The World Meteorological Organization (WMO) Tropical Cyclone Programme has been established to minimise the loss of life and damage caused by tropical cyclones.

Who is Watching for Tropical Cyclones?

Where to get the Latest Tropical Cyclone Information

What is a Tropical Cyclone?

When do we get Tropical Cyclones?

What are the Different Categories of Tropical Cyclone?

Important Facts about Tropical Cyclones and New Zealand

Who is Watching for Tropical Cyclones?

Internationally there are six Regional Specialised Meteorological Centres (RSMCs), together with six Tropical Cyclone Warning Centres (TCWCs), which have regional responsibility to provide tropical cyclone warnings and information.

MetService operates TCWC Wellington, so all official warnings and information about tropical cyclones that could affect New Zealand originate from us.

Map of WMO Tropical Cyclone Monitoring Centres

Where to get the Latest Tropical Cyclone Information

For New Zealand:

• MetService produces a Tropical Cyclone Potential Bulletin which appears as a 'Tropical Cyclone Activity' link in the warnings banner and on the warnings home page of metservice.com if there are any current cyclones in the tropics or any lows that could potentially develop into a tropical cyclone. Details of the latest updates, positions and expected movements can be found on the Tropical Cyclone Activity page. 
• Any and all severe weather effects on New Zealand caused by tropical cyclones are covered by our Severe Weather Watches and Warnings. 

For our region and globally:

• You can see all current Tropical Cyclone Warnings from RSMC Nadi in Fiji here and from Australia's TCWC here 
• Other national weather services may issue warnings for the impact of tropical cyclones on their national territory, local areas and coastal waters.
• WMO's Severe Weather Information Centre provides a global snapshot of all tropical cyclones and depressions, typhoons and hurricanes currently being monitored by WMO monitoring centres.

What is a Tropical Cyclone?

A tropical cyclone is a low pressure system which forms over warm waters in the Tropics and has gale force winds (63km/h or more) at low levels near the centre (winds turning clockwise in the Southern Hemisphere), with organised convection (i.e. thunderstorm activity). The diagram below shows the structure of a tropical cyclone in the Southern Hemisphere. 

Diagram of the structure of a tropical cyclone in the Southern Hemisphere

The gale force winds around the cyclone can spread out hundreds of kilometers from the centre or eye of the cyclone, and if the winds reach 64 knots (118km/h) then the system is called a severe tropical cyclone. See the table below for what severe tropical cyclones are called in other parts of the world.

When winds exceed 64 knots (118km/h) a tropical cyclone is known as:

When do we get Tropical Cyclones?

In the Southern Hemisphere, the tropical cyclone season usually lasts from November to April, with a peak in February. In the Northern Hemisphere, most tropical cyclones occur between June and November with a peak in September. However, in the north-west Pacific it is not unusual to have the occasional tropical cyclone outside of this period. Tropical cyclones have been observed in the South Atlantic, but this is a very rare occurrence.

What are the different categories of Tropical Cyclone?

This table outlines how Tropical Cyclones are categorised under the Australian tropical cyclone intensity scale used by the Fiji Meteorological Service and the Australian Bureau of Meteorology (any tropical cyclones that impact New Zealand originate from these regions, and are no longer 'tropical' when they reach us).

Important Facts about Tropical Cyclones and New Zealand

• On average, about 10 tropical cyclones form in the South Pacific tropics between November and April each year, and about one of those will affect New Zealand as an ex-tropical cyclone (most commonly in February or March).
• The characteristics and structure of any tropical cyclone will change dramatically by the time it reaches New Zealand, and it will almost certainly be re-classified as an ex-tropical cyclone. This is due to it moving over colder waters and encountering strong upper level winds as it moves south out of the tropics. See our blog Tropical cyclones: extra-tropical transition for more information about extra-tropical transition. Although these systemswill no longer be classified as tropical cyclones, we will often still refer to them by their given name  e.g.  ‘Cyclone Gita' for communication purposes.
• Re-classification as an ex-tropical cyclone does not necessarily mean the system has weakened, but rather that it has transformed into a completely different type of weather system. Ex-tropical cyclones may still have considerable potential for severe weather, and under the right meteorological conditions they can intensify and acquire lower pressures than they had before being re-classified. Many of New Zealand’s most severe and impactful storms have been ex-tropical cyclones.
• In the tropics, the strongest winds and most intense rain associated with a tropical cyclone usually occur just outside the ‘eye’ (cyclone centre). However, after the cyclone has undergone extra-tropical transition, it loses its symmetric cloud pattern and the strongest winds and heaviest rain can be hundreds of kilometres from the cyclone's centre, usually in a large area south of the centre. This means that the position of the cyclone centre is no longer a good indicator of where the most severe weather will be. For example, During Cyclone Bola in 1988, the heaviest rain and strongest winds over New Zealand occurred well away from the centre of the cyclone. It is wrong to assume that because the cyclone centre is missing New Zealand, the severe weather will also miss New Zealand!