclone is analyzed as degenerating back to a vigorous tropical wave around the 0600 UTC 11 September synoptic hour.

The wave continued to produce locally heavy rain and gusty winds that spread across Hispaniola on the 11th. These conditions advanced westward before diminishing over the northwest Caribbean Sea and adjacent portions of Mexico on the 15th. Satellite pictures indicate that some of the activity could also have spread into the southeastern Gulf of Mexico.

Meteorological Statistics

The 60 knot maximum one-minute surface wind speed at 0600 UTC is primarily based on hourly reports around that time from two observation sites in Martinique. At Le Vauclin, on the southeast coast, 10-min winds of 56 knots with a gust to 85 knots were recorded. At the Morne Des Cadets observatory, the maximum 10-min wind was 54 knots with a gust to 84 knots (the maximum one-minute wind speed is usually a little higher than the associated 10-min average). The nearby ship PJRB (name unknown) reported a sustained wind of 44 knots at 0600 UTC, and this likely does not represent the storm's maximum at that time.

In addition, the meteorological service of the Dominican Republic reported gusts to 54 knots on the 11th, but this could have come after Debby ceased to exist as a tropical cyclone.

The ship PJRB reported 1000.7 mb at 0600 UTC on the 10th. This pressure is believed to be incorrect. A pressure of 1007.0 mb would have been more appropriate for that time at their location.

An isohyet analysis by the meteorological service of Martinique indicates that about one-half of that island had at least 4 inches of rain. the largest total was 7.24 inches at Saint Joseph/Rabuchon.

Casualty and Damage Statistics

A total of four people were killed (two in landslides) and 24 injured in St. Lucia. There was one drowning in Martinique and another (a fisherman) off the southwest coast of Puerto Rico. Three deaths (apparently related to downed power lines) were reported in the Dominican Republic. The deaths in the Dominican Republic occurred on the 11th, possibly after Debby had reverted back to a tropical wave.

The Associated Press indicated that the worst damage occurred in St. Lucia where rains caused landslides that blocked main roads and covered the town of Pont St. Jacques (spelling uncertain). Two inches of silt covered the roadways at the main international airport. The rains caused floods that washed away hillside shacks, eight bridges, and portions of some roadways. Water was chest-high in the village of Anse La Raye. Debby's winds damaged banana plantations in St. Lucia.

In Martinique, some towns were flooded while wind-felled trees blocked some roads. The banana crop was also damaged in Martinique. About 20000 residents lost power and schools were closed.

The meteorological service of the Dominican Republic reported that some rivers flooded.

The number of people left homeless in the area affected by Debby was estimated to be in the hundreds.