by Jeff Masters, wunderblog, February 1, 2011
Tropical Cyclone Yasi continues to intensify as it speeds westwards towards vulnerable Queensland, Australia. Yasi, now a Category 4 storm with 140 mph winds, is under light wind shear of 5-10 knots, and over warm ocean waters of 29 °C (84 °C). The sea surface temperatures over the region of ocean Yasi is traversing (10° - 20° S, 145° - 160° E) were 1.2 °C above average during December 2010, the latest month we have data for from the UK Hadley Center. This is the highest value on record, going back to the early 1900s. Low wind shear and record warm sea surface temperatures will continue to affect Yasi for the next day, and the cyclone should be able to maintain Category 4 strength until landfall Wednesday evening (local time.)
Queensland faces three major threats from Yasi. The cyclone will bring torrential rainfall to a region with saturated soils that saw record flooding earlier this month. The latest rainfall rates in Yasi's eyewall as estimated by NASA's Tropical Rainfall Measuring Mission (TRMM) satellite are 20 mm (0.8") per hour. The GFS model is predicting that a wide swath of Queensland will receive 5-10 inches of rain over the next week, due to the combined effects of Yasi and a moist flow of tropical air over the region. Fortunately, Yasi is moving with a rapid forward speed, about 21 mph, and is not expected to linger over Queensland after landfall. The heaviest rainfall will miss Queenland's most populated regions to the south that had the worst flooding problems earlier this month, including the Australia's third largest city, Brisbane.
Yasi will bring highly destructive winds to a region of coast near the city of Cairns (population 150,000.) Townsville (population 200,000) is farther from the expected landfall of the eyewall, and should see lesser winds. Strong building codes have been in place in Queensland since the 1960s, which will help reduce the damage amounts.
A dangerous storm surge in excess of ten feet can be expected along the left front quadrant of the storm where it comes ashore. The critical thing will be when Yasi hits relative to the tidal cycle. The tidal range between low and high tide along the coast near Cairns will be about 2 meters (6 feet) during the evening of February 2. If Yasi hits at low tide, a 10-foot storm surge will only bring the water levels four feet above mean tide, but a strike at high tide would bring water levels a full ten feet above mean tide. High tide is at 9 p.m. EST (local) time in Cairns on February 2.
Yasi is comparable to Cyclone Larry of 2006, which hit Queensland as a Category 4 storm with 135 mph winds. Larry killed one person and caused $872 million in damage (2011 U.S. dollars.) Yasi is a much larger storm than Larry, though, and will bring heavy rains to a region with soils already saturated from record rains. Yasi is likely to be a billion-dollar disaster for Australia.
Figure 3. Tropical Cyclone Yasi at midnight GMT on February 1, 2011. At the time, Yasi was a Category 3 storm with 115-mph winds. Image credit: NASA.
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