Tropical Cyclone Alan
Cyclone Alan near its peak intensity
Meteorological history
FormedApril 17, 1998
DissipatedApril 26, 1998
Category 1 tropical cyclone
10-minute sustained (FMS)
Highest winds75 km/h (45 mph)
Lowest pressure992 hPa (mbar); 29.29 inHg
Tropical storm
1-minute sustained (SSHWS/NPMOC)
Highest winds85 km/h (50 mph)
Overall effects
Fatalities10
Areas affectedFrench Polynesia
IBTrACS
Part of the 1997–98 South Pacific cyclone season

Tropical Cyclone Alan was considered to be one of the worst natural disasters experienced in French Polynesia. The system, first noted as a tropical disturbance on April 17, 1998 east of the Northern Cook Islands, initially moved erratically prior to its designation as Alan upon developing into a tropical cyclone on April 21. The following day, Alan reached its estimated peak intensity with 10-minute maximum sustained wind speeds of 75 km/h (45 mph), posing several forecasting challenges due to difficulties in establishing its location and intensity. On April 23, Alan appeared to become sheared with the low-level circulation center displaced about 20 km (10 mi) from the nearest atmospheric convection. However, over the next day, after the mid-level trough of low pressure shearing the system moved faster than expected, Alan became better organized. While situated about 555 km (345 mi) west-northwest of the French Polynesian island of Tahiti, the system drifted south-southeastward. Early on April 25, Alan passed near to or over the French Polynesian Society Islands of Maupiti, Bora-Bora, and Raiatea. However, strong wind shear pulled the system apart, with visible imagery confirming the system's degeneration into a depression with a fully exposed low-level circulation. The system affected French Polynesia with high winds and torrential rain, causing several landslides on the Society Islands. The landslides resulted in the collapse of two bridges, and also overturned trees, in turn obstructing roads. Overall, ten people died as a result of the system, while thirty others were injured, with a majority of the casualties a result of landslides. On the islands of Raiatea, Taha'a and Huahine, several churches, schools and clinics were damaged, while water and electricity supplies were cut off. On the islands, around 750 houses were destroyed, with 430 and 150 wrecked on Huahine and Ra'iātea, respectively.

Meteorological history

Map plotting the storm's track and intensity, according to the Saffir–Simpson scale
Map key
Saffir–Simpson scale
  Tropical depression (≤38 mph, ≤62 km/h)
  Tropical storm (39–73 mph, 63–118 km/h)
  Category 1 (74–95 mph, 119–153 km/h)
  Category 2 (96–110 mph, 154–177 km/h)
  Category 3 (111–129 mph, 178–208 km/h)
  Category 4 (130–156 mph, 209–251 km/h)
  Category 5 (≥157 mph, ≥252 km/h)
  Unknown
Storm type
triangle Extratropical cyclone, remnant low, tropical disturbance, or monsoon depression

On April 17, 1998, the Fiji Meteorological Service (FMS) started to monitor the development of a tropical disturbance east of the Northern Cook Islands.[1][2] Before the FMS initiated advisories on the system on April 19, the system moved erratically; it later developed into a weak depression while it located about 555 km (345 mi) north of Manihiki Atoll in the Northern Cook Islands.[2] Over the next couple of days, the system slowly moved northwards, gradually organizing, with gale-force winds developing in its southern quadrant on April 20.[2][3] Over the next day, the system developed into a category 1 tropical cyclone on the Australian tropical cyclone intensity scale, before the FMS named the system Alan at 1800 UTC.[1] By this time, the cyclone, located about 300 km (185 mi) east-southeast of Manihiki, started to move westwards.[4] On April 22, the FMS estimated that Alan had reached its peak intensity with 10-minute maximum sustained wind speeds of 40 kn (75 km/h; 45 mph), just as the system started to pose several forecasting challenges due to difficulties in establishing its location and intensity.[4][5] Later that day the Naval Pacific Meteorology and Oceanography Center (NPMOC) started to issue warnings on Alan and accidentally assigned it the designation 32P as opposed to 36P, estimating peak 1-minute sustained wind speeds of 85 km/h (50 mph).[3][6]

On April 23, the system appeared to become sheared, with the low-level circulation center displaced about 20 km (10 mi) from the nearest atmospheric convection.[4][7] As a result, the FMS reported Alan's weakening into a depression, while the NPMOC subsequently issued their final advisory on the system.[1][2][7] Over the next day, as a mid-level trough of low pressure shearing Alan moved faster than expected, the depression became better organized and developed favorable upper-level outflow.[2][8] As a result, the NPMOC initiated advisories on the system again on April 24, and later that day, the FMS reported that Alan re-intensified into a Category 1 tropical cyclone.[8][5] While located about 555 km (345 mi) west-northwest of the French Polynesian island of Tahiti, the system moved south-southeastward under the influence of an upper-level anticyclone located to its east.[2][8] Early on April 25, Alan passed near to or over the French Polynesian Society Islands of Maupiti, Bora-Bora, and Ra'iātea.[2] As the system moved near Bora Bora, a maximum wind gust of 147 km/h (91 mph) and a pressure observation of 1002 hPa (29.6 inHg) were recorded,[4] indicating the minimal strength of Alan; however, the wind gust was stronger than that expected of similarly intense tropical cyclones.[4] It was later speculated that squalls within Alan's convective feeder band may have been responsible for the stronger wind gusts.[4] On April 25, strong wind shear pulled the system apart, with visible imagery revealing a fully exposed low-level circulation.[1] As a result, the FMS reported that the system weakened into a depression at 1800 UTC that day, while located about 185 km (115 mi) west-northwest of Tahiti.[1][2] The NPMOC subsequently reissued their final advisory early the next day as the system drifted south-southeastward while located west of Tahiti.[2][9]

