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A PRELIMINARY ASSESSMENT OF THE LANDFALL OF TYPHOON
CHATAAN ON CHUUK, GUAM, AND ROTA
By
Charles `Chip' Guard, NWS Forecast Office Guam
Mark A. Lander, University of Guam
Bill Ward, NWS Forecast Office Guam

1. Background:
On June 27, a disturbance embedded in an active monsoon trough
began to organize near 4N 154E, or about 350 miles southwest of Pohnpei and 275 miles
southeast of Chuuk Lagoon. This disturbance was upgraded by the Joint Typhoon
Warning Center (JTWC) to Tropical Depression (TD) 08W at 0000 UTC on 28 June and
to Tropical Storm (TS) 08W, 24 hours later. TS 08W was named Tropical Storm
Chataan (pronounced tsa-Ta-an) by RSMC-Tokyo at 0600 UTC on 29 June. The
monsoon trough in which Chataan was embedded brought heavy rains and strong winds
to a large portion of the tropical western North Pacific, including Pohnpei State and
Chuuk State.

From 27 June to 1 July, the system drifted slowly and erratically north, then east toward
Pohnpei, then northwest toward Chuuk. On 1 July, Chataan, located near 6N156E, was
forecast to maintain a northwest to west-northwest track and pass northeast of Chuuk
Lagoon. However, the tropical storm, still poorly organized, began to take a more
westward track, passing very close to, or just south of, the weather station at Weno Island
(WMO 91334) in Chuuk Lagoon on the afternoon and evening of 3 July. On 1 and 2
July, the rain field associated with Chataan was very asymmetric, with an area of
extremely heavy rainfall and deep convection on the west side of the circulation. This
area of convection moved slowly over the Lagoon on 2 and 3 July, dropping nearly 20
inches of rain. Figure 1 is an infrared satellite picture of Chataan near Chuuk.

After passing Chuuk, Chataan resumed a northwest track and began to intensify. Chataan
was upgraded to a typhoon at 1800 UTC on 3 July, while moving toward the Mariana
Islands on a track predicted to pass between Guam and Saipan. On the afternoon of
4 July, the typhoon came into the range of the Andersen Air Force Base (AFB)
WSR-88D NEXRAD radar. Shortly thereafter, Chataan took a more westward track
toward the Rota Channel and northern Guam. The center of the eye moved just slightly
north of Guam, with the southern half of the eye and eyewall cloud traversing the island.
The eye entered the northeast side of the island at about 2130 UTC on 4 July and exited
the northwest side of the island about 0000 UTC on 5 July. The eye lasted from 2 hours
over northern Guam to just minutes over central parts of the island. Most of the southern
half of the island remained in the eyewall cloud, being pummeled continuously by strong
winds and heavy rains. Figure 2 is a visible satellite image of Typhoon Chataan while it
was over Guam. Figure 3 is a radar image of Chataan over the island.

After passing over Guam, Chataan continued on a northwest track. It eventually recurved
to the northeast and made landfall at 1500 UTC 10 July on the Boso Peninsula of Japan.
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Shortly thereafter, it became extratropical about 60 miles east of Tokyo Bay. Figure 4
shows the track of Typhoon Chataan during the period it affected Chuuk State, Pohnpei
State, Guam, and Rota.

The following paragraphs summarize the meteorological and hydrological aspects of
Chataan's passage through Micronesia. For Guam, the information is used to make a
determination of the most likely intensity of Typhoon Chataan as it crossed the island.

2. Chuuk

a.
Eye Passage: An eye had not yet developed as Chataan approached and crossed
Chuuk Lagoon. The cyclone center passed very close to Weno Island, Chuuk Lagoon,
Chuuk State at around 0500 UTC on 3 July. This path is based on a combination of
surface wind and pressure observations from the Weather Service Office (WSO) at
Chuuk, which suggest that the circulation center was just south of Weno Island. This
path differs slightly from the JTWC track shown in Figure 4, which has the track slightly
north of Weno Island.

b. Wind:
Peak winds observed at WSO Chuuk were from 240 degrees at 29 knots (33
mph) with gusts to 44 knots (51 mph) at 0550 UTC on 3 July. This occurred just after the
center passed the island. However, Chataan could have been somewhat more intense as
the maximum winds may not have passed over the island. The JTWC intensity derived
from satellite data was 40-45 knots (45-50 mph).

