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TROPICAL CYCLONE REPORTS
Hurricane Adolph
Tropical Storm Barbara
Tropical Storm Cosme
Tropical Storm Erick
Hurricane Dalila
Tropical Depression Six-E
Hurricane Flossie
Hurricane Gil
Tropical Storm Henriette
Tropical Storm Ivo
Hurricane Juliette
Hurricane Kiko
Tropical Storm Lorena
Tropical Depression Fourteen-E
Tropical Storm Manuel
Hurricane Narda
Hurricane Octave
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Tropical Cyclone Report
Hurricane Gil
4 - 9 September 2001
Jack Beven
National Hurricane Center 25 October 2001
Gil was an eastern North Pacific hurricane with maximum winds
that reached 85 kt, but that will be best remembered for its
interaction with and absorption of Tropical Storm Henriette.
a. Synoptic history
A tropical wave moved westward from the African coast
on 14-15 August. The system showed signs of organization as it
approached the Lesser Antilles on 21 August, with the northern
portion of the wave spawning Tropical Storm Dean on 22 August. The
southern portion of the wave continued westward, crossing Central
America into the Pacific on 24 August. Little development occurred
for the next several days as the wave continued westward.
Convection associated with the wave increased on 30 August while
large-scale low-level cyclonic turning developed over the tropical
eastern Pacific - an environment similar to the western Pacific
monsoon environment. The wave gradually organized over the next few
days, and Tropical Depression Eight-E formed near 0600 UTC 4
September about 850 n mi southwest of Cabo San Lucas, Mexico
(Table 1 and Figure 1).
The cyclone strengthened and became Tropical Storm
Gil six hours later. At that time, Tropical Depression Nine-E
(which became Henriette) formed about 765 n mi to the east of
Gil.
Gil moved westward with its speed varying from 3-8 kt
from 4-6 September. Steady strengthening occurred during this time,
with Gil becoming a hurricane early on the 6th. It
reached a peak intensity of 85 kt by 1800 UTC that day when an eye
was visible in both conventional and microwave satellite imagery.
At that time the stronger Gil was 465 n mi southwest of the larger
Henriette.
Gil turned northwestward early on 7 September. Later
that day it accelerated northward as Henriette began to pass to the
north and the two cyclones began interacting in earnest. Gil
weakened during this time because the rapid northward motion into
the northeasterly upper-level outflow from Henriette caused shear.
By 0000 UTC on the 8th, Henriette was passing 330 n mi
north of Gil and the two cyclones began to rotate around each
other. This caused Gil to move at 20-25 kt on a north-northwest
track that changed to west by 0000 UTC on the 9th. Gil
weakened steadily during this time and had become a depression by
early on the 9th.
While Henriette dissipated as a tropical cyclone
shortly after 1200 UTC on the 8th about 210 n mi west of
Gil, the remnant low-to-mid level vorticity center continued to
move around the south and east sides of Gil. This caused Gil to
turn southwestward early on the 9th. Once Henriette was
fully absorbed, Gil slowed from a 20-25 kt motion to a westward
drift in a few hours. Associated convection totally dissipated
during the merger and did not return afterwards. This resulted in
Gil dissipating late on the 9th about 1000 n mi east of
the Hawaiian Islands. The remnant low cloud swirl moved generally
west-northwestward to northwestward for several days, finally being
absorbed by a cold front well to the north of the Hawaiian Islands
on the 19th.
The merger of two tropical cyclones or the absorption
of one tropical cyclone by another are uncommon events in the
National Hurricane Center's area of responsibility. The last
documented case of such an occurrence in the eastern North Pacific
was when Tropical Storm (later to become Hurricane) Norbert
absorbed Tropical Depression Eighteen-E in September 1990 (Avila
1991).
b. Meteorological statistics
The "best track" of Gil is given in Table 1 and
Figure 1. Figure 2 and Figure 3
show the best track maximum sustained (1
min average) surface (10 m elevation) wind speed and minimum
central pressure, as well as the associated observations. These
include Dvorak technique position and intensity estimates from the
Tropical Analysis and Forecast Branch (TAFB), the NOAA/NESDIS
Satellite Analysis Branch (SAB), and the Air Force Weather Agency
(AFWA).
The combination of Gil and Henriette helped trigger a
strong surge of southwesterly and southerly flow to the east and
southeast of the cyclones. The ship Pacific
Highway (call sign H3AK) encountered this flow, reporting
40 kt winds, a 1005.3 mb pressure, and 22 ft seas at 0000 UTC 7
September while about 205 n mi southeast of the center of Gil.
c. Casualty and damage statistics
No reports of damages or casualties have been
received by the National Hurricane Center (NHC).
d. Forecast and warning critique
Fujiwhara interaction between tropical cyclones
and absorption of one cyclone by another are rare events in the NHC
area of responsibility. Thus, it might be expected that this
complex scenario would produce worse-than-average track forecasts,
and this was indeed the case for Gil, although not for
Henriette. Table 2 shows the average errors
during the tropical storm and hurricane stages of Gil for the
official NHC track forecast and a selection of objective guidance
models.The average track forecast errors
(with the number of cases in parentheses) were 55 (16), 104 (14),
133 (12), 143 (10), and 151 (6) n mi for 12, 24, 36, 48, and 72 h
respectively. This compares to the 1991-2000 average errors of 37,
68, 99, 128, and 185 n mi for 12, 24, 36, 48, and 72 h. Examination
of individual forecasts shows two main sources of error. Early
forecasts moved the cyclone steadily toward the west-northwest or
west and failed to capture either the slow motion on 5-6 September
or the northward turn on the 7th and 8th.
Later forecasts did capture the northward turn and the subsequent
westward turn. However, these forecasts were generally too
slow.
