Tropical cyclone season
The 2011–12 Australian region cyclone season was a below average tropical cyclone season, with 7 cyclones forming rather than the usual 11. It began on 1 November 2011, and ended on 14 May 2012. The regional tropical cyclone operational plan defines a "tropical cyclone year" separately from a "tropical cyclone season"; the "tropical cyclone year" began on 1 July 2011 and ended on 30 June 2012.
The scope of the Australian region is limited to all areas south of the equator, east of 90°E and west of 160°E. This area includes Australia, Papua New Guinea, western parts of the Solomon Islands, East Timor and southern parts of Indonesia.
Tropical cyclones in this area are monitored by five Tropical Cyclone Warning Centres (TCWCs): the Australian Bureau of Meteorology in Perth, Darwin, and Brisbane; TCWC Jakarta in Indonesia; and TCWC Port Moresby in Papua New Guinea. The Joint Typhoon Warning Center issues unofficial warnings for the region, designating tropical depressions with the "S" suffix when they form west of 135°E, and the "P" suffix when they form east of 135°E.
Seasonal forecasts.
During each tropical cyclone year, Australia's Bureau of Meteorology National Climate Centre (BoM), the Guy Carpenter Asia-Pacific Climate Impact Centre, New Zealand's National Institute of Water and Atmospheric Research (NIWA) and partners issue seasonal forecasts for the Australian region and its various subregions. Since a tropical cyclone can move through a region, the actual number of tropical cyclones in a region include any that form in or move into a region from another.
Bureau of Meteorology.
In October 2011, ahead of the tropical cyclone season starting on 1 November, the BoM issued seasonal forecasts for the whole Australian region, one for each of the subregions Western, North-Western, Northern and Eastern Australia with each one covering the whole tropical cyclone year. For each forecast they took into account, the current neutral ENSO conditions to weak La Niña conditions that had been observed during that summer. For the whole region they predicted that there was an 80% chance that the season would be above average.
For the Western region between 90°E and 125°E, the BoM forecast that the area would see activity near to or slightly above the average, with a 35% chance of a below average cyclone activity. For the North-Western subregion between 105°E and 130°E, it was predicted that there was a 60% chance of above average tropical cyclone activity, while TCWC Perth noted that there was a likelihood of two tropical cyclones and one severe tropical cyclone impacting Western Australia. The Northern Territory which was defined as being between as being 125°E and 142.5°E had a 60% chance of being above average, while the Eastern region had a 65% chance of having an above average tropical cyclone season. During February, after Cyclones Heidi and Iggy had threatened Western Australia, it was reported that there was a likelihood of at least one more coastal impact and a significant risk that it could be a severe tropical cyclone. The BoM also issued a seasonal forecasts for the South Pacific region between 142.5°E and 165°E and one for the Eastern Southern Pacific region between 165°E and 120°W. The outlook called for a westward shift in tropical cyclone activity with a 65% chance that the Western Pacific Region will see more than the average number of tropical cyclones while there was a 40% chance that the Eastern Pacific Region will see above average number.
Systems.
Severe Tropical Cyclone Alenga.
On 3 December, TCWC Perth and TCWC Jakarta started monitoring a tropical low that had developed about 1220 km (760 mi) to the southwest of Jakarta, Indonesia. Over the next few days, the low moved towards the southeast, while the system's low-level circulation centre gradually developed further, in an area of moderate vertical wind shear. During the next day, the low moved out of the Australian region and moved into the South-West Indian Ocean, where it was named Alenga by the Mauritius Meteorological Service on 5 December. During that day, RSMC La Réunion reported that Alenga reached an initial 10-minute peak windspeed of , as it started to recurve and move towards the Australian region. The system then weakened slightly, before re-intensifying and surpassing its initial peak intensity, as it moved back into the Australian region during 7 December. Later that day, the JTWC reported that Alenga had become equivalent to a category one hurricane on the Saffir–Simpson hurricane scale (SSHS). During the next day, TCWC Perth reported that Alenga had become a Category 3 Severe Tropical Cyclone, and reached its 10-minute peak windspeeds of before the JTWC also reported that Alenga had intensified further and peaked with 1-minute windspeeds of , and had become equivalent to a category 3 severe tropical cyclone, on the SSHWS. Alenga then started to rapidly weaken as it moved into an area of higher vertical wind shear, with convection diminishing and the low-level circulation centre becoming elongated. On 9 December, TCWC Perth reported that Alenga had degenerated into a tropical low, while the JTWC followed suit and issued their final advisory on Alenga. TCWC Perth then tracked the remnants for another three days, in their tropical weather outlooks, before dropping it on 12 December. Alenga brought severe storms to Perth on 12 and 13 December.
Tropical Low Fina.
On 18 December, TCWC Brisbane reported that a tropical low had developed within the South Pacific convergence zone, to the southeast of Port Moresby, in Papua New Guinea. Over the next few days, the low moved towards the south-southwest under the influence of a weak upper ridge of high pressure in an area of weak vertical wind shear. On 20 December, as convection had consolidated over the systems low level circulation centre, the JTWC issued a Tropical Cyclone Formation Alert on the system. During the next day TCWC Brisbane reported that the low had become a category one tropical cyclone and named it Fina during the next day as it had 10-minute windspeeds of . However, as soon as Fina was named, it started weakening as an upper-level trough of low pressure moving through eastern Australia, increased windshear over the system, which meant that Fina's low level circulation centre became exposed, with convection displaced over from the centre. As a result of this, the JTWC decided to cancel their Tropical Cyclone Formation Alert late on 22 December, while TCWC Brisbane reported that Fina had degenerated into an extratropical cyclone. Over the next few days Cyclone Fina developed storm force winds and produced dangerous surfing conditions in southeast Queensland over the Christmas holiday. In post-analysis in early 2012, the BoM downgraded Fina to a tropical low.
