2003 NWS TYPHOON PONGSONA ASSESSMENT FINAL 59p.PDF

NOAA/NWS METEOROLOGICAL ASSESSMENT

FOR TYPHOON PONGSONA IN: POHNPEI STATE, FSM;

CHUUK STATE, FSM; GUAM; AND, ROTA, CNMI

The National Oceanic and Atmospheric Administration (NOAA) National Weather

Service (NWS) Meteorological Assessment Team for

Typhoon Pongsona

Charles ‘Chip’ Guard, NWS Forecast Office Guam

Arthur N. L. Chiu, University of Hawaii at Manoa

Mark A. Lander, University of Guam

April 2003

1. Background: On 2 December 2002, a disturbance began to organize near 6.5N 165E,

or about 370 miles east of Pohnpei. This late season disturbance was associated with an

El Niño-induced westerly wind burst that would produce Pongsona and a twin

disturbance in the Southern Hemisphere that later became Cyclone Yolande (04P). This

type of tropical cyclone development has been described in various ways by Lander

(1990, 1994); Dickenson and Molinari (2002). At 1100 UTC on 2 December, the Joint

Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert indicating

that the circulation associated with the disturbance was likely to become a significant

tropical cyclone in the subsequent 12 to 24 hours. Seven hours later, JTWC issued the

first warning on Tropical Depression (TD) 31W, located at that time near 7.4N 163.5E.

At 0000 UTC on 3 December, the JTWC upgraded the Depression to Tropical Storm

(TS) 31W as it continued on a northwest track. TS 31W was named Tropical Storm

Pongsona (pronounced Bong-sahn-WAH or Pong-sahn-WAH) by RSMC-Tokyo at 1200

UTC on 3 December, as it took a more westward track. Figure 1 shows Pongsona as it

was developing on 3 December with 35-40 knot (39-44 mph) sustained winds.

TS Pongsona passed about 70 n mi (81 miles) south of Ujelang in the western Marshall

Islands on 3 December at 2100 UTC, and then began to move to the west-southwest in

the direction of Pohnpei State. After nearly 24 hours of this motion, Pongsona took a

more westward track and passed some 60 n mi (69 miles) to the north of Pohnpei Island

with 55-knot (63-mph) winds. At 0600 UTC on 5 December, Pongsona was upgraded by

JTWC to typhoon status, and a few hours later, it passed to the north of Oroluk Atoll in

northern Pohnpei State. From 0300 UTC to 0900 UTC on 6 December, Pongsona,

packing 75- to 80-knot (86- to 92-mph) winds, moved just north of the Hall Islands in

northern Chuuk State. After pounding and submerging parts of the small islets, the

typhoon headed toward the Mariana Islands.

In response to the close agreement of nearly all tropical cyclone computer forecast

models, Pongsona was predicted by JTWC to take a northward turn and pass east of

Guam, toward Saipan and Tinian, with only slow intensification. On the early morning

of 8 December, the typhoon came into the range of the Andersen Air Force Base (AFB)

Weather Surveillance Radar—1988 Doppler (WSR-88D), which indicated that the

typhoon was continuing to move on a more west-northwest track toward Rota and Guam.

While Pongsona did eventually turn to the north, the northward turn was delayed some 24

hours, taking it west of Saipan and Tinian, but much closer to Guam and Rota. The

center of the eye passed to within 10 n mi (12 miles) of Guam’s northeast coastline, with

the southwestern semicircle of the eye wall cloud traversing most of the island. In the

18-hour period from 1800 UTC 7 December until its peak intensity at 1200 UTC 8

December, Pongsona intensified from 105 knots (121 mph) to 130 knots (150 mph),

reaching the super typhoon status of 130 knots (150 mph) while the center of the eye was

northwest of Guam and the southeastern eye wall cloud was just off of the northwestern

part of the island.

After passing over Guam, Pongsona continued on a northwest track, where it also

pummeled Rota, especially the southwestern part of the island. After passing west of

Rota, the intense typhoon moved to the north, west of Tinian and Saipan. On

9 December, it recurved to the northeast near 18N 144E, and on 11 December, it became

extratropical. Figure 2 shows the track of Typhoon Pongsona: during its development;

during the period it affected Chuuk State, Pohnpei State, Guam, and Rota, Tinian, Saipan,

and the northern islands of the Commonwealth of the Northern Mariana Islands (CNMI);

and, during its recurvature and extratropical transition. Ujelang in the western Marshall

Islands was affected early in the storm’s life but is not inhabited.

