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TROPICAL CYCLONE REPORTS
Tropical Storm Allison
Tropical Depression Two
Tropical Storm Barry
Tropical Storm Chantal
Tropical Storm Dean
Hurricane Erin
Hurricane Felix
Hurricane Gabrielle
Tropical Depression Nine
Hurricane Humberto
Hurricane Iris
Tropical Storm Jerry
Hurricane Karen
Tropical Storm Lorenzo
Hurricane Michelle
Hurricane Noel
Hurricane Olga
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Tropical Cyclone Report
Hurricane Erin
1 - 15 September 2001
Erin was the third of a series of four "interrupted track"
tropical cyclones during the 2001 Atlantic hurricane season. After
re-forming, it strengthened to a category three hurricane on the
Saffir-Simpson Hurricane Scale, just to the east of Bermuda.
a. Synoptic History
Erin can be traced back to a tropical wave that emerged from
western Africa on 30 August. The system almost immediately showed
signs of tropical cyclone formation, with curvature in the bands of
associated deep convection. Dvorak classifications commenced at
1800 UTC on the 30th. There was little change in the
system's organization, and deep convection was sporadic for the
next day or so. On 1 September the cloud pattern began to become
better organized, and based on the satellite presentation as well
as drifting buoy data that showed a definite closed surface
circulation, it is estimated that a tropical depression formed by
1800 UTC that day, located about 600 n mi west-southwest of the
Cape Verde Islands. With a mid-tropospheric ridge in place to its
north, the tropical cyclone moved on a west to west-northwestward
heading at 14-18 kt over the following three days.
At first, there was weak to moderate vertical shear over the
area, and the cyclone was able to strengthen into Tropical Storm
Erin by 0600 UTC 2 September. Erin's maximum winds increased to
near 50 knots by 0600 UTC 3 September. Later on the 3rd,
vertical shear associated with an upper-level low to the northwest
caused the low-level center of the storm to become exposed to the
southwest of the main area of deep convection, indicating that Erin
had weakened. On the 4th, the shear appeared to lessen
somewhat, and Erin re-strengthened slightly. However the system
failed to become much better organized, and was soon on a weakening
trend. By 5 September, southwesterly shear caused the tropical
cyclone to degenerate into an area of disturbed weather.
About a day later, a surface circulation re-developed in the
northern part of the area of disturbed weather that was associated
with Erin. The re-generated tropical depression moved
north-northeastward, then north-northwestward, and regained
tropical storm strength about 550 n mi north-northeast of the
northern Leeward Islands at 1800 UTC 7 September. After a mid-level
ridge to the north of Erin was temporarily weakened by a passing
trough, the ridge re-built, moving the storm toward the northwest
and north-northwest. Erin strengthened into a hurricane late on the
8th. While passing east of Bermuda on the 9th, Erin
continued to strengthen, and it reached its peak intensity of 105
kt around 1800 UTC that day. A few hours later, the eye of the
hurricane passed within about 90 n mi east-northeast of Bermuda,
which was Erin's point of closest approach to the island.
After brushing Bermuda, the hurricane continued to move mainly
toward the north-northwest. On 10 September, Erin began to weaken,
however the weakening was slower than usual over the ensuing days,
due in part to slightly warmer than normal waters over the western
subtropical Atlantic. A series of short-wave troughs weakened the
western portion of the Atlantic subtropical ridge. This caused the
motion of the hurricane to turn toward the right, with a decrease
in forward speed, on the 11th. Erin's heading veered
toward the east-northeast and east on the 12th. Then, a
broad, amplifying mid- to upper-level trough over eastern Canada
accelerated Erin toward the northeast. The center passed just east
of Cape Race, Newfoundland at 0000 UTC, while the system was
weakening to just below hurricane strength. Then, Erin lost its
tropical characteristics. The extratropical storm accelerated
north-northeastward and passed over southern Greenland on 16
September, and merged with high-latitude cyclonic flow over eastern
Greenland on the 17th.
b. Meteorological Statistics
The "best track" map of Erin's path is given in
Figure 1, and the
best track positions and intensities are listed in
Table 1. Time
series of the wind and pressure histories of Erin are shown in
Figure 2 and Figure 3, respectively.
Observations in these figures 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 flight-level and dropwindsonde observations from
flights of the 53rd Weather Reconnaissance Squadron of
the U. S. Air Force Reserve Command.
