Category 1 Rusty Australia's strongest tropical cyclone of 2013

Posted by: Dr. Jeff Masters, 01:36 PM GMT em 25 de Fevereiro de 2013

Australia's most dangerous tropical cyclone of the season so far is Tropical Cyclone Rusty, which has intensified to Category 1 strength and is lumbering southeastwards towards the northwestern coast of Australia at 6 mph. Rusty is expected to intensify further into a powerful Category 3 storm, and is predicted to make landfall near the town of Port Hedland (population 15,000) on Tuesday near 18 UTC (1 pm EST in the U.S.) Rusty formed on Saturday evening when westerly winds blowing near the Equator combined with easterly winds blowing south of New Guinea to create an unusually large tropical storm with a huge, 100-mile diameter cloud-free center. Ordinarily, a storm this large takes a long time to wind up, but Rusty intensified quickly, taking advantage of low wind shear of 5 - 10 knots, and near-record warm ocean temperatures of 31 - 32°C (88 - 90°F). It's not often that a tropical cyclone gets 31 - 32°C waters to feed off of; these temperature are about 1.5°C (2.7°F) warmer than average for this time of year. The hot ocean temperatures are largely due to Australia's hottest month in its history--the nationally-averaged monthly maximum temperature during January 2013 was the highest ever recorded. For the waters off the northwest Australian coast (15°S - 20°S, 115°E - 120°E), only two years since ocean temperature records began in 1960 have seen February ocean temperatures this warm (1.57°C above average in 2005, and 1.62°C above average in 1983.) The warm waters also extend to great depth; the tropical cyclone heat potential (TCHP) in this region is over 90 kJ/cm**2, a value commonly associated with rapid intensification. With its slow movement, large circulation, and near-record warm waters to feed off, Rusty is going to dump some prodigious rains on the coast of northwestern Australia over the next few days. Radar out of Port Hedland shows very heavy rains already affecting the coast, and sustained wind as high as 38 mph have been observed there today.


Figure 1. Radar image of Rusty showing the large cloud-free center and an intense band of precipitation to it southwest moving ashore over the coast of Australia near Port Hedland. image credit: Bureau of Meteorology.


Figure 2. Tropical Cyclone Rusty at 0555 UTC on February 24, 2013 as seen by NASA's Aqua satellite. At the time, Rusty was a tropical storm with 50 mph winds, and had an usually large cloud-free center more than 100 miles in diameter. Image credit: NASA.

Rusty the strongest tropical cyclone to affect Australia so far in 2013
It's been a rather quiet tropical cyclone season for Australia so far in 2012 - 2013; only two weak tropical storms have hit the country. Tropical Cyclone Peta hit the northwest coast on January 23 as a tropical storm with 45 mph winds. Peta dumped heavy rains of 4 - 10" (102 - 254 mm) in the Port Hedland area, very close to where Rusty is expected to make landfall. Peta's rains caused widespread flooding but no major damage. More serious were the rains from the remnants of Tropical Cyclone Oswald, which hit the Queensland coast in Northeast Australia on January 21 as a tropical storm with 40 mph winds. The remnants of Oswald pushed southwards along the coast and generated record rains that caused massive flooding in Queensland. Six people died and damage was estimated at $2.5 billion. The heaviest rains fell near Tully, where approximately 1 meter (39") of rain fell in 48 hours.


Figure 3. Tropical Cyclone Oswald at 0425 UTC on January 21, 2013. Oswald hit Queensland, Australia as a tropical storm with 40 mph winds. Image credit: NASA.

Jeff Masters
[link to www.wunderground.com]