Former cyclone Gita getting faster, despite being downgraded

Tuesday, 20 February 2018

The sun rises to New Zealand's east at 7am on Tuesday, with Former Tropical Cyclone Gita still hidden in darkness. Inset: Infrared satellite image shows Gita in the Tasman Sea.

Gita may have been downgraded to an ex-tropical cyclone, but that doesn't mean its necessarily slowing down.

MetService says people should not interpret the re-classification as a sign the storm is weakening. 'It's actually getting faster,' meteorologist Rob Kerr said.

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'The environment it's moving into will mean it will actually speed up, get a bit faster, and the central pressure will drop even more.'

The ex-cyclone's wind speeds are currently 55 knots (102 kilometres per hour), but could pick up to 65 knots (120kmh) around the low, and then weaken again before reaching New Zealand.

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Kerr said Taranaki and Farewell Spit could expect to see swells up to six metres.

Cyclones produce gales, torrential rain and storm surges of varying intensity, depending on how low the storm's central pressure becomes.

The destructive winds, heavy rainfall and storm surges can cause extensive damage and lead to flooding, landslides and dangerous sea conditions.

WHAT MAKES A CYCLONE?

Kerr said one of the criteria for a storm maintaining cyclone status is that the main convector band must remain within one degree of the latitude or longitude of the eye of the storm.

Vertical shear could weaken cyclones by interfering with the eye of the storm.

'Gita's been sheared off in its vertical column, so it's no longer a straight thing,' Kerr said.

CYCLONE? HURRICANE? TYPHOON?

What's the difference between a 'cyclone' like ex-cyclone Gita and a 'typhoon' or a 'hurricane'?

Kerr said it depended where the storm was based. 'The same beast, different names,' he said.

In the South Pacific and Indian Oceans, storms like Gita are known as cyclones. In the Atlantic and northeast Pacific Oceans, they are known as hurricanes. And in the northwest Pacific Ocean, they are known as typhoons.

MEASURING CYCLONES

The Saffir-Simpson hurricane wind scale is widely used internationally, but New Zealand uses the Australian tropical cyclone category system.

Category one cyclones produce winds of more than than 90-125 kilometres per hour. Category five cyclones have winds surpassing 280kmh.

Gita intensified into a category 5 severe storm at its peak, before weakening.

CATEGORY ONE: Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings. Strongest winds gust 90-125kmh.

CATEGORY TWO: Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings. Strongest winds are regarded as destructive, gusting to 125-164kmh.

CATEGORY THREE (SEVERE): Some roof and structural damage. Some caravans destroyed. Power failures likely. Strongest winds are regarded as very destructive, gusting to 165-224 kmh.

CATEGORY FOUR (SEVERE): Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures. Strongest winds gust to 225-279kmh.

CATEGORY FIVE (SEVERE): Extremely dangerous with widespread destruction. Strongest winds gusts of more than 280kmh.