Erin's peak intensity of 105 kt at 1800 UTC 9 September is a
compromise of the following observations taken around that time: 90
percent of the maximum flight-level (700 mb) winds of 118 kt (106
kt); the surface reduction of the lowest 150 m of a dropwindsonde
sounding, yielding 97 kt; and a surface value from that same
sounding of 111 kt. Although the minimum central pressure value of
968 mb around the time of peak intensity would not typically
support quite so high a wind speed, Erin was situated in an area of
higher than average (for a tropical cyclone) environmental
pressures. About 24 h after peak intensity, during 1900-2000 UTC 10
September, radar data and Stepped-Frequency Microwave Radiometer
measurements from a NOAA aircraft showed that Erin had a concentric
eyewall structure.
Bermuda reported a peak wind gust of 36 kt at 2300 UTC 9
September, about when the center of Erin was nearest to that
island. Cape Race, Newfoundland reported sustained winds of 38 kt
with a gust to 46 kt at 2100 UTC 14 September, and a sustained wind
of 46 kt with a gust to 58 kt at 0200 UTC 15 September. St. John's,
Newfoundland reported a gust to 45 kt at 0352 UTC. Grates Cove,
Newfoundland reported a gust to 56 kt at 0449 UTC 15 September, and
Bonavista Newfoundland reported sustained winds of 34 kt with a
gust to 58 kt at 0536 UTC 15 September. Note that the observations
after 0000 UTC 15 September occurred during the extratropical stage
of Erin. Maximum rainfall totals from Newfoundland were 131 mm (5.1
in) at Sagona Island, 102 mm (4.0 in) at Burgeo, and 78 mm (3.1 in)
at Bonavista.
Ship and buoy reports of winds of tropical storm force
associated with Erin are listed in Table 2.
There were a couple of
drifting buoy observations of hurricane force winds near the
tropical cyclone, at 0000 and 0300 UTC 4 September, which have been
excluded because they were not consistent with nearby observations
in time and/or space (the best track intensity was 40-45 kt around
these times).
Figure 4 is a striking, high-resolution
(250 m) visible image of
the inner core of Erin just before its peak intensity, taken with
the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard
the polar-orbiting NASA Terra satellite. Note the presence of two
distinct vortices inside the eye. Such features have been observed
before in intense hurricanes.
c. Casualty and Damage Statistics
There were no reports of damage or casualties associated with
Erin.
d. Forecast and Warning Critique
Table 3 lists the average track forecast errors for Erin, for
selected numerical guidance models and the official forecasts. It
can be seen that the average official track forecasts for this
tropical cyclone were better than the most recent ten-year averages
at all forecast times. However, the AVNI and GFDI model forecasts
had even lower average errors at almost all forecast times,
especially at 72 h.
Average absolute official intensity errors were 5, 10, 15, 16,
and 11 kt for the 12, 24, 36, 48, and 72 h forecasts, respectively.
For comparison, the average official intensity errors over the
10-yr period of 1991-2000 are 7, 11, 14, 16, and 20 kt,
respectively. So, the average NHC intensity forecasts for Erin were
about the same as the long-term averages for 12 through 48 h, and
somewhat better than average at 72 h. In general, Erin's strength
was over-predicted during the first few days of its existence. In
particular, the NHC forecasts never anticipated that the system was
going to dissipate in the deep tropics. After Erin's re-formation,
the NHC intensity forecasts were mostly under-predictions, and
strengthening to a major hurricane was not forecast until just 12 h
before it happened. For comparison, the average absolute errors of
the SHIPS model intensity forecasts for Erin were 7, 11, 16, 20,
and 24 kt.
Table 5 lists the watches and warnings associated
with Erin. The
government of Bermuda issued a hurricane watch 30 h before Erin's
closest point of approach, and a hurricane warning 24 h before the
closest point of approach to the island.
e. Acknowledgments
The high-resolution MODIS image of Erin was provided by Liam
Gumley of the Space Science and Engineering Center of the
University of Wisconsin-Madison. MODIS data acquired by direct
broadcast from the NASA Terra spacecraft at the Space Science and
Engineering Center, University of Wisconsin-Madison. Paul Vukits of
the NOAA Marine Prediction Center provided analyses and high seas
forecasts that were used to produce the extratropical portion of
the best track.
Table 1: Best track for Hurricane Erin, 1 - 15 September 2001.
