An extraordinary Cat 5 in Australian waters

We've got a truly exceptional Category 5 tropical cyclone in the waters off of the Western Australia coast to discuss, so the continuation of my blog on whether the global number of Category 4 and 5 hurricanes are increasing will have to wait. Tropical Cyclone Glenda is a Category 5 storm on the Saffir-Simpson scale, with maximum sustained winds of 160 mph and a central pressure estimated at 898 mb by the U.S. Navy (910 mb by the Australian Bureau of Meteorology). A central pressure of 898 mb is the lowest pressure ever estimated for a Southern Hemisphere cyclone, at least that I could find record of. Reliable records go back to the 1980s. The lowest pressure ever estimated for a Southern Hemisphere cyclone was 900 mb, for Inigo of 2003 and Gwenda of 1998. Both were Category 4 cyclones on the Saffir-Simpson scale, and hit Western Australia in a location near where Glenda is expected to strike on Thursday. The lowest pressure measured at the surface in a Southern Hemisphere cyclone was 905 mb at North Rankin A gas platform during Cyclone Orson on 22-23 April 1989. Orson had 160 mph maximum sustatined winds at the time, making it a Category 5 storm.


Figure 1. Tropical Cyclone Glenda is moving southwest, parallel to the Australian coast. This visible light image courtesy of the U.S. Navy.

Two weeks after suffering an estimated $1 billion in damage from Cyclone Larry, Australia must brace for another strike from a major hurricane. The region of Western Australia likely to be threatened by Glenda is not heavily populated, but is home to many important mining, oil, and gas operations. Over 1,500 people were eveacuated earlier this year when Tropical Cyclone Clare battered the area with 70 mph winds. Oil and gas operations are already shutting down as Australia battens down again. Glenda is in a very favorable environment for continued intensification, with water temperatures averaging 30 C (86F) underneath, and very light wind shear. It is possible that later today Glenda will reach the highest winds ever recorded in the Southern Hemisphere, equalling the record of 150 knots (172 mph) estimated for Cyclone Daryl/Agnielle in November 1995. Increasing wind shear on Wednesday should act to weaken Glenda, but she is still expected to be a formidable Category 3 or higher storm at landfall (Category 5 on the Australian intensity scale, which goes by wind gust). Glenda is the sixth tropical cyclone this season for the Western Australia area. On average, five of these storms form during the season, which runs November through April.

Jeff Masters