Tropical Cyclone Report
Tropical Storm Otto
29 November - 3 December 2004
Stacy R. Stewart
National Hurricane Center 7 December 2004
Otto was a tropical storm that originated
from a mid-latitude baroclinic system. The cyclone remained over
the open Atlantic Ocean about midway between Bermuda and the Azores
Islands.
a. Synoptic History
A cold front emerged off the east coast of
the United States on 21 November and moved slowly eastward before
stalling about midway between Bermuda and the Azores Islands early
on 25 November. Later that day, a strong upper-level trough to the
north dug southward and initiated the development of an
extratropical low along the front about 1000 n mi southwest of the
Azores Islands by 0000 UTC 26 November. Due to the strong
baroclinic effects of the upper-low, the occluded surface low
pressure system quickly deepened and became a large gale area later
that day. The upper-level trough continued to dig southward and
eventually developed a cutoff low to the south of the occluded
surface low on 26 November.
The surface and upper-level lows moved
generally southwestward at 5-10 kt in tandem for the next three
days. Late on 28 November, weak ridging to the north of the
upper-low formed over the surface low and convective banding
features began to develop near the low-level center. In addition,
surface data and analyses from the Tropical Prediction Center's
Tropical Analysis and Forecast Branch (TAFB) indicated that the
frontal structure associated with the cyclone had dissipated.
Dvorak satellite classifications from the various satellite
agencies at that time suggest that Subtropical Storm Otto formed
about 1000 n mi east-southeast of Bermuda around 1200 UTC 29
November. The "best track" chart of the cyclone's path is given in
Figure 1,
while the best track positions and intensities are listed
in Table 1. Wind and pressure plots are shown in
Figure 2 and Figure 3,
respectively.
After Otto acquired subtropical
characteristics, the cyclone moved northwestward through a weakness
in the mid-level subtropical ridge located to its north. Convection
continued to increase over the low-level center and Advanced
Microwave Sounding Unit (AMSU) satellite-derived temperature data
indicate the system acquired enough warm-core characteristics to be
considered a tropical storm at 1200 UTC 30 November about 700 n mi
east of Bermuda. For the next 2 days, the cyclone meandered over
relatively cool water (sea-surface temperatures of
21-23° C) and beneath a region of relatively weak
vertical shear between the upper-low located to its south and
another weaker upper-low situated to its northeast. The marginally
favorable upper-level flow pattern allowed Otto to maintain some
central deep convection. The tropical storm turned slowly
southeastward on 1 December as north to northwesterly mid- to
upper-level flow increased on the east side of an amplifying ridge
off the U.S. east coast. However, the vertical shear also increased
causing the deep convection to become displaced to the southeast of
the center, which induced a slow weakening trend. Otto weakened
into a tropical depression on 2 December. Mid-level dry air
entrainment, coupled with the increasing vertical shear, eventually
eroded the convection and Otto degenerated into a non-convective
remnant low pressure system on 3 December about 800 n mi southeast
of Bermuda. For the next 2 days, the broad remnant drifted slowly
southward and southwestward before dissipating around 1200 UTC 5
December about 800 n mi northeast of the northern Leeward
Islands.
b. Meteorological Statistics
Observations in Otto (Figure 2 and Figure 3)
include satellite-based Dvorak technique intensity estimates from
the Tropical Analysis and Forecast Branch (TAFB), the Satellite
Analysis Branch (SAB), and the U. S. Air Force Weather Agency
(AFWA), as well as microwave satellite imagery from NOAA
polar-orbiting satellites, the NASA Tropical Rainfall Measuring
Mission (TRMM), the NASA QuikSCAT, and Defense Meteorological
Satellite Program (DMSP) satellites.
Ship reports of tropical storm force winds
associated with Otto are given in Table 2.
Otto's peak intensity of 45 kt during the
subtropical storm stage (Figure 4)
is based on a blend of ship
reports and Dvorak satellite intensity estimates. Ship LAXP4
(Star Hansa) reported a sustained wind of 51 kt at 1800 UTC
29 November about 190 n mi northeast of the center. However,
quality control checks by the NOAA Ocean Prediction Center indicate
that the wind speeds had a slight high bias. In addition, Ship
V7BI3 (Sabine) reported a sustained wind of 52 kt at 0900
UTC and 51 kt at 0600 UTC 1 December about 200 n mi northwest of
the center. Quality control checks revealed that those reports had
a severe high bias, and were also considerably higher than the 30-
to 35-kt wind reports from nearby ships. Therefore, the wind
reports from ship V7BI3 were not used in determining the 'best
track' intensity estimates, nor were they included in Table 2.
