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Severe Tropical Cyclone Evan was considered to be the worst tropical cyclone to affect the island nation of Samoa since Cyclone Val in 1991. The system was first noted on December 9, 2012, as a weak tropical depression about to the northeast of Suva, Fiji. Over the next couple of days the depression gradually developed further before it was named Evan on December 12, as it had developed into a tropical cyclone. During that day the system moved toward the Samoan Islands and gradually intensified, before the system slowed down and severely affected the Samoan Islands during the next day with wind gusts of up to . The storm moved east and impacted the French islands of Wallis and Futuna before affecting Samoa and American Samoa. On December 16 Evan turned to the south and paralleled western areas of Fiji. ==Meteorological history== 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 about to the northeast of Suva, Fiji. Over the next two days, the depression gradually developed further in an area of low vertical windshear and favorable sea surface temperatures of between , as it was steered eastwards by an upper level ridge of high pressure.〔 Late on December 11, the United States Joint Typhoon Warning Center (JTWC) started to issue warnings on the system and designated it as Tropical Cyclone 04P, after 1-minute sustained winds had become equivalent to a tropical storm.〔 At around this time the precipitation radar on NASA's Tropical Rainfall Measuring Mission observed a complete eyewall surrounding the system's eye, which NASA speculated could mean that Evan was stronger than originally thought. RSMC Nadi reported early the next day that the system had become a category 1 tropical cyclone on the Australian tropical cyclone intensity scale and named it Evan, while the system was located about to the west of Pago Pago on the American Samoan island of Tutuila. After the system was named, the upper level ridge continued to steer Evan eastward, toward the Samoan islands, as the cyclone quickly intensified. During December 12, as the system started to impact the Samoan Islands with gale and storm force wind speeds, RSMC Nadi reported that the system had become a category 2 tropical cyclone with 10-minute sustained wind speeds of .〔 〕 The JTWC also reported that day that the system had become equivalent to a category 1 hurricane on the Saffir-Simpson hurricane scale (SSHS) with 1-minute sustained wind speeds of 120 km/h (75 mph).〔 Over the next 12 hours, the system developed a cloud-filled eye on visible imagery, while the system's 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. The system continued to intensify and started to recurve toward the west on December 16; at 1800 UTC (0700 UTC+13, December 17) the JTWC reported that Evan had reached an intensity of , which made it equivalent to a category 3 hurricane on the SSHS. Early on December 17 the JTWC reported that Cyclone Evan had reached its peak intensity with 1-minute windspeeds of 230 km/h (145 mph), which made it equivalent to a category 4 hurricane on the SSHS.〔 During that day Evan started to gradually weaken as it moved around the coast of the Fijian island of Viti Levu. With RSMC Nadi reporting by 1800 UTC (0700 UTC+13, December 18) that the system had weakened into a category 3 severe tropical cyclone.〔 By December 19, wind shear from the northwest had exposed the system's low level circulation center completely, pushing the bulk of convection and thunderstorm activity about to the southeast of the center. Evan had also moved into cooler sea surface temperatures, below 27 °C (81 °C), so evaporation and thunderstorm development had waned, needing to falling cloud heights because of less evaporation.〔http://www.nasa.gov/mission_pages/hurricanes/archives/2012/h2012_Evan.html〕 On December 19, RSMC Nadi reported that Cyclone Evan had weakened below cyclone intensity and declassified it as a tropical cyclone, before the JTWC issued their final warning later that day as the system started to transition into an extratropical cyclone. Over the next couple of days the remnant low continued moving southwards and moved below 25°S and into the Wellington tropical cyclone warning centre's (TCWC Wellington) area of responsibility. During December 21, the system started to move towards the southwest as it directed a moist tropical airmass onto the North Island during the next day.〔 The remnants of Evan were last noted by TCWC Wellington during December 25. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cyclone Evan」の詳細全文を読む スポンサード リンク
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