A case study of Tropical Cyclone Wilma, January 2011
Tropical Cyclone Wilma developed in the southwest Pacific basin during January 2011 and caused significant damage and extreme weather both in Tonga and in New Zealand. It has been suggested that Wilma was the first true tropical cyclone to reach New Zealand (without having undergone extratropical transition) but it is shown here that this is not the case. Analysis of the storm’s life cycle reveal that Wilma began extratropical transition around 1000 km north of New Zealand and was essentially extratropical in nature by the time it affected the North Island. In its early depression phase, Wilma moved slowly eastwards in the vicinity of Wallis Island and the area between Tokelau and Samoa before recurving southwards. It was named Wilma around 1800 UTC on 23 January with a central pressure of 995hPa. Wilma moved southwestward and intensified to hurricane intensity near Tongatapu, Tonga around 0600 UTC on 25 January and reached peak intensity with 10-minute average winds of 100 knots and estimated central pressure of 939hPa, early on 26 January. On 27 January, Wilma shifted onto a more southerly course and began to weaken. At 0000 UTC on 28 January, its intensity dropped to marginal hurricane, 65 knots with a central pressure of 970hPa, about 500 km north of North Island. Wilma then recurved onto a southeasterly track to pass just off the east coast of Northland, New Zealand. Wilma was an unusual storm, both since it formed well east of the usual cyclone genesis region during a strong La Niña event, and since its interaction and absorption into the mid-latitude flow did not occur until well after the extratropical transition process had begun.