The following sections summarize the meteorological and hydrological aspects of

Pongsona’s passage through Micronesia. For Guam and Rota, the information is used to

make a determination of the most likely intensity of Typhoon Pongsona as it traversed the

islands. This analysis is in support of the NWS-directed assessment of the winds over

Guam and Rota for Pongsona.

2. Pohnpei State

a. Track: A visible eye had not yet developed as Typhoon Pongsona approached

Oroluk Atoll in Pohnpei State early on 6 December. The cyclone center passed Oroluk at

around 0000 UTC on 6 December. This path is based on a combination of surface wind

and pressure observations from the Automatic Meteorological Observing Station

(AMOS) weather instrument on Oroluk (WMO98345) and from satellite imagery (e.g.

Figure 3).

b. Wind: Pongsona did not affect Pohnpei Island much, where a peak wind of only 39

knots (45 mph) was observed at 1058 UTC on 5 December. At the Oroluk AMOS, the

highest sustained wind was from 210º at 31 knots (36 mph) at 0100 UTC on

6 December. This occurred just after the center passed the island to the east. The highest

gust measured was from 230º at 54 knots (62 mph) at 1300 UTC on 5 December.

c. Pressure: The minimum sea level pressure at Pohnpei was 999.8 hPa at 0258 UTC

and at 0354 UTC on 5 December. The minimum sea level pressure at Oroluk was 990.8

hPa at 1100 UTC on 5 December.

d. Rainfall: Rainfall at Oroluk was not measured as the sensor on the AMOS was

inoperative. Twenty-four-hour rainfall at Pohnpei was 2.24 inches from 0600 UTC

4 December 2002 to 0600 UTC 5 December 2002. The greatest 6-hour rainfall was 0.63

inches ending at 1800 UTC 4 December.

e. Storm Surge: Storm surge and inundation were not observed on Pohnpei. There may

have been some inundation of taro patches and other low-lying coastal areas on Oroluk

from high waves and surf.

f. Flooding and Mudslides: There were no reports of flooding or mudslides on

Pohnpei.

g. Damage Assessment: To our knowledge, no formal damage assessment was

conducted and damage was minimal.

3. Chuuk State

a. Track: A visible eye had not yet developed as Pongsona moved north of Chuuk

Lagoon. The cyclone center passed 90 n mi (104 miles) north of Weno Island, Chuuk

Lagoon, Chuuk State at around 0600 UTC on 6 December. This path is based on a

combination of surface wind and pressure observations from the Weather Service Office

(WSO) at Chuuk, from the AMOS at Ulul Island (WMO91328) in Namonuito Atoll, and

from satellite imagery (Figure 3). Observations and reports from Fananu and Murilo

Atolls in the Hall Islands suggest that the typhoon passed very close to these islands with

typhoon force winds. Seawater covered much of the islands.

b. Wind: The maximum sustained wind observed at WSO Chuuk was from 290º at 43

knots (49 mph) at 0859 UTC with maximum gusts from 240º at 59 knots (68 mph) at

0936 UTC on 6 December. The AMOS at Ulul in Namanuito Atoll (WMO98325), about

175 n mi (201 miles) west-southwest of the typhoon, indicated a sustained wind from

310º at 30 knots (35 mph) at 1201 UTC and a peak gust from 310º at 48 knots (55 mph)

at 1201 UTC and 1301 UTC. At Satawan Atoll (WMO91338), 210 n mi (242 miles)

south of the typhoon, the maximum observed sustained wind measured by the AMOS

was from 210º at 31 knots (36 mph) at both 1201 UTC and 1301 UTC. The peak

observed gust was 51 knots (58 mph) from 310 degrees at 2001 UTC on 5 December.

c. Pressure: The minimum pressure at WSO Chuuk was 993.2 hPa at 0553 UTC on

6 December. The pressure at Ulul was 992.8 hPa at 1301 UTC on 6 December. At

Satawan, the minimum pressure was 999.5 hPa at 0401 UTC on 6 December.

d. Rainfall: Rainfall reported at the Weather Service Office at Weno Island, Chuuk is

shown in Table 1.

Table 1. Rainfall amounts in inches at WSO Chuuk for 1-hour, 6-hour, 12-hour, and 24-

hour periods on 5 and 6 December.