c. Pressure: The minimum pressure at WSO Chuuk was 999.5 hPa at 0555 UTC on 3
July. This corresponds to a tropical depression with about 30-kt intensity using the
Atkinson-Holliday wind-pressure relationship. This pressure could support slightly
stronger winds using other wind-pressure relationships. While this intensity is consistent
with the maximum observed winds recorded at the Chuuk WSO, the maximum wind may
not have been observed at the weather station.

d.
Rainfall: Rainfall at Chuuk was extremely heavy, especially in the 24-hour
period prior to the tropical storm passage. A large, persistent mesoscale convective area
on the west side of Chataan produced very heavy rain, which persisted for a long period
over the Lagoon due to its large size and the slow movement of the storm. From 1 July
1200 UTC until 2 July 1200 UTC, the speed of motion was around 7 knots (8 mph).
Over the same period, 19.92 inches of rain were recorded at the weather station at Weno
Island, Chuuk. In the 12 hours from 01 July 1800 UTC to 02 July 0600 UTC, the
weather station recorded 14.21 inches. Figure 5 is a microwave satellite picture of
Chataan that shows the high rainfall rates associated with the tropical storm as it was over
the Lagoon. Table 1 shows the daily rainfall value recorded at the Chuuk Weather
Service Office, from Xavier High School on the east side of Weno Island, and from
Lukunoch, Ta, and Ettal Islands in the southern Mortlock Island group from 21 June to 3
July 2002. Chuuk Lagoon locations received more than twice the amount of the
Mortlock Island locations over the 13-day period.
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e. Storm Surge: Storm surge and inundation associated directly with the tropical storm
were not directly assessed, but were likely less than those produced on southern and
western exposures by the monsoon surge to the south. This persistent surge produced a
strong southwest swell several days earlier, creating hazardous surf on the southern and
western reefs of the Lagoon. The persistent winds could have also generated some waves
within the Lagoon. Inundation could have been up to 1 foot at high tide in some
locations. Waves, surf, and inundation was likely more serious on the Southern and
Northern Mortlock Islands.

f. Flooding and Mudslides:
Table 1 illustrates that the islands of Chuuk Lagoon got
several days of heavy rain, saturating the soil. Due to the very heavy rainfall on 1 and 2
July and the saturated soil, there was extensive flooding of low-lying areas and at least 30
devastating mudslides. These mudslides on several Chuuk Lagoon high islands killed 47
people, creating the greatest natural loss of life on Chuuk in recorded history.

Table 1.
Daily rainfall measurements at the Chuuk Weather Service Office, at Xavier
High School, at Lukunoch, at Ta, and at Ettal from 21 June through 3 July 2002. Values
are in inches. Measurement times differ at various locations. WSO Chuuk times are
midnight to midnight, while those at Xavier High School are taken during the daytime.
Month/
Day
WSO
Chuuk
Xavier
High
School
Lukunoch
Mortlock
Islands
Ta
Mortlock
Islands
Ettal
Mortlock
Islands
6/21 0.10 Trace 0.23
0.22 0.68
6/22 0.50 0.13 0.46
0.36 0.06
6/23 0.80 Trace 0.06
0.71 0.95
6/24 1.34 1.00 0.24
0.00 0.00
6/25 0.61 1.55 0.72
0.13 0.00
6/26 1.29 0.94 0.50
0.00 0.00
6/27 0.78 0.15 5.00
0.28 0.77
6/28 2.77 0.85 2.21
1.46 3.25
6/29 4.63 4.25 5.34
3.55 3.75
6/30 0.11 2.58 2.50
4.70 2.11
7/01 4.16 2.34 2.25
2.50 0.00
7/02 19.74 11.28 0.00
0.16 0.16
7/03 0.65 8.11 0.00
0.00 0.00
13-dy 37.48 33.18 14.51
14.07
11.73


g. Damage Assessment: Damage assessments may have been conducted by survey
teams on Chuuk, but the authors have not been privy to them. Based on the measured
and satellite-estimated winds, serious wind damage was likely limited to traditional huts
made of thatch and to poorly constructed tin and wooden structures. Poorly constructed
signs were also damaged or destroyed. A few un-guyed wooden power poles and a few
power and phone lines were likely blown down. Wooden power poles with wood rot and
termite infestation were the most vulnerable. In addition, some damage to pulpy trees
such as banana and papaya undoubtedly occurred. Some smaller branches of mango and
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breadfruit trees were likely broken. Many ripe coconuts, some breadfruit and a few green
coconuts were likely blown from trees. Saltwater spray and some inundation could have
seriously damaged taro and tapioca crops. This level of damage corresponds to a strong
Category A or a weak Category B tropical storm, or 35-45 knots (40-50 mph).

