AIR Analyzes Impact of Tropical Storm Laurence on Western Australia

December 17, 2009

According to a report from catastrophe risk modeling firm AIR Worldwide, “severe Tropical Cyclone Laurence made a second landfall this evening [Dec. 16] along the Kimberley coast of north Western Australia State. Although the storm is small in area, around 20 kms [12.5 square miles] it packed maximum wind “gusts of 220 kilometers per hour (app. 138 mph) close to the cyclone’s center.

“As of 8:00 pm Wednesday, December 16th, local time (12:00 UTC), Laurence was estimated to be about 60 kilometers [38 miles] southeast of Cockatoo Island and 100 kms [62 miles] north northeast of Derby, moving at eight kilometers [5 mph] per hour.”

AIR noted that the Australian Bureau of Meteorology categorizes tropical cyclones according to 3-second gust wind speeds rather than the 1-minute sustained winds used by the U.S.’s Saffir Simpson scale or the 10-minute sustained winds used by many other meteorological agencies around the world. Consequently, while the Australian five-category scale may appear at first glance to be similar to the Saffir-Simpson Scale, the actual intensity of tropical cyclones assigned the same category in the two scales can be quite different.

“In the case of Severe Tropical Cyclone Laurence, the storm was designated Category 3 on the Australian scale (average maximum winds of 118 to 159 km/h) [74 to 100 mph] when it made landfall, having weakened from a Category 5 status earlier,” explained Dr. Tim Doggett, principal scientist, AIR Worldwide. “On the Saffir-Simpson Scale, however, Laurence probably would have been designated not higher than Category 2 at its peak intensity.”

AIR also noted that for the past two days “Laurence has brought heavy rain and destructive winds across Western Australia’s Kimberley region, which is sparsely settled.” Dr. Doggett added: “Laurence is expected to weaken as it moves inland overnight, but the heavy rain is expected to continue, with daily rainfall reaching an excess of 10 centimeters |app. 4 inches]. It should come closest to the town of Derby (population 4,000 in 2006), about 5:00 am Thursday morning local time. The storm’s remnants are forecast to reach Broome (population 12,000 in 2006) on Friday.”

Tropical Cyclone Laurence developed in the Timor Sea last Sunday, December 13th, and “intensified rapidly, bringing as much as 25 cm of rain (10 inches) to Darwin in a 24-hour period over the weekend,” AIR’s report continued. “It made a brief landfall on Monday as a weak (Australian) Category 3 storm (Saffir-Simpson Category 1) about 640 kms [400 miles] southwest of Darwin, and then moved back out to sea. Since then it has moved parallel to the Kimberley coast until making a second landfall today.” The storm “is expected to move inland and to the west, with a likelihood of moving offshore yet again, redeveloping, and possibly making landfall once more.”

AIR also, indicated that, “because this region of Australia is so sparsely populated,” it doesn’t “expect significant insured losses. However, AIR is continuing to monitor Laurence and will provide additional information if warranted.”

Source: AIR Worldwide – www.air-worldwide.com

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