Period

(hour)

Amount

(inches)

Date/Time ending at

(UTC)

1.37

06/0100

3.29

06/0200

5.05

06/0800

7.13

06/0900

e. Storm Surge: Eye witness reports at Fananu Island and Nomwin Island in the Hall

Islands indicated “high tides washing up over lands”. “Taro patches and the low-lying

parts of the islands, especially the coastal areas, are covered under water.” With

reference to beach erosion, “especially the Hall Islands, including Fananu and Nomwin,

have their beaches wash out from inland due to high tides and coastal waves”. At Weno

Island in Chuuk Lagoon, “parts of the roads on Weno, especially the boat pool area and

Mechitiw, are eroded from high tides washing up on lands”.

f. Flooding and Mudslides: There were not reports of mudslides on the high islands of

Chuuk Lagoon. There was some flooding on Weno Island as indicated from the

statement: “The same low-lying areas near the airport are again submerged under water

due to runoffs from the higher areas and the heavy rainfall.”

g. Damage Assessment and other storm effects: The weather station at Weno, Chuuk

reported “minor injuries from fallen coconut trees and broken glasses in the Hall Islands

of Fananu and Ruo”. In addition, “taro patches on Fananu inundated by saltwater,

reported total destruction of houses…also animals, such as hogs, dogs, [and] chickens

raised for food, totally wiped out”. Overall, “the Hall Islands, including Murilo, Ruo,

Fananu, and Nomwin, were mostly devastated by Typhoon Pongsona, as it passed just to

the north of them.”

4. Guam

a. Methodology for wind assessment on Guam and Rota

1). Techniques:

Because direct wind measurement systems either failed or did not provide adequate

coverage, the assessment of the maximum winds over Guam and Rota required an in-

depth look at varying types of data. These data included: available sustained wind and

wind gust measurements; sea level pressure measurements; storm surge/inundation

measurements including run-up and still water levels; rainfall measurements; available

weather radar and meteorological satellite data; and, the damage to structures,

infrastructure and vegetation. By meshing observations and theory, it is possible to

progressively whittle down the likely maximum winds to a narrow range, and to propose

a single value as the most likely maximum 1-minute average, 10-meter height wind speed

as the typhoon traversed Guam and Rota. Subsequently in this assessment, the terms

intensity and maximum 1-minute average, 10-meter height sustained wind will be used

interchangeably. Most Guam locations discussed in this assessment are shown in

Figure 4.

a). Winds: The techniques used to evaluate the wind observations are those described

in: Powell and Houston (1996a, b); Houston, Forbes, and Chiu (1999, 2002); and, Guard

and Lander (1999). Techniques of Fujita (1971, 1992) were used to assess anomalous

hurricane transients and tornados, and to determine first wind and second wind

contributions. Because of the differing gust factors over land and water, the maximum

sustained wind over land will be referred to as the over-water equivalent (OWE) value.

This OWE is associated with a discrete gust as provided in warnings by the Joint

Typhoon Warning Center. All winds are converted to 1-minute average, 10-meter

elevation. Two-minute winds were converted to 1-minute winds using a conversion

factor from Krayer and Marshall (1992): w 1min =1.08 w 2min . Conversion to 10-meter height

(when necessary) was accomplished using a logarithmic wind profile (Holton 1992).

Knots were converted to miles-per-hour (mph) using the factor: w mph =1.15 w kts .

b). Pressure: Sea level pressure data were the most complete data set available over

Guam. The minimum observed sea level pressure over Andersen Air Force Base was

very close to the minimum pressure in Pongsona at that time. Various wind-pressure

relationships (e.g., Atkinson and Holliday (1975), Kraft (1961), Callaghan and Smith,

1998)) were considered. The rationale for choosing a specific wind-pressure relationship

is shown in Figure 5. The intensity can also be estimated by applying the observed

pressure gradient, and eye and eye wall cloud characteristics to a parametric wind model

based on a tropical cyclone wind-pressure profile (e.g., Holland (1980)). The observed

minimum sea level pressure in the eye can also be used in concert with satellite data to

assess potential wind-pressure relationships.

c). Storm surge data: Storm surge refers to the height of the inundation in reference to

mean sea level. The storm surge measurements associated with Pongsona can be

compared with storm surge measurements of past cyclones affecting a location. In the

case of Pongsona, the Typhoon Paka storm surge data were very useful. Considerable

historical data describing the effects of the coral reefs on the actual coastal inundation are

available for Guam (JTWC (1991); Guard et al. (1999)) . Guard and Lander (1999), but

these data are not available for Rota. Lander and Guard (1999) present coastal wave-

cyclone intensity relationships for Guam derived from historical typhoon data and

parametric wave model computations for typhoons of various size, intensity, and

location.

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