Date/Time (UTC) | Position | Pressure (mb) | Wind Speed (kt) | Stage |
Lat. (°N) | Lon. (°W) |
01 / 1800 | 12.5 | 34.3 | 1006 | 30 | tropical depression |
02 / 0000 | 12.9 | 35.9 | 1005 | 30 | " |
02 / 0600 | 13.2 | 37.5 | 1002 | 40 | tropical storm |
02 / 1200 | 13.5 | 39.0 | 1003 | 45 | " |
02 / 1800 | 14.1 | 40.6 | 1003 | 45 | " |
03 / 0000 | 14.7 | 42.2 | 1003 | 45 | " |
03 / 0600 | 15.3 | 43.7 | 1003 | 50 | " |
03 / 1200 | 15.9 | 45.3 | 1003 | 50 | " |
03 / 1800 | 16.3 | 47.1 | 1002 | 40 | " |
04 / 0000 | 16.7 | 48.7 | 1002 | 40 | " |
04 / 0600 | 17.0 | 50.3 | 1000 | 45 | " |
04 / 1200 | 17.0 | 52.0 | 1000 | 45 | " |
04 / 1800 | 17.2 | 53.4 | 1000 | 45 | " |
05 / 0000 | 17.4 | 54.8 | 1000 | 45 | " |
05 / 0600 | 17.8 | 55.9 | 1000 | 45 | " |
05 / 1200 | 18.1 | 57.0 | 1013 | 35 | " |
05 / 1800 | 19.0 | 57.9 | 1014 | 25 | low pressure area |
06 / 0000 | 20.1 | 58.1 | 1015 | 15 | " |
06 / 0600 | 21.2 | 58.2 | 1015 | 15 | " |
06 / 1200 | 22.2 | 58.3 | 1014 | 20 | " |
06 / 1800 | 23.2 | 58.4 | 1013 | 25 | re-formed td |
07 / 0000 | 23.9 | 58.1 | 1012 | 25 | " |
07 / 0600 | 24.4 | 57.6 | 1008 | 30 | " |
07 / 1200 | 24.9 | 57.8 | 1008 | 30 | " |
07 / 1800 | 25.3 | 58.0 | 1005 | 35 | tropical storm |
08 / 0000 | 26.0 | 58.3 | 1004 | 35 | " |
08 / 0600 | 26.5 | 58.8 | 999 | 40 | " |
08 / 1200 | 27.2 | 59.2 | 999 | 45 | " |
08 / 1800 | 28.4 | 59.8 | 994 | 60 | " |
09 / 0000 | 29.7 | 60.4 | 987 | 75 | hurricane |
09 / 0600 | 30.6 | 61.3 | 982 | 90 | " |
09 / 1200 | 31.5 | 62.2 | 979 | 95 | " |
09 / 1800 | 32.4 | 62.8 | 968 | 105 | " |
10 / 0000 | 33.3 | 63.3 | 969 | 105 | " |
10 / 0600 | 34.2 | 64.1 | 969 | 105 | " |
10 / 1200 | 34.9 | 64.7 | 969 | 100 | " |
10 / 1800 | 35.7 | 65.4 | 970 | 90 | " |
11 / 0000 | 36.4 | 65.7 | 973 | 80 | " |
11 / 0600 | 36.9 | 65.9 | 976 | 80 | " |
11 / 1200 | 37.4 | 65.6 | 976 | 80 | " |
11 / 1800 | 37.8 | 65.1 | 976 | 80 | " |
12 / 0000 | 38.0 | 64.3 | 976 | 80 | " |
12 / 0600 | 38.0 | 63.3 | 979 | 75 | " |
12 / 1200 | 37.9 | 62.6 | 979 | 75 | " |
12 / 1800 | 37.9 | 62.0 | 979 | 75 | " |
13 / 0000 | 38.1 | 61.4 | 979 | 75 | " |
13 / 0600 | 38.3 | 61.0 | 982 | 70 | " |
13 / 1200 | 38.8 | 60.6 | 982 | 70 | " |
13 / 1800 | 39.6 | 60.2 | 982 | 70 | " |
14 / 0000 | 40.6 | 59.3 | 982 | 70 | " |
14 / 0600 | 42.0 | 58.1 | 987 | 65 | " |
14 / 1200 | 43.3 | 56.7 | 987 | 65 | " |
14 / 1800 | 44.7 | 55.2 | 984 | 65 | " |
15 / 0000 | 46.7 | 52.7 | 981 | 60 | tropical storm |
15 / 0600 | 49.0 | 51.0 | 981 | 60 | extratropical |
15 / 1200 | 52.0 | 49.0 | 978 | 55 | " |
15 / 1800 | 55.0 | 47.5 | 976 | 55 | " |
16 / 0000 | 58.0 | 46.0 | 972 | 55 | " |
16 / 0600 | 60.0 | 44.5 | 976 | 55 | " |
16 / 1200 | 61.5 | 42.0 | 981 | 55 | " |
16 / 1800 | 63.0 | 38.9 | 985 | 50 | " |
17 / 0000 | 65.0 | 35.0 | 995 | 40 | " |
17 / 0600 | | | | | merged |
09 / 1800 | 32.4 | 62.8 | 968 | 105 | minimum pressure |
Table 2: Selected ship observations of tropical storm or greater
winds associated with Hurricane Erin, 1 - 15 September 2001.
