Lisa briefly reached hurricane force in the
central North Atlantic Ocean and did not affect land.
a. Synoptic History
Lisa originated from a tropical wave which moved westward from Africa
into the eastern tropical Atlantic Ocean on 29 September. The associated cloudiness was fairly well organized and centered at
about 10°N latitude. By the next day, it was an almost indistinguishable part of the Intertropical Convergence
Zone(ITCZ) which was active across the entire tropical Atlantic. By 3 October, the system became better
defined as its convection increased and the ITCZ cloudiness dissipated to its east and west. On the 4th,
midway between Africa and the Lesser Antilles, there were signs of a low level circulation and it is estimated
that a tropical depression formed at 0000 UTC on the 5th. The
best track of Lisa begins at this time as plotted
in Fig. 1 (44K GIF) and listed in Table 1.
The depression strengthened into Tropical Storm Lisa on the 5th,
although it was in an environment of strong vertical shear, as evidenced by the low-level
center being exposed to the west of the
associated deep convection. This shear was caused by an upper level low located to the northwest of the
storm. The presence of this low also weakened the ridge to the north, causing the storm's motion to begin a
turn toward the north. During the next two days, a strong baroclinic trough in the westerlies evolved into a deep
low in the central North Atlantic. This resulted in an acceleration toward the northeast. The forward speed
reached in excess of 50 knots by the afternoon of the 9th. The vertical shear relaxed over the storm and it
gradually strengthened. Lisa turned northward on the 9th, steered by the deep low to its west and a 1032 mb
high to its east. This strong east-west pressure gradient also resulted in increasing the surface winds well to
the east of the center and Lisa briefly strengthened to a 65-knot hurricane on
the 9th, before merging with an extratropical frontal system in the far North Atlantic.
On the 10th, it was no longer possible to identify a well-defined circulation on satellite imagery.
b. Meteorological statistics
Figs. 2 (29K GIF) and 3 (36K GIF)
show plots of satellite-based pressure and wind estimates and the best-track pressure and wind curves.
Dvorak estimates were provided by the U.S.
Air Force Weather
Agency (AFGWC), the Tropical Analysis and Forecast Branch (TAFB) of the Tropical Prediction Center,
NWS and the Synoptic Analysis Branch (SAB) of NESDIS.
A NOAA drifting buoy (16.6N, 46.9W) in the central tropical Atlantic
provided a wind observation of 35 knots at 0850 UTC on the 5th and another of 36
knots at 2138 UTC. These observations were essential in determining that Lisa had become a
tropical storm, as satellite-based intensity estimates were well below storm strength at these times. The
estimate that Lisa acquired 65-knot hurricane-force winds on the 9th was based
on satellite intensity estimates and on a report of 61 knots from the ship ZCBD9
located at 46.9N, 33.3W at 1800 UTC (approximately 240 n mi east of the center). The system was rapidly
transforming into an extratropical system during this time and it is not certain that the strongest winds were
near the center.
c. Casualty and damage statistics
There were no reports of casualties or damage received.
d. Forecast and warning critique
The average official track forecast errors for Lisa were 89 (15 cases), 200 (13 cases), 265 (11
cases), 314 (9 cases), and 417 n mi (5 cases), respectively, for the 12-, 24-, 36-, 48-, and 72-hour forecast
periods. These are all larger than the 1988-1997 average official errors of 47, 88, 127, 166, and 248 n mi for
the same forecast periods. The GFDL and UKMET track guidance models had slightly smaller
errors than the official forecasts, but it is understandable that a forward speed in excess of 50 knots could result in rather large
track errors. The CLIPER average errors for Lisa exceeded 1000 n mi at 72 hours.
The official intensity forecast errors showed a negative bias. It was not anticipated that Lisa
would become a hurricane under the vertical wind shear that characterized the environmental conditions during
much of the storms's existence.
There were no watches or warnings issued.