h. Outer Islands:
The outer islands also received some damage, especially those of the
Southern Mortlocks (Satawan, Lukonoch, Ettal), the Northern Mortlocks (Nama), the
Hall Islands (Nomwin, Murilo), and Namonuito Atoll (Ulul). The Northern and Southern
Mortlocks likely received most damage from the monsoon surge south and west of the
developing storm. The Hall Islands and Namunuito Atoll received the most damage from
the intensifying storm. From 28 June 1700 UTC until 29 June 1700, the Satawan
HANDAR measured 11.45 inches of rain. Table 1 also shows that several of the islands
of the Mortlock group received heavy rain. However, it was the persistent wind and
inundation from high seas that did most of the damage.

3. Guam

a. Eye passage:
Over Guam, the eye extended from Andersen AFB southward to the
center of the island. It lasted about 2 hours at Andersen and only a few minutes at Piti. A
diagram of the eye is shown in Figure 2. The center of the eye was a few miles north of
the island in the Rota Channel. The north eyewall likely did not reach Rota Island, but
remained in the Channel. Below is the estimated time of eye duration at various locations
on Guam.

Andersen AFB
0730L to 0930L
Tiyan
0806L to 0910L
Hagatna (Agana)
0845L to 0915L
Sinajana 0830L
to
0900L
Mangilao 0810L
to
0845L
Piti
0820L
to
0840L

b. Wind:
The maximum 2-minute sustained surface wind on the island was 200? at 65
knots (75 mph) with a peak gust of 90 knots (105 mph) reported at Andersen AFB at
2339 UTC and at 2355 UTC on 4 July. Because of the type of instrument used (hot wire
anemometer), the winds were logged as estimated. The maximum sustained wind at
WFO Tiyan Guam was 220? at 65 knots (74 mph) at 2326 UTC on 4 July and the peak
gust was 78 knots (90 mph) at 2338 UTC. On the west side of the island, the tidal station
at Apra Harbor recorded a peak gust of 92 knots (106 mph). The HANDAR at Apra
Harbor (different location from the tidal station) recorded a peak wind of 86 knots
(99 mph) at 0000 UTC on 5 July. At Merizo on the south end of the island, the peak gust
recorded was 82 knots (94 mph) at 2050 UTC on 4 July. Unfortunately, the data from the
Mangilao (east-central side) and Inarajan-Dan Dan (southeast side) HANDARS were not
recoverable. A private weather station in Sinajana recorded a gust to 97 mph there. The
wind measurements support a typhoon of 75-80 knots (85-90 mph) with gusts to 90-100
knots (105-115 mph).
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c. Pressure: Minimum sea-level pressure within the eye of Chataan was extremely
uniform, with 961.1 hPa being recorded at WFO Tiyan and at Andersen AFB. This
corresponds to 82 knots (94 mph) sustained winds based on the Atkinson-Holliday wind
pressure relationship.

d. Rainfall:
Rainfall from Typhoon Chataan was very uniform across the island where
local rainfall measurements were taken. However, extremely high rates of river flow and
the resulting road damage, erosion, and silting in Fena Lake suggest that the rain rates
and amounts were significantly higher across the mountainous areas of southern Guam.
This has been confirmed by preliminary measurements from USGS measuring sites on
Guam. Preliminary data from these stations indicated hourly rainfall rates as high as 5.9
inches in one hour, 12.2 inches in 3 hours, 15.5 inches in 6 hours, 17.9 inches in 12
hours, and 20.4 inches in 24 hours. These are extreme rainfall rates that may exceed a
100-year rainfall event. Rainfall amounts at various stations around Guam are listed
below in Table 2.


