Tropical Cyclones  

  By the time that you read this article there may have been the first tropical cyclone within the Australian area for this cyclone season; although cyclones as early as November are not common. 
  Tropical Cyclones are low pressure systems that form over warm tropical waters and have at least gale force winds (sustained winds of 34 knots or greater and gusts in excess of 48kts) near the centre. Technically they are defined as a non-frontal low pressure system of synoptic scale developing over warm waters having organised convection (upwards motion in the atmosphere) and a maximum mean wind speed of 34kts or greater extending more than halfway around near the centre and persisting for at least six hours. 
  The gale force winds can extend hundreds of kilometres from the cyclone centre. If the sustained winds around the centre reach 64 kts (gusts in excess 89 kts), then the system is called a severe tropical cyclone. These are referred to as hurricanes or typhoons in other countries. 
  The circular eye or centre of a tropical cyclone is an area characterised by light winds and, often, by clear skies. Eye diameters are typically 20 nautical miles but can range from under 5nm to over 55nm. The eye is surrounded by a dense ring of cloud about 16km high known as the eye wall that marks the belt of strongest winds and heaviest rainfall. 
  Tropical cyclones derive their energy from the warm tropical oceans and do not form unless the sea-surface temperature is above 26.5°C, although once formed, they can persist over lower sea-surface temperatures. Tropical cyclones can persist for many days and may follow quite erratic paths. They usually dissipate over land or colder oceans. 
  There is a wealth of information on the Bureau of Meteorology website to help the mariner to understand cyclones <http://www.bom.gov.au/weather/cyclone/> 
  A hierarchy of products are issued by the Bureau of Meteorology scaling down in either time or distance that can help mariners with their cyclone decision making. In late October a seasonal outlook is issued giving indications of cyclone activity in the upcoming season (November to April). 
  For a mariner preparing for a voyage during cyclone season there is a three-day tropical cyclone outlook. This outlook gives a daily potential for cyclone development. Development Potential is an estimate of the probability of tropical cyclone development for each day ... Low = 10% or less, Moderate = 20% - 40%, High = 50% or more. 
  Both outlooks can be found at <http://www.bom.gov. au/weather/cyclone/tc-outlooks.shtml> 
  Once a cyclone has formed in the Australian area and, provided that it is not expected to cause land-based gales within 48 hours, a tropical cyclone information bulletin will be issued and then renewed every six hours. 
  The Tropical Cyclone Information Bulletin will contain a range of information about the cyclone along with a statement of longer-term threats. 
  See <http://www.bom.gov.au/weather/cyclone/about/ warnings/WarningsInformation_TC_InfoBulletin.shtml> Cyclones mentioned in the tropical cyclone information bulletin would also be covered in the High Seas forecasts and warnings for mariners. Tropical Cyclone Information Bulletins are for the Coral Sea cyclones and Indian Ocean Cyclones. Tropical cyclones in the Northern zone will be a threat to some land area within 48 hours; hence Darwin does not issue Tropical Cyclone Information Bulletins. 
  The Tropical Cyclone Forecast Track Map (see diagram) is a graphical product that provides a track of the cyclone showing recent movement, and forecast movement (with uncertainty indicated) 48 hours hence. It also shows the current extent of damaging winds and a graphical representation of the areas affected by current cyclone watch and cyclone warning. 
  If a cyclone is expected to affect coastal communities with the onset of gales within 48 hours, but not within 24 hours a Tropical Cyclone Watch is issued. 
  If a cyclone is expected to affect coastal communities with the onset of gales within 24 hours or they are already occurring a Tropical Cyclone Warning is issued. 
  Marine warnings for coastal waters will be issued up to 24 hours in advance of the expected conditions. Routine marine forecasts will also contain information regarding wind speeds, seas and swells expected with tropical cyclones extending out to the fourth day on the evening issue of the forecast. 
  Computer model guidance of weather and wind patterns are available from the Bureau’s site at <http://www.bom. gov.au/jsp/marine/wind/index.jsp> and give wind speed and directions out to seven days. 
  There are also non-Bureau sailing weather sites that email a return of computer model weather data in brief messages for mariners. One that readers can try is: in the body of an email put the following spot:25.5S,159.25E (adjusting the latitude and longitude to your requirements) and send to <query@saildocs.com>. 
  Within a few minutes you will get a spot forecast for the location specified every six hours out to five days emailed back to you. 
  The wind forecasts that are produced from computer models may not always reflect official forecasts, especially in rapidly changing conditions. The accuracy can decline the further out in time the forecast extends. 
  If you are operating in water affected by cyclones there are many heavy weather sailing guides for mariners’ reference to ‘evade’ an imminent oncoming cyclone. Most ports in cyclone areas have a cyclone action plan that mariners should follow.

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of QLD and is a Master V. He gives education courses on Marine Meteorology.