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Severe Tropical Cyclone Evan (04P) Hits Samoa

22.5S 154.6W

December 17th, 2012 Category: Tropical Cyclones

Tropical Cyclone Evan (TC 04P) – December 17th, 2012

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Track of Tropical Cyclone Evan (TC 04P) - December 17th, 2012 © Univ. of Wisconsin

Track of TC 04P

Severe Tropical Cyclone Evan (RSMC Nadi designation: 04F, JTWC designation: 04P) is considered to be the worst tropical cyclone to hit the island nation of Samoa since Severe Tropical Cyclone Val.

The first cyclone of the 2012–13 South Pacific cyclone season, Evan developed from a tropical disturbance on December 9 north-northeast of Fiji. The storm moved east and impacted Samoa and American Samoa; Evan also hit the French islands of Wallis and Futuna and is currently active.

On December 9, the Fiji Meteorological Service’s Regional Specialized Meteorological Centre in Nadi (RSMC Nadi) started to monitor a weak tropical depression, that had developed within the South Pacific Convergence Zone (SPCZ), about 700 km (430 mi) to the northeast of Suva. Over the next two days, the depression gradually developed further in an area of low vertical windshear and favourable sea surface temperatures of about 28 – 30 °C (82 – 86 °F), as it was steered eastwards by an upper level ridge of high pressure.

At 1800 UTC on December 11, the United States Joint Typhoon Warning Center (JTWC) started to issue advisories on the system and designated it as Tropical Cyclone 04P, after 1-minute sustained winds had become equivalent to a tropical storm while the system’s low level circulation centre was rapidly consolidating. RSMC Nadi then reported early the next day that the system had become a category one tropical cyclone on the Australian tropical cyclone intensity scale and named it Evan, while located about 410 km (250 mi) to the west of Pago Pago on the American Samoan island of Tutuila.

Throughout December 12, Evan continued to be steered eastwards towards the Samoan islands by the upper level ridge, as it quickly intensified further with RSMC Nadi reporting at 1200 UTC that the system had become a category two tropical cyclone. At 1800 UTC the JTWC reported that the system had become equivalent to a category one hurricane on the Saffir-Simpson hurricane scale (SSHS) with 1-minute sustained windspeeds of 120 km/h (75 mph), while it was located about 40 km (25 mi) to the southeast of Apia, Western Samoa.

Over the next 12 hours the system developed a 17 km (11 mi) cloud filled eye on visible imagery, while the systems forward motion started to slow down as it entered a weak steering environment with the upper level ridge of high pressure to the north of the system weakened and a subtropical ridge of high pressure developed to the south of the system.

At 0600 UTC RSMC Nadi reported that Evan had become a category 3 severe tropical cyclone, as it passed over the island of Upolu. During that day the system continued to intensify as started to recurve towards the west, before at 1800 UTC the JTWC reported that Evan had reached its initial peak intensity of 185 km/h (115 mph), which made it equivalent to a category 3 hurricane on the SSHS. On December 16, Evan completed a cyclonic loop, and by December 17, the system strengthened into a Category 4–equivalent cyclone on the SSHS.