Table 2. Rainfall rates from several different Guam sites. USGS measurements are
preliminary and are shown as approximations.
Location
1-hr ending at
6-hr ending at
12-hr ending at 24-hr ending at
WFO Guam
2.48 (042359)
5.27 (042354)
8.64 (050159)
10.44 (051700)
Andersen AFB
7.08 (051400)
Sinajana
10.90
Merizo
2.24
(042250) 5.89
(050050)
USGS (Prelim) ~5.9
~15.5
~17.9
~20.4


e. Storm Surge:
The maximum storm surge and inundation (height of run-up) appears to
be about 12 feet, focused in the bay at the village of Umatac and 10 feet at the causeway
to Commercial Port. Over all, the height of the inundation on windward coastlines
ranged from 5 to 8 feet. These heights are consistent with a Category 2 typhoon on
Guam.

It should be noted that the maximum inundation observed on the south and southeast
coasts occurred with Typhoon Halong (10W) on 10 July (See Figures 6 and 7). Halong's
closest point of approach was 90 nm (100 miles) south of the island at 0100 UTC on 10
July. Halong caused considerable beach erosion at Talofofo and Inarajan Bays. Surf
exceeded 15 feet from Pago Bay to Talofofo Bay and 20 feet at the Inarajan Salt Water
Pools. There was considerable erosion at Talofofo Bay, as the water rose 10 feet above
normal heights. The sea wall at Inarajan held up very well. These waves are consistent
with a medium-sized Category 2 typhoon passing to the south of Guam.

f. Damage Assessment: A damage assessment of structures, infrastructure, and
vegetation was conducted over most areas of the island. Several observations stand out.
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1). Damage to structures: Structural damage was constrained to poorly built structures
or wooden structures weakened by wood rot or termite infestation. Even with these
structures, most damage was to roofs. Concrete, sheet metal, and well-built wooden
structures stood up well to the winds. Window damage was minimal, suggesting that the
winds were insufficient to carry large debris or to dislodge window frames.

2). Damage to infrastructure: Most of the infrastructure weathered the storm well.
Several un-guyed or rotten wooden poles were downed. A few un-guyed hollow-spun
concrete poles were blown down or dragged down. A few of these poles that were un-
guyed and set in concrete broke at the base due to strong vibrations created by the
oscillating power lines. A few secondary power transmission and distribution lines were
blown down, but the majority of these lines were dragged down as poles fell. Primary
power transmission and distribution lines held up well. Even tertiary distribution lines to
residences held up relatively well.

During Chataan's passage, many rivers flooded to levels not seen by long-time residents
before. These swollen rivers scoured gorges and over-ran bridges, eroding areas of
pavement on some of the roads. The heavy rains caused tremendous erosion, dislodging
many old trees, and caused numerous landslides. The heavy runoff in the Fena
Watershed caused severe silting in the Fena Reservoir, making lake waters unsuitable for
drinking water production for several days.

3). Damage to vegetation: Palm trees weathered the winds relatively well. There were
no indications of palms loosing their crowns. Some palms were blown down, but
primarily because the heavy rains softened the ground sufficiently. In coastal areas, a
few palm trees were felled due to undermining by ocean waters. Some palms had twisted
crowns, but these were near tall building or risen elevations, where the winds accelerated
and formed strong eddies. Brittle trees, such as mango, African tulip, and shower trees
lost many branches. There were some areas where vegetation was badly scoured, but
these were generally in hilly/mountainous areas where funneling or up- and down-slope
wind accelerations occurred. The island had a general green appearance rather than a
brown appearance--Category 2 rather than Category 3. Damage to vegetation was
consistent with that of a Category 2 typhoon, but some low-end Category 3 damage
occurred in hilly/mountainous areas.

4). Other aspects of damage: There was little damage to the paint on automobiles, again
suggesting that the winds were insufficient to carry debris capable of pitting the paint.
Automobile damage was largely due to fallen trees and branches, but there were no
reports of flipped automobiles. This suggests weaker than Category 3 winds. One small
aircraft at the airport was flipped over.

g. Satellite data: Satellite analyses provided a Dvorak T-number and CI-number of 5.0
at 1130 UTC on 4 July, 5.5 at 1430 UTC on 4 July, and 5.5 at 1430 UTC on 5 July. The
5.0 value corresponds to a sustained wind of 90 knots (105 mph) and the 5.5 value
corresponds to a sustained wind value of 102 knots (117 mph).
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h. Radar data: In general, the strongest winds captured by the WSR-88D Doppler
radar, both inbound and outbound, were 99-101 knots (114-116 mph). These winds were
observed at 3,500 feet to 5,200 feet, and are representative to the observed gusts. There
was a single observation of an inbound wind of 128 knots (147 mph) at 3,500 feet, but
this wind appears to have been very transient (associated with a suction vortex). Radar
observations suggest an intensity of 80-85 knots (100-105 mph).

i. Overall intensity assessment: Based on all of the available data, the over-water
intensity of Chataan as it passed across Guam was 85-90 knots (95-105 mph) with gusts
of 105-110 knots (120-125 mph).


