a. Synoptic History
Tropical Storm Sebastien originated from a
tropical wave
that moved westward across the Atlantic Ocean from the 13th through the
19th of October. Associated thunderstorms remained unorganized and
mostly to the south of 15N during that period. On the 20th, deep
convection became more concentrated about 350 n mi to the east of Barbados.
Dvorak technique
T-numbers were first assigned on that date by meteorologists at the National
Hurricane Center's
Tropical Analysis and Forecast Branch (TAFB, formerly TSAF as in figures)
and the NESDIS
Synoptic Analysis Branch (SAB),
and within 24 hours increased to 2.5. Based on their analyses, it is estimated that
the system became a tropical depression
at 1200 UTC on the 20th and a tropical
storm at 0000 UTC on the 21st (Table 1,
Fig. 1 [74K GIF]).
From the 20th to the 22nd, Sebastien was embedded within the
outer part of the circulation associated with a large, deep low-
pressure system centered just northeast of Puerto Rico. The
associated steering currents moved Sebastien toward the northwest
to north at about 15 knots during that period. Southwesterly
vertical wind shear precluded significant development and, based on
a ship observation (see Fig. 2
[63K GIF]), Sebastien reached its peak intensity of 55 knots,
at 1800 UTC on the 22nd. At that time, its exposed low-level cloud
center
was located about 350 n mi to the northeast of the northern Leeward Islands.
Sebastien began moving with the low-level flow toward the
southwest on the 23rd. This pushed Sebastien directly into the
upper-level southwesterly winds, displacing the coldest cloud tops
about 100 n mi to the east and northeast of the center. Sebastien
weakened to a tropical depression during the evening of the 23rd
while it moved under the upper-level cyclonic circulation. Its
dissipating circulation of low clouds was near the U.S. Virgin
Islands about 24 hours later, while some remnant deep convection
persisted well to the east.
b. Meteorological Statistics
The "best track"
intensities were obtained from the data presented in
Figs. 2 and 3
(63K GIF). Those figures show Sebastien's estimated central pressure
and maximum one-minute wind speed, respectively, versus time.
Sebastien did not cause tropical storm conditions over land.
The basis for the estimated maximum wind speed of 55 knots
is a 52 knot wind observed on the ship with call sign ELSE5
while it was located 60 n mi northeast of the center at 1800 UTC on the
22nd. The maximum flight-level wind at 1500 feet came at about
this time and was 50 knots. The only other reports of 34 knots or
higher wind speeds came from the ship Sea Wolf. It had 36 and 38
knot winds at 1200 and 1400 UTC, respectively, on the 21st, 35 to
45 n mi to the east of the circulation center.
c. Casualty and Damage Statistics
No reports of casualties or damages were received by the NHC.
d. Forecast and Warning Critique
Sebastien was a tropical storm for less than 3 days, so a
meaningful quantitative evaluation of forecast accuracy could not
be made. However, it is noted that neither the objective guidance
nor the official forecast accurately indicated the rapid initial
motion or the subsequent southwestward turn. In fact, most of the
guidance never showed a southwestward motion even though Sebastien
headed in that direction for its final 48 hours.
A tropical storm watch for the U.S. and
British Virgin Islands was issued at 0900 UTC on the 23rd and discontinued 24 hours later.
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