Preparations and impact

Cyclone Alan produced high winds and torrential rain over French Polynesia between April 24 and 25, causing several landslides on the Society Islands,[2][10][11] resulting in the collapse of two bridges and obstruction of roads by fallen trees.[12] Damage associated with the system was mainly confined to the Leeward Islands of Bora Bora, Tahaa, Ra'iātea and Huahine, which had all been damaged by Cyclone Osea earlier in the season.[2][4] Overall, ten people died as a result of the system, while thirty others were injured, with the majority of the casualties occurring due to landslides.[4][13] On Bora Bora, wind gusts of up to 147 km/h (91 mph) were reported, while on Tahaa, wind gusts of up to 95 km/h (60 mph) were reported.[1][4] On the islands of Ra'iātea, Tahaa and Huahine, several churches, schools and clinics were damaged, while water and electricity supplies were cut off.[4][12] On the islands, around 750 houses were destroyed, with 430 and 150 wrecked on Huahine and Ra'iātea, respectively.[12]

On Huahine, the extent of the damage, along with a sharp dip on the Uturoa barograph trace, suggested that a tornado may have been spawned in one of Alan's convective bands.[4] In the system's aftermath a, 60-person relief team was deployed from Tahiti to the affected islands to assist with emergency operations.[10] The French President Jacques Chirac and Prime Minister Lionel Jospin both sent messages of support to the Leeward Islands, and in French Polynesia, the system was considered to be one of the worst natural disasters in the islands' history.[4][14] A few days after Cyclone Alan impacted the islands, Cyclone Bart caused an additional ten deaths and some minor damage to the French territory.[1][4][15] During December 1998, the European Community Humanitarian Office responded to a government appeal, providing €105,000 for tents, medical aid, food and clothing.[16]

See also

References

  1. 1 2 3 4 5 6 7 Tropical Cyclone Season Summary 1997–98 (PDF) (Report). Fiji Meteorological Service. Archived from the original (PDF) on March 26, 2009. Retrieved February 8, 2014.
  2. 1 2 3 4 5 6 7 8 9 10 11 Padgett, Gary (1998). "Monthly Global Tropical Cyclone Summary April 1998". Archived from the original on June 5, 2011. Retrieved May 29, 2011.
  3. 1 2 Padgett, Gary. "Monthly Global Tropical Cyclone Tracks April 1998". Archived from the original on February 22, 2014. Retrieved February 8, 2014.
  4. 1 2 3 4 5 6 7 8 9 10 11 12 13 Chappel, Lori-Carmen; Bate, Peter W (June 2, 2000). "The South Pacific and Southeast Indian Ocean Tropical Cyclone Season 1997–98" (PDF). Australian Meteorological Magazine. Australian Bureau of Meteorology. 49: 121–138. Archived (PDF) from the original on July 7, 2013. Retrieved July 23, 2013.
  5. 1 2 "1998 Tropical Cyclone Alan (1998108S10203)". International Best Track Archive for Climate Stewardship. Retrieved October 9, 2019.
  6. ↑ "Tropical Cyclone 32P (Alan) Warning 1 April 22, 1998 21z". United States Naval Pacific Meteorology and Oceanography Center. Archived from the original on December 25, 2017. Retrieved February 8, 2014.
  7. 1 2 "Tropical Cyclone 32P (Alan) Warning 3 April 23, 1998 21z". United States Naval Pacific Meteorology and Oceanography Center. Archived from the original on December 25, 2017. Retrieved February 9, 2014.
  8. 1 2 3 "Tropical Cyclone 32P (Alan) Warning 4 April 24, 1998 12z". United States Naval Pacific Meteorology and Oceanography Center. Archived from the original on February 9, 2014. Retrieved February 9, 2014.
  9. ↑ "Tropical Cyclone 32P (Alan) Warning 7 April 26, 1998 03z". United States Naval Pacific Meteorology and Oceanography Center. April 26, 1998. Archived from the original on December 25, 2017. Retrieved February 9, 2014.
  10. 1 2 "At least eight killed by French Polynesia cyclone". Pacific Islands Report. Pacific Islands Development Program/Center for Pacific Islands Studies. April 27, 1998. Archived from the original on February 21, 2014. Retrieved February 9, 2014.
  11. ↑ De Changy, Florence (April 28, 1998). "Un cyclone en Polynésie française provoque la mort de huit personnes". Le Monde (in French). Retrieved March 29, 2015.
  12. 1 2 3 Padgett, Gary (1998). "Monthly Global Tropical Cyclone Summary May 1998". Archived from the original on February 22, 2014. Retrieved May 29, 2011.
  13. ↑ "French Polynesia cyclone deaths rise to 10". Pacific Islands Report. Pacific Islands Development Program/Center for Pacific Islands Studies. May 1, 1998. Archived from the original on February 21, 2014. Retrieved February 9, 2014.
  14. ↑ "Cyclone meurtrier sur la Polynésie". La Dépêche. April 27, 1998. Archived from the original on February 22, 2014. Retrieved February 11, 2014.
  15. ↑ May, Jutta (September 17, 2008). "Event Information: Tropical Cyclone Bart". Pacific Disaster Net. Archived from the original on April 22, 2012. Retrieved June 2, 2011.
  16. ↑ "Humanitarian aid news: Asia" (Press release: IP/98/1110). European Union. February 22, 1988. Archived from the original on February 22, 2014. Retrieved February 9, 2014.
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