TROPICAL CYCLONE REPORTS
Tropical Storm Allison
Tropical Depression Two
Tropical Storm Barry
Tropical Storm Chantal
Tropical Storm Dean
Tropical Depression Nine
Tropical Storm Jerry
Tropical Storm Lorenzo
Tropical Cyclone Report
24 November - 4 December 2001
Lixion A. Avila
National Hurricane Center
17 December 2001
Tenacious late season Hurricane Olga roamed over the Atlantic
Ocean for about a week.
a. Synoptic History
Hurricane Olga had a non-tropical origin and formed from the
interaction of a cold front and a small area of disturbed weather
in the north Atlantic. Five previous tropical cyclones and gale
centers had already originated from a similar pattern in that area
during this season. Olga was an uncommon event but not unique.
There have been other late- or off-season tropical cyclones which
moved southwestward across the Atlantic, such as Hurricane Lili
during December of 1984. The long and winding track of Olga is
displayed in Figure 1.
Surface observations and satellite imagery showed falling
pressures and increasing weather in the area between Bermuda and
the Leeward Islands as early as the 21st of November. A
cold front reached the area on the 22nd, resulting in
the formation of an extratropical low. The extratropical low
gradually intensified and it is estimated that it transformed into
a subtropical storm at 0000 UTC on the 24th, about 780 n
mi east-southeast of Bermuda. At this time, satellite imagery
showed the typical pattern of a subtropical cyclone with most of
the weather within a comma-shaped cloud band located to the east of
a low-level circulation center and a large area of gale force winds
extending far to the north. During the next several hours, the
thunderstorm activity markedly increased near an inner center of
circulation which began to form, suggesting that the system was
acquiring tropical characteristics. However, the wind field
indicated that the system was a hybrid.
The shower activity continued to become more concentrated near
the center and a post-analysis of satellite imagery indicates that
by 1200 UTC 24 November, the inner portion of the system had
already transformed into a tropical storm. In fact, there was a
hint of an eye feature on visible satellite imagery at 1545 UTC on
that day (Figure 2).
However, one can argue that the system was not
purely tropical, because it remained embedded within the much
larger extratropical circulation.
The cyclone, now Tropical Storm Olga, moved slowly toward the
northeast. As a high pressure ridge built to the north, Olga turned
toward the west and west-southwest and gradually intensified. By
then, satellite imagery showed well defined hooking bands
surrounding a central dense overcast. Olga became a hurricane by
1200 UTC on the 26th and reached its peak intensity of
80 knots and a minimum pressure of 973 mb around 0600 UTC 27
From the 26th to the 28th, Olga made a
double cyclonic loop as it interacted with a large-scale deep-layer
cyclonic circulation that was pretty much isolated from the main
belt of westerlies. Once it completed the second loop on the
28th, Olga became steered southwestward by another
strong area of high pressure to the northwest. It then encountered
strong upper-level northerly and then westerly shear, displacing
the convection from the center of circulation. Olga weakened to
tropical depression status by 1200 UTC 30 November, but a temporary
relaxation of the shear, allowed deep convection to redevelop near
the center. Satellite imagery suggested that Olga regained tropical
storm status at 0000 UTC 2 December. By then, Olga was moving
northward and began to make another loop, this time
Olga weakened as it was making this final loop and another high
pressure ridge to the north steered the tropical cyclone
west-southwestward. Olga gradually lost its associated deep
convection and low-level circulation, and dissipated by 0000 UTC on
the 5th about 600 n mi east of Nassau in the Bahamas. A
vorticity maximum, embedded within a trough continued westward
across the Bahamas, the Florida Straits, Cuba and the eastern Gulf
of Mexico producing gusty winds and locally heavy showers.
b. Meteorological Statistics
Table 1 gives the best track positions
and intensities of Olga
at six-hourly intervals. Figure 1 shows a plot of this track.
Figure 3 and Figure 4 depict the curves of
maximum one-minute average (10 m above sea-level) wind speed and
minimum sea-level pressure,
respectively, as functions of time. Also plotted are the
observations on which the curves are based. These consist of ship
observations and data from satellite-based Dvorak- and
Hebert-Poteat technique estimates by the Tropical Analysis and
Forecast Branch (TAFB), Satellite Analysis Branch (SAB), and the
U.S. Air Force Weather Agency (AFWA). The peak intensity associated
with Olga was based on Dvorak objective T-numbers which were the
highest around 0600 UTC 27 November.
There were several reports from vessels along the path of Olga
and these are included in Table 2. The most significant observation
came from a German sailing yatch Manana Tres
which apparently went through the center of the cyclone and
reported a minimum pressure of 989 mb at 0900 UTC 24 November. This
was the basis for starting advisories. The ship Liberty Sun
encountered the center of Olga during the
25th and the 26th, and captured the wind
shift and the pressure change associated with the tropical cyclone
as summarized in Table 2.