4. Rota

a. Eye Passage:
Based on extrapolation of the radar data and other data obtained from
the island of Rota, no eye passage occurred there.

b. Wind:
The observer at the Rota Airport 2253 UTC 120? at 40 knots (45 mph) with
gusts to 65 knots (75 mph). The HANDAR, also located at the Airport, measured
maximum winds at 0051 UTC on 5 July from 110? at 38 knots (44 mph) with gusts to 54
knots (62 mph). Winds were likely 15 to 20 mph higher on the southeastern and southern
coasts.

c. Pressure: The minimum sea-level pressure recorded at Rota was 984.7 hPa at 042351
UTC by the HANDAR and 29.17 inches of mercury by the SAWRS observers at the Rota
Airport, also at 042351 UTC. This is a significantly higher pressure than the 961.1 hPa
pressure observed on northern Guam, and suggests that this location on Rota was well
outside the eye and likely outside the eyewall.

d. Rainfall:
Rainfall at Rota was fairly meager, with a maximum 1-hour value of 0.55
inches ending at 042251 UTC and a 24-hour value of 1.52 inches ending at 051400 UTC.
This also indicates that this Rota location was well outside the eyewall. More rain likely
fell on southeastern and southern exposures and over the Sabana.

e. Storm Surge:
Storm surge was not observed, however, based on the location and
intensity of Chataan, storm surge and inundation of 2 to 3 feet is likely on windward
coastal areas.

f. Damage Assessment:
No damage assessment was performed by NWS personnel.

g. Overall wind assessment: Rota likely experienced maximum winds of 65 knots
(75 mph) with gusts to 80 knots (90 mph) or the winds of a low-end Category 1 typhoon.


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5. Pohnpei State

a. Pohnpei:
No significant damage was reported on Pohnpei Island.

b. Outer Islands: Observers on Nukuoro reported damaging winds up to 45 mph and
torrential rains on 29 June. From 28 June 1200 UTC until 29 June 1200 UTC, the
HANDAR at Nukuoro recorded 17.98 inches of rain. Just after 29 June 1300 UTC, that
HANDAR was blown down.

Sapwuafik residents, located 90 miles east of Chataan on 30 June, indicated by radio that
Chataan was "pounding the island with torrential rain, strong winds, high surf, and rising
ocean waters that wash onto the atoll."
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Figure 1. 07/02/0031 UTC July 2002 GMS-5 infrared image of Tropical Storm Chataan
as it passes over Chuuk. (Courtesy Naval Research Laboratory, Monterey, CA)
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Figure 2. 07/04/2131 UTC visible imagery of Typhoon Chataan as it moves over Guam.
(Courtesy Naval Research Laboratory, Monterey, CA)
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Figure 3.
Radar reflectivity image of Typhoon Chataan as it was crossing Guam. The
colors indicate the relative intensity of the rainfall, with red being the heaviest and blue
being the lightest. (0857L 5 July 2002 data from the Andersen Air Force Base WSR-88D
Doppler weather radar.)









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Figure 4. Estimated track of Typhoon Chataan as it passed through Micronesia. (From
HURREVAC)
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Figure 5. TRMM satellite picture showing the rainfall rates of Tropical Storm Chataan
over Chuuk. Lavender color represents rain rates exceeding 1.4 inches per hour.
(Courtesy US Naval Research Laboratory)

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Figure 6. 07/10/0631 UTC July 2002 GMS-5 visual satellite imagery of Typhoon
Halong as it passes south of Guam. (Courtesy US Naval Research Laboratory)
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Figure 7. Estimate of Typhoon Halong's track to the south of Guam on 10 July.
(Courtesy Joint Typhoon Warning Center)