Preliminary Report
Tropical Storm Arlene
11 - 18 June 1999

1999 PRELIMINARY REPORTS

Tropical Storm Arlene (TS)
Hurricane Bret (4)
Hurricane Cindy (4)
Hurricane Dennis (2)
Tropical Storm Emily (TS)
Hurricane Floyd (4)
Hurricane Gert (4)
Tropical Storm Harvey (TS)
Hurricane Irene (2)
Hurricane Jose (2)
Tropical Storm Katrina (TS)
Hurricane Lenny (4)

a. Synoptic History

During its initial stages of development Arlene was not a purely tropical system. Although initially cold-core, by the time depression status had been attained on 11 June, the overall structure more-closely resembled a tropical, rather than a subtropical cyclone. At some (unknown) point, the system became warm-core, as revealed by reconnaissance data on the 15th.

Arlene's complex development can be traced to a mid- to upper-level cold low that developed near the tail end of a diffuse front in the central Atlantic. Water-vapor imagery first showed the circulation of the upper low a few-hundred miles north of Puerto Rico late on 8 June. Simultaneously, a fairly large-amplitude tropical wave passed through the tail end of the frontal zone southeast of the upper low, and a low-level cloud swirl became visible near 22N, 61W, close to the wave axis, and southeast of the upper low. The low-level cloud swirl then moved slowly north-westward over the next two days without development due to westerly shear from the upper low.

Throughout this period, fairly steady convection had been maintained in the diffluence region to the east of the upper low. By 0600 UTC on the 10th, the low-level circulation moved underneath the cold low, near 24N, 63W. Shortly thereafter, the upper low began to move off to the east into the convective area. As the upper low accelerated northeastward late on the 10th, satellite microwave imagery revealed the rapid downward development of a vortex in the convection, which led to the formation of a new low-level center. During the morning of the 11th, the convection acquired a well-defined banding pattern, and it is estimated that a tropical depression formed at 1800 UTC on the 11th, about 465 nautical miles southeast of Bermuda. The original low-level cloud swirl continued moving away to the west and gradually dissipated.

Almost immediately after reaching depression status, the cyclone slowed and began a northward drift for 24 h. By 1200 UTC on the 12th, Dvorak satellite classifications from the Tropical Analysis and Forecast Branch (TAFB) of the Tropical Prediction Center and the NOAA/NESDIS Satellite Analysis Branch (SAB) indicated that tropical storm strength had been attained. Arlene intensified for 12 h until westerly shear began to expose the low-level circulation center. The maximum intensity was reached at 0000 UTC on the 13th, when the winds were estimated to be 58 mph and the minimum central pressure was estimated to be 1006 mb. From the 13th to the 15th, Arlene moved generally west-northwestward while weakening slightly under the westerly shear.

Steering currents became poorly defined and Arlene moved little on the 15th. The best track indicates that Arlene executed a small cyclonic loop, although this apparent motion may have been due a reformation of the center closer to the convection on the east side of the cyclone. A northwesterly motion resumed late on the 15th, followed by a gradual turn to the north then northeast over the next three days as Arlene moved around the western edge of the subtropical ridge. Arlene's closest approach to land was at 0600 on the 17th, when the cyclone passed about 100 nautical miles to the east of Bermuda. Convection began to diminish on the 16th as the environmental shear changed to northeasterly and Arlene moved over cooler waters. Synoptic-scale upper-level confluence and subsidence in the immediate environment of Arlene also acted to suppress convection. The low-level circulation weakened to depression status at 0000 UTC on the 17th, and dissipated ahead of an approaching frontal zone on the 18th.

b. Meteorological Statistics

There was only a limited amount of in-situ aircraft reconnaissance data from the Air Force Reserve Hurricane Hunter Squadron, from 1200 UTC on the 15th to 0000 UTC on the 17th. The maximum winds from the reconnaissance aircraft were observed from 15/1200-16/0600 UTC, when the surface winds were estimated to be 52 mph. As is typical for storms in the subtropics, central pressures measured by reconnaissance were somewhat higher than satellite-based estimates (Figure 3). Arlene's minimum central pressure is estimated from satellite imagery and ship reports to be 1006 mb at 13/0000 UTC, although the lowest pressure measured by aircraft reconnaissance was 1008 mb at 15/1200 UTC.

There are no known ship or land reports of winds in excess of 39 mph associated with Arlene.

c. Casualty and Damage Statistics

There have been no reports of casualties or damage from Arlene.