Large waves associated with Olga spread
onto the U.S. east coast, the Bahamas and most of the Caribbean
Islands. In fact, some of the France's buoys reported waves in
excess of 12 feet as far south as Guadeloupe.
c. Casualty and Damage Statistics
The only report of damage came from the Manana
Tres which indicated "lots of damage" during its encounter
d. Forecast and Warning Critique
Average official track errors for Olga were
63, 115, 141, 173, and 215 n mi for the 12, 24, 36, 48 and 72 h
With the exception of the 72 h forecasts, these errors were larger
than last the 10-yr period as indicated in Table 3.
interaction of Olga with the large-scale deep-layer cyclonic
circulation was considered in the official forecast, the tropical
cyclone ended both loops and moved toward the west and
west-southwest faster than anticipated. This probably caused large
speed errors during that period. The NCEP Global model (AVNI)
outperformed every other model and the official forecast.
The absolute wind speed errors for Olga were 4, 7, 11,
14 and 12 kt for the 12, 24, 36, 48, and 72 h forecasts,
respectively. For comparison the past 10-yr errors are 7, 11 14, 16
and 20 kt, for the same forecast periods.
Operationally, Olga was
carried as a subtropical storm until 1500 UTC 26 November; while
the best track shows the system becoming a tropical cyclone two
days earlier - at 1200 UTC on the 24th. The lack of
well-defined criteria separating subtropical from fully-tropical
cyclones continues to be troublesome in these designations. In the
initial advisory on the system on the 24th, it was noted
that the low had a "small tropical-like center embedded within a
huge non-tropical cyclone", and therefore was designated a
subtropical cyclone (one that shares characteristics of tropical
and non-tropical cyclones). The primary goal was to focus the
marine community's attention on the large area of strong winds,
which were largely unrelated to the developing inner core. Over the
next two days the wind field gradually contracted in response to
the persistent inner convection and the system was designated Olga
on the 26th. An alternative interpretation, and the one
taken in the construction of the post-storm best track, is that a
small tropical cyclone had formed on the 24th with the
development of the inner core, and that this tropical cyclones was
embedded within a large-scale cyclonic environment.
Table 1: Best track for Hurricane Olga, 24 November - 4 December 2001.
|Lat. (°N)||Lon. (°W)
|23 / 0600||30.5||51.0||1003||25||extratropical
|23 / 1200||30.2||51.0||1000||25||"
|23 / 1800||29.7||50.7||994||30||"
|24 / 0000||29.3||50.3||990||50||subtropical storm
|24 / 0600||29.3||50.0||990||50||"
|24 / 1200||29.5||49.8||989||50||tropical storm
|24 / 1800||30.3||49.5||989||50||"
|25 / 0000||31.3||50.6||989||50||"
|25 / 0600||31.8||52.0||989||50||"
|25 / 1200||31.7||53.5||989||55||"
|25 / 1800||30.8||55.0||987||60||"
|26 / 0000||30.3||55.3||979||60||"
|26 / 0600||30.1||55.6||979||60||"
|26 / 1200||30.6||55.9||979||65||hurricane
|26 / 1800||31.1||56.0||977||65||"
|27 / 0000||32.1||56.2||975||75||"
|27 / 0600||32.6||57.0||973||80||"
|27 / 1200||31.5||57.2||974||80||"
|27 / 1800||32.0||56.0||975||75||"
|28 / 0000||32.3||55.8||975||75||"
|28 / 0600||32.6||55.7||975||75||"
|28 / 1200||32.4||56.2||977||75||"
|28 / 1800||31.4||57.0||979||70||"
|29 / 0000||30.3||57.6||987||65||"
|29 / 0600||29.3||58.7||990||60||tropical storm
|29 / 1200||28.4||60.1||992||60||"
|29 / 1800||27.3||61.8||994||55||"
|30 / 0000||26.2||63.2||998||45||"
|30 / 0600||25.3||64.7||1001||35||"
|30 / 1200||24.5||66.2||1003||30||tropical depression
|30 / 1800||23.7||67.1||1004||30||"
|01 / 0000||23.4||67.7||1004||30||"
|01 / 0600||23.1||68.0||1005||30||"
|01 / 1200||22.8||68.3||1004||30||"
|01 / 1800||23.0||68.6||1004||30||"
|02 / 0000||23.4||68.9||1003||35||tropical storm
|02 / 0600||24.3||68.9||1003||35||"
|02 / 1200||25.5||69.2||1001||40||"
|02 / 1800||26.4||69.5||1001||40||"
|03 / 0000||27.0||69.7||1004||35||"
|03 / 0600||27.4||69.9||1005||35||"
|03 / 1200||27.8||69.9||1005||35||"
|03 / 1800||27.7||69.6||1005||35||"
|04 / 0000||27.7||69.1||1006||30||tropical depression
|04 / 0600||27.8||68.2||1006||30||"
|04 / 1200||27.8||68.0||1006||30||"
|04 / 1800||27.0||67.4||1006||30||"
|05 / 0000||dissipated
|27 / 0600||32.6||57.0||973||80||minimum pressure
Table 2: Selected ship observations of tropical storm or greater
winds associated with Hurricane Olga, 24 November - 4 December 2001.