It is also interesting to note that during Otto's weakening period,
several normally reliable Objective Dvorak (ODT) satellite
intensity estimates (Figure =2)
were much higher than nearby ship and
QuikSCAT wind data were indicating. This may be due to the
relatively shallow nature of Otto (convective cloud top
temperatures were rarely observed below -50° C) and its
possibly possessing more subtropical than tropical
characteristics.
It is difficult to ascertain exactly when
an extratropical low acquires enough warm-core characteristics to
be classified as a subtropical low, or when a subtropical low
becomes a tropical cyclone. Therefore, it is possible that Otto may
have become a tropical cyclone earlier on 29 November.
c. Casualty and Damage Statistics
There were no reports of damages or
casualties associated with Tropical Storm Otto.
d. Forecast and Warning Critique
No meaningful forecast verification
statistics are available due to Otto's short lifetime. However, the
few cases that made up the 12-, 24-, 36-, and 48-h average errors
of 34, 71, 103, and 159 n mi, respectively, were better than the
10-yr average of 44, 78, 112, 146 n mi through 36 h. The larger
track error at 48 h was due to the first two official forecasts,
which called for Otto to move eastward, as suggested by the various
NHC consensus models. Average official intensity errors were 3, 3,
5, and 5 kt for the 12, 24, 36, and 48 h forecasts, respectively.
These errors were exceptionally good as compared to the average
official intensity errors over the 10-yr period 1994-2003 of 6, 10,
12, and 15 kt, respectively.
No tropical cyclone watches or warnings
were required for Tropical Storm Otto. However, the NOAA Ocean
Prediction Center and the TPC's Tropical Analysis and Forecast
Branch issued marine gale warnings for their respective Atlantic
High Seas Forecast areas of responsibility about 18 h before the
development of the pre-Otto extratropical low.
Table 1: Best track for Tropical Storm
Otto, 29 November - December 2004.