Ship Name or Call Sign | Date/Time (UTC) | Lat. (°N) | Lon. (°W) | Wind dir/speed (deg/kt) | Pressure (mb) |
41559 | 4/0000 | 17.2 | 48.9 | 060/39 | 1009.4 |
41559 | 4/0200 | 17.2 | 48.9 | 090/41 | 1009.0 |
41559 | 4/0400 | 17.2 | 48.9 | 120/37 | 1010.3 |
41559 | 4/0900 | 17.2 | 48.9 | 140/35 | 1010.8 |
8POI | 9/1800 | 32.5 | 59.7 | 140/34 | 1017.3 |
PCUI | 9/1800 | 32 | 65 | /35 | |
WDA367 | 12/1200 | 36.3 | 64.8 | 340/35 | 1008.0 |
WDA367 | 12/1500 | 36.2 | 63.7 | 310/35 | 1006.0 |
WDA367 | 12/1800 | 36.0 | 62.7 | 270/37 | 1004.0 |
WDA367 | 13/0000 | 36.1 | 60.6 | 230/43 | 1004.0 |
WGMJ | 13/0000 | 36.4 | 65.7 | 360/34 | 1010.8 |
WDA367 | 14/0000 | 39.1 | 52.5 | 120/35 | 1016.0 |
WGMJ | 14/0000 | 36 | 66 | /35 | |
UCTR | 14/1500 | 43.2 | 51.7 | 230/48 | 1006.0 |
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Table 3: Preliminary forecast evaluation (heterogeneous sample)
for Hurricane Erin, 1-15 September 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) |
12 | 24 | 36 | 48 | 72 |
CLIP | 44 (40) | 102 (36) | 162 (32) | 224 (28) | 401 (24) |
GFDI | 32 (39) | 53 (35) | 71 (31) | 87 (27) | 169 (23) |
LBAR | 40 (40) | 82 (36) | 137 (32) | 200 (28) | 346 (24) |
AVNI | 31 (34) | 53 (30) | 71 (26) | 87 (22) | 165 (22) |
BAMD | 41 (40) | 74 (36) | 114 (32) | 160 (28) | 262 (24) |
BAMM | 36 (40) | 67 (36) | 90 (32) | 102 (28) | 191 (24) |
BAMS | 44 (40) | 87 (36) | 123 (32) | 160 (28) | 273 (24) |
NGPI | 40 (38) | 65 (34) | 100 (30) | 119 (26) | 219 (22) |
UKMI | 38 (37) | 81 (33) | 141 (29) | 208 (25) | 353 (22) |
GUNS | 28 (35) | 50 (31) | 74 (27) | 96 (23) | 193 (20) |
NHC Official | 32 (40) | 60 (36) | 80 (32) | 96 (28) | 183 (24) |
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. |
Table 4: Watch and warning summary, Hurricane Erin, 1 - 15 September 2001.
Date/Time | Action | Location |
8/1500 | Hurricane watch issued | Bermuda |
8/2100 | Hurricane watch replaced by hurricane
warning | Bermuda |
9/2100 | Hurricane warning changed to tropical storm
warning | Bermuda |
10/1500 | Tropical storm warning discontinued | Bermuda |
Figure 1:
Best track for Hurricane Erin, September 2001. Track during the
extratropical stage is based on analyses from the NOAA Marine Prediction
Center.
Figure 2:
Best track maximum sustained surface wind speed curve for
Hurricane Erin, September 2001, and the observations on which the
best track curve is based. Aircraft observations have been adjusted for
elevation using 90%, 80%, and 80% reduction factors for observations from
700 mb, 850 mb, and 1500 ft, respectively. Dropwindsonde observations include
actual 10 m winds (sfc), as well as surface estimates derived from the mean
wind over the lowest 150 m of the wind sounding (LLM), and from the sounding
boundary layer mean (MBL). Estimates during the extratropical stage are
based on analyses from the NOAA Marine Prediction Center.
Figure 3:
Best track minimum central pressure curve for
Hurricane Erin, September 2001, and the observations on which the best track
curve are based. Estimates during the extratropical stage are
based on analyses from the NOAA Marine Prediction Center.
Figure 4:
Moderate Resolution Imaging Spectroradiometer (0.66 microns) image of the
inner core of Hurricane Erin, from the Terra (Earth Observing System AM-1)
polar-orbiting satellite at 1530 UTC, 9 September 2001.
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