|Ship Name or Call Sign||Date/Time (UTC)||Lat. (°N)||Lon. (°W)||Wind dir/speed (deg/kt)||Pressure (mb)
|Dorothea Schulte||24/0000||31.9||51.5||010/38 ||1008.0
|Dorothea Schulte||24/0600||32.3||50.3||010/39 ||1008.0
|Irvin Primrose||24/0600||34.2||52.2||040/45 ||1012.5
|Manana Tres||24/0900||29.5||51.0|| ||989.0
|Liberty Sun||24/1200||33.1||58.4||350/36 ||
|Dorothea Schulte||24/1200||32.6||49.1||070/39 ||1005.0
|Irvin Primrose||24/1200||34.2||53.8||040/45 ||1013.5
|Dorothea Schulte||24/1800||32.9||48.0||070/39 ||1005.1
|Lykes Liberator||25/0900||37.4||52.7||050/47 ||1013.5
|Lykes Liberator||25/1200||37.3||53.8||040/44 ||1015.2
|Liberty Sun||25/1800||30.8||56.1||360/55 ||990.0
|Liberty Sun||25/2100||30.1||55.8||340/38 ||983.5
|Liberty Sun||26/0000||29.7||55.5||270/31 ||981.4
|Liberty Sun||26/0600||30.1||54.5||275/45 ||990.0
|Liberty Sun||26/0900||30.4||53.9||170/29 ||993.0
|Liberty Sun||26/1200||30.7||53.4||160/38 ||995.0
|Safmarine Infanta||26/1200||24.1||58.2||320/37 ||1004.0
|City of Alberni||29/1800||27.2||65.5||020/35 ||1008.0
|Lykes Liberator||25/1200||37.3||53.8||040/44 ||1015.2
Table 3: Table 3. Forecast evaluation (heterogeneous sample) for
Olga, 24 November- 4 December, 2001. Forecast errors for tropical
storm and hurricane stages (n mi) are followed by the number of
forecasts in parentheses. Errors smaller than the NHC official
forecast are shown in bold-face type.
|Forecast Technique||Period (hours)
|CLIP||86 (28)||195 (24)||281 (20)||406 (18)||488 (18)
|GFDI||49 (25)||89 (21)||161 (17)||302 (15)||576 (14)
|LBAR||76 (28)||162 (24)||242 (20)||352 (18)||483 (18)
|AVNI||44 (25)||67 (21)||88 (17)||134 (14)||186 (12)
|BAMD||60 (28)||119 (24)||170 (20)||232 (18)||318 (18)
|BAMM||62 (28)||111 (24)||154 (20)||199 (18)||233 (17)
|BAMS||74 (28)||128 (24)||169 (20)||225 (18)||298 (18)
|NGPI||51 (26)||69 (22)||74 (18)||116 (16)||195 (14)
|UKMI||63(19)||110 (16)||119 (14)||130 (13)||159 (8)
|GUNS||50 (19)||80 (16)||105 (14)||159(13)||269(8)
|GUNA||47 (19)||71 (16)||92 (14)||143 (13)||219 (8)
|NHC Official||63 (27)||115 (23)||141 (19)||173 (17)||215 (17)
|NHC Official (1991-2000 mean)||44 (2049)||82 (1835) ||118 (1646) ||151 (1475) ||226 (1187)
|*Output from these models was unavailable at time of forecast issuance.
Best track positions for Hurricane
Olga, 24 November to 4 December, 2001. Dotted line with X marks the
locations of the vorticity maximum associated with the remnants of Olga.
GOES visible satellite image showing Olga as a tropical storm at
1545 UTC 24 November, 2001. Note the eye feature trying to form.
Best track maximum sustained surface wind speed curve for
Hurricane Olga, 24 November to 4 December, 2001, and the observations
on which the best track curve is based.
Best track minimum central pressure for Hurricane Olga,
24 November to 4 December, 2001, and the
observations on which the best track curve is based.