Cyclone Gabrielle is expected to hit New Zealand on Sunday - bringing destructive winds, severe gales and monster waves.
Forecaster Weatherwatch.co.nz has dubbed it as one of the worst storms of this century.
In the past day, it has intensified into a “severe category three” cyclone.
Tropical cyclones are classified into five categories, ranging from category 1 – with 10-minute mean wind speeds (MWS) of 63-87km/h - to category 5, which can bring a 10-minute MWS of more than 200km/h.
Category 3 is a severe tropical cyclone, with 10-minute mean wind speeds (MWS) of 119-157km/h and very destructive winds carrying a maximum three-second gust speed of 165-224km/h.
Winds at such high speeds can wreak havoc on buildings and turn airborne debris into potentially lethal missiles.
Torrential levels of rainfall and tide-raising storm surges can combine to cause disastrous floods.
Tropical cyclones are like giant atmospheric heat engines, drawing moisture from the warm ocean as fuel and generating enormous amounts of energy as clouds form.
Rotating thunderstorms form spiral rainbands around their centre where the strongest winds and heaviest rain are found – creating a wildly-destructive “eye wall” around the eye.
While we know them as tropical cyclones in New Zealand and the southwest Pacific – such systems are called typhoons in South East Asia and China, and hurricanes in the North Atlantic, where they rotate in the opposite direction, or counter-clockwise.
When Cyclone Gabrielle reaches New Zealand it will be reclassified as an ex-tropical cyclone.
MetService states ex-tropical cyclones 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 an 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.
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