Tropical Low 03U (Benilde).
On 21 December, TCWC Perth reported within their tropical weather outlooks that a tropical low was developing about 770 km (410 mi) to the northeast of the West Island of the Cocos (Keeling) Islands. Over the next few days they continued to monitor it, as the system slowly moved west-southwestward. On 28 December it moved into RSMC La Réunion's area of responsibility, where it commenced a strengthening trend.
Tropical Cyclone Grant.
An area of thunderstorms north-northeast of Darwin was designated Tropical Low 04U by TCWC Darwin on 21 December. Tropical Low 04U developed into a Category 1 tropical cyclone on the morning of 25 December, and reached Category 2 strength in the afternoon of 25 December. A cyclone watch was declared between Dundee Beach and Nhulunbuy, whilst a cyclone warning was issued for areas between Cape Fourcroy and Milingimbi including the Tiwi Islands.
A gale warning had been issued for Darwin and the Tiwi Islands on 24 December.
Upon intensification to Category 1 on 25 December the cyclone was moving slowly towards the Cobourg Peninsula. The system intensified further on 25 December, reaching Category 2 status in the afternoon. The system crossed Van Diemen Gulf and made a second landfall east of Point Stuart in the early morning of 26 December. Shortly thereafter the cyclone was downgraded to Category 1 and by the afternoon of that day it was downgraded to an ex-tropical low.
On 28 December in the evening, the system moved back onto the waters of the Gulf of Carpentaria, and was downgraded to a tropical low. The system continued to move eastward across the Cape York Peninsula and into the Coral Sea, even as the storm weakened, until Grant underwent an extratropical transition on 2 January 2012. Later on the same day, the extratropical remnant of Grant dissipated.
Severe Tropical Cyclone Heidi.
Early on 9 January, both the JTWC and TCWC Perth started to monitor a tropical low that had developed within a monsoon trough, about 950 km (590 mi) to the north of Port Hedland in Western Australia. Over the next two days, the low gradually moved southwards and developed in favourable conditions into a tropical cyclone, before late on 10 January, it was assigned the designation 06S by the JTWC and named Heidi by TCWC Perth.
In preparation for the cyclone Port Hedland Port Authority closed the port, oilfields and gas production off the North-West coast were halted, and flights from Perth to Port Hedland were cancelled.
In the early morning of 12 January the system made landfall just east of Port Hedland and after a few hours the system was downgraded to Category 1 cyclone. Port Hedland received of rain by 9:00 am with gusts of and Wallal Downs recording of rain. More than 3,500 homes and businesses in Port Hedland were left without power. Late on 12 January the system was downgraded to a tropical low about northeast of Tom Price. Flood Warnings continued for the Ashburton and De Grey River areas. On 13 January, the remnants of Heidi continued to move south through the Gascoyne region with widespread rainfall totals of up to . Significant rainfall was recorded in the Gascoyne region through early morning 14 January at the of Mount Vernon with of rain and with of rain. In the early morning of 15 January a severe weather warning was cancelled for northeast Gascoyne. However showers from the remnants of the tropical low still persisted.
Ahead of Cyclone Heidi making landfall during 12 January, various tropical cyclone alerts and warnings were issued for parts of the Pilbara Coast between Wallal and Dampier.
Track data subsequently released by the BoM indicated that Heidi's winds were equivalent to a Category 3 system.
Tropical Low 10U.
On 22 January, a tropical low formed in a monsoonal trough over the Arafura Sea. The low then slowly moved to the south and west over the next few days as gale-force westerly winds developed to the north. This storm made 2 landfalls. The first one was on 24 January around 10:00 am when the storm made landfall over Melville Island. Later that night, the storm made its final landfall in the late afternoon hours of 24 January.
This storm brought heavy rain and strong winds to Darwin. This led to the temporary closure of the Territory Wildlife Park and the George Brown Botanic Gardens on 26 January. Flights out of Darwin International Airport had delays and cancellations. Coastal regions around Darwin also experienced significant beach erosion.
Tropical Cyclone Iggy.
On 21 January, JTWC reported that an area of convection with a weak low level circulation had developed about from Port Hedland. By 24 January, TCWC Perth reported that a tropical low had formed, and that it was north northwest of Exmouth moving west at . The tropical low was then given the identification "11U". Late on 25 January, the JTWC upgraded it into a tropical depression and designated as "09S". Soon after the system strengthened and the TCWC Perth had named the system Tropical Cyclone "Iggy". The system was located northwest of Exmouth, moving west at .
During its formative stages, Iggy was responsible for triggering severe weather, including tornadoes and flooding in Java, Bali, and in Lombok, Indonesia, causing the deaths of at least 16 people, and the destruction of thousands of houses.
Early on 26 January, the system began to recurve and headed slowly towards the Pilbara coast. Early on 27 January, the system was upgraded to a Category 2 cyclone, while located northwest of Exmouth moving east southeast. By late 27 January a cyclone warning had been issued from Mardie to Ningaloo. On 28 January, Iggy reached its peak intensity, with a minimum central pressure of 970 mbars. By 29 January, the system became stationary. Wind shear and lower sea surface temperatures, which the cyclone remained stationary over inhibited intensification.