Date/Time (UTC) | Position | Pressure (mb) | Wind Speed (kt) | Stage |
Lat. (°N) | Lon. (°W) |
26 / 0000 | 28.0 | 40.0 | 1009 | 30 | extratropical |
26 / 0600 | 27.3 | 41.0 | 1006 | 35 | " |
26 / 1200 | 26.5 | 41.9 | 1002 | 35 | " |
26 / 1800 | 26.8 | 42.5 | 1000 | 35 | " |
27 / 0000 | 26.8 | 43.2 | 1000 | 35 | " |
27 / 0600 | 26.7 | 43.8 | 1000 | 35 | " |
27 / 1200 | 26.4 | 44.3 | 1000 | 35 | " |
27 / 1800 | 26.2 | 44.4 | 1000 | 35 | " |
28 / 0000 | 26.3 | 44.1 | 1000 | 35 | " |
28 / 0600 | 27.4 | 43.9 | 1000 | 35 | " |
28 / 1200 | 28.2 | 44.0 | 1000 | 35 | " |
28 / 1800 | 28.7 | 45.1 | 999 | 35 | " |
29 / 0000 | 28.8 | 46.5 | 998 | 35 | " |
29 / 0600 | 29.0 | 47.8 | 998 | 40 | " |
29 / 1200 | 29.6 | 47.9 | 998 | 40 | subtropical storm |
29 / 1800 | 30.2 | 48.8 | 998 | 45 | " |
30 / 0000 | 30.7 | 49.7 | 997 | 45 | " |
30 / 0600 | 30.9 | 50.5 | 997 | 45 | " |
30 / 1200 | 31.3 | 51.0 | 997 | 40 | tropical storm |
30 / 1800 | 31.7 | 51.0 | 997 | 40 | " |
01 / 0000 | 31.8 | 50.9 | 997 | 40 | " |
01 / 0600 | 31.9 | 50.8 | 997 | 40 | " |
01 / 1200 | 31.6 | 50.4 | 997 | 40 | " |
01 / 1800 | 31.0 | 50.1 | 997 | 35 | " |
02 / 0000 | 30.5 | 49.9 | 997 | 35 | " |
02 / 0600 | 29.9 | 50.1 | 996 | 35 | " |
02 / 1200 | 29.1 | 50.4 | 995 | 30 | tropical depression |
02 / 1800 | 28.5 | 50.1 | 996 | 30 | " |
03 / 0000 | 27.9 | 50.1 | 997 | 30 | " |
03 / 0600 | 27.2 | 50.2 | 998 | 30 | remnant low |
03 / 1200 | 26.5 | 50.3 | 1000 | 30 | " |
03 / 1800 | 26.2 | 50.6 | 1002 | 25 | " |
04 / 0000 | 25.8 | 50.2 | 1005 | 25 | " |
04 / 0600 | 25.4 | 50.1 | 1008 | 25 | " |
04 / 1200 | 25.0 | 50.4 | 1010 | 25 | " |
04 / 1800 | 25.1 | 50.9 | 1011 | 25 | " |
05 / 0000 | 25.5 | 51.1 | 1012 | 20 | " |
05 / 0600 | 25.9 | 51.1 | 1013 | 20 | " |
05 / 1200 | 26.1 | 51.4 | 1014 | 20 | " |
05 / 1800 | | | | | dissipated |
02 / 1200 | 29.1 | 50.4 | 995 | 30 | minimum pressure |
Table 2: Selected ship reports with winds of
at least 34 kt for Tropical Storm Otto,
29 November - 3 December 2004.
Ship Name or Call Sign | Date/Time (UTC) | Lat. (°N) | Lon. (°W) | Wind dir/speed (deg/kt) | Pressure (mb) |
V2OO7 | 26 / 0000 | 33.0 | 41.1 | 070 / 37 | 1015.0 |
V2OO7 | 26 / 1800 | 33.1 | 37.4 | 070 / 37 | 1017.0 |
V3WY2 | 28 / 1800 | 32.1 | 44.9 | 070 / 35 | 1011.7 |
LAXP4 | 29 / 0000 | 32.6 | 51.3 | 060 / 35 | 1014.0 |
V3WY2 | 29 / 0000 | 31.5 | 46.4 | 070 / 35 | 1006.6 |
V3WY2 | 29 / 0600 | 31.0 | 48.0 | 070 / 39 | 1005.0 |
LAXP4 | 29 / 1200 | 33.0 | 48.9 | 060 / 40 | 1010.0 |
V3WY2 | 29 / 1200 | 30.3 | 49.6 | 070 / 35 | 1001.3 |
WAUW | 29 / 1200 | 33.5 | 42.8 | 090 / 35 | 1018.0 |
LAXP4 | 29 / 1800 | 33.5 | 48.1 | 060 / 51 | 1008.8 |
PCGQ | 29 / 1800 | 35.7 | 50.3 | 070 / 37 | 1014.1 |
WAUW | 29 / 1800 | 33.3 | 44.7 | 090 / 37 | 1013.5 |
LAXP4 | 30 / 0000 | 33.8 | 47.8 | 070 / 41 | 1009.2 |
LAXP4 | 30 / 0600 | 34.4 | 46.7 | 080 / 37 | 1010.2 |
LAXP4 | 30 / 1200 | 35.1 | 45.7 | 080 / 37 | 1012.2 |
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Figure 1:
Best track positions for Tropical
Storm Otto, 29 November - 3 December 2004.
Figure 2:
Selected wind observations and best
track maximum sustained surface wind speed curve for Tropical Storm
Otto, 29 November - 3 December 2004. Objective
Dvorak estimates represent linear averages over a three-hour period
centered on the nominal observation time. Surface plots include
ship reports and non-rainflagged QuikSCAT wind vector data.
Subtropical Dvorak satellite classifications are denoted by a small
dot plotted to the left of or below the data symbol.
Figure 3:
Selected pressure observations and
best track minimum central pressure curve for Tropical Storm
Otto, 29 November - 3 December 2004. Objective
Dvorak estimates represent linear averages over a three-hour period
centered on the nominal observation time. Surface
pressure data plots were obtained from ship and buoy observations
within 120 n mi of the center.
Figure 4:
TRMM microwave satellite image at
1752 UTC 29 November 2004, near Otto's peak intensity
of 45 kt as a subtropical storm. The yellow
"X" denotes the location of ship LAXP4 at 1800 UTC when
it reported a sustained wind of 51 kt. Quality
control checks indicate the wind report was about 4-6 kt high
(image courtesy of the Naval Research
Laboratory, Monterey, CA).
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