The average intensity forecast errors were 8, 14, 19,
17, and 13 kt for the 12, 24, 36, 48, and 72 h forecasts
respectively. These can be compared to the 10-year average errors
of 7, 12, 16, 19, and 21 kt for those forecast times. Examination
of the individual forecasts shows that early forecasts on Gil
underestimated how much it would intensify, while later forecasts
underestimated how much it would weaken during the interaction with
Henriette.
Warnings and watches were not required for Gil.
References
Avila, L. A., 1991: Eastern North Pacific hurricane
season of 1990. Mon. Wea. Rev., 119, 2034-2046.
Table 1: Best track for Hurricane Gil, 4 - 9 September 2001.
Date/Time (UTC) | Position | Pressure (mb) | Wind Speed (kt) | Stage |
Lat. (°N) | Lon. (°W) |
04 / 0600 | 15.4 | 122.6 | 1006 | 30 | tropical depression |
04 / 1200 | 15.4 | 123.4 | 1004 | 35 | tropical storm |
04 / 1800 | 15.4 | 124.2 | 1002 | 40 | " |
05 / 0000 | 15.4 | 124.5 | 997 | 50 | " |
05 / 0600 | 15.4 | 124.7 | 994 | 55 | " |
05 / 1200 | 15.3 | 125.0 | 994 | 55 | " |
05 / 1800 | 15.2 | 125.5 | 990 | 60 | " |
06 / 0000 | 15.1 | 126.0 | 986 | 65 | hurricane |
06 / 0600 | 15.1 | 126.5 | 983 | 70 | " |
06 / 1200 | 15.0 | 127.3 | 979 | 75 | " |
06 / 1800 | 14.9 | 128.0 | 975 | 85 | " |
07 / 0000 | 15.1 | 128.5 | 975 | 85 | " |
07 / 0600 | 15.4 | 128.7 | 975 | 85 | " |
07 / 1200 | 15.9 | 129.1 | 979 | 80 | " |
07 / 1800 | 16.5 | 129.5 | 979 | 75 | " |
08 / 0000 | 17.4 | 129.5 | 987 | 65 | " |
08 / 0600 | 18.8 | 129.8 | 990 | 60 | tropical storm |
08 / 1200 | 20.6 | 130.3 | 997 | 50 | " |
08 / 1800 | 22.1 | 132.1 | 1003 | 40 | " |
09 / 0000 | 22.4 | 134.5 | 1006 | 30 | tropical depression |
09 / 0600 | 21.5 | 136.2 | 1008 | 30 | " |
09 / 1200 | 20.7 | 137.2 | 1008 | 30 | " |
09 / 1800 | 20.7 | 137.5 | 1009 | 25 | " |
10 / 0000 | | | | | dissipated |
| | | | | |
06 / 1800 | 14.9 | 128.0 | 975 | 85 | minimum pressure |
Table 2: Preliminary track forecast evaluation for Hurricane Gil - heterogeneous sample. Errors in nautical miles for
tropical storm and hurricane stages with number of forecasts in
parentheses. Bold numbers represent forecasts which were
better than the official forecast.
Forecast Technique | Period (hours) |
12 | 24 | 36 | 48 | 72 |
CLIP | 50 (16) | 91 (14) | 131 (12) | 151 (10) | 148 (6) |
GFDI | 49 (15) | 95 (13) | 133 (11) | 139 (9) | 119 (5) |
GFDL* | 52 (16) | 91 (14) | 133 (12) | 154 (10) | 119 (6) |
GFNI | 47 (10) | 110 (8) | 220 (8) | 334 (6) | 544 (2) |
GFDN* | 37 (5) | 75 (5) | 144 (4) | 255 (4) | 508 (2) |
LBAR | 57 (16) | 117 (14) | 181 (12) | 237 (10) | 391 (6) |
AVNI | 43 (15) | 73 (13) | 112 (11) | 136 (9) | 157 (5) |
AVNO* | 40 (15) | 67 (14) | 100 (12) | 116 10) | 136 (6) |
BAMD | 55 (16) | 95 (14) | 119 (12) | 138 (10) | 255 (6) |
BAMM | 47 (16) | 83 (14) | 105 (12) | 112 (10) | 122 (6) |
BAMS | 52 (16) | 83 (14) | 102 (12) | 110 (10) | 116 (6) |
NGPI | 43 (15) | 92 (14) | 147 (12) | 186 (10) | 294 (6) |
NGPS* | 34 (8) | 66 (7) | 116 (6) | 153 (5) | 266 (3) |
UKMI | 42 (14) | 74 (12) | 95 (10) | 98 (8) | 224 (5) |
UKM* | 43 (8) | 71 (7) | 86 (6) | 102 (5) | 174 (3) |
P91E | 52 (16) | 97 (14) | 138 (12) | 148 (10) | 231 (6) |
P9UK | 49 (8) | 92 (7) | 131 (6) | 155 (5) | 293 (3) |
GUNS | 35 (13) | 77 (12) | 100 (10) | 87 (8) | 115 (5) |
| | | | | |
NHC Official | 55 (16) | 104 (14) | 132 (12) | 143 (10) | 151 (6) |
NHC Official 10-Year Average (1991-2000) | 37 (2273) | 68 (2034) | 99 (1802) | 128 (1584) | 185 (1203) |
*Output from these models was unavailable at time of forecast issuance. |
Figure 1:
Best track for Hurricane Gil, 4-9 September 2001.
Figure 2:
Best track maximum sustained 1-minute 10 meter wind speed curve for
Hurricane Gil, 4-9 September 2001.
Figure 3:
Best track minimum central pressure curve for Hurricane Gil, 4-9 September
2001.
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