Vania has this afternoon been downgraded to an ex-tropical cyclone 24 hours earlier than previously predicted reports WeatherWatch.co.nz.

We have extensive coverage of the approching tropical systems – see below for all the details on both Vania and Zelia.

The tropical storm, which was yesterday predicted by the Fiji Meteorological Service to stay at cyclone status until Sunday night has had less favourable conditions to maintain it’s cyclone strength and this afternoon Vania has been reclassified as a tropical depression.

WeatherWatch.co.nz head weather analyst Philip Duncan says despite Vania becoming an ex-tropical cyclone the storm will still pack a punch.  “When tropical cyclones start to move out of the tropics they move over cooler waters.  These storms then go through a major transition where the entire structure of the storm changes”.

But Mr Duncan says it is still a serious system to monitor as once they move out of the tropics they can again deepen and strengthen.  “These lows are still packed with moisture once they leave the tropics, so while the winds have eased considerably from 48 hours ago, the low is going to pull tropical moisture down to New Zealand early next week”.

Bob McDavitt, MetService weather ambassador, told WeatherWatch.co.nz that the isobars – the lines on weather maps that highlight air pressure levels – are like onion rings.  “Those compacted central isobars are something like onion rings. The outer layers “unravel” nicely so that surrounding barometers do not measure the isobars that hug the centre, until the centre makes landfall somewhere”.

“If a tropical cyclone encounters an injection of cold air as it approaches NZ then it may feed on the density difference – the cold air turns the tropical moisture to rain, and that rain-making process lowers the pressure, so that the system re-deepens and becomes extra windy” says Mr McDavitt.

But of more concern for forecasters will be newly developed Tropical Cyclone Zelia.  The depression, which has remained off the Queensland coast for a number of days, deepened yesterday and was classified as a Category 1 cyclone late on Friday by the Bureau of Meteorology in Australia.

A category 1 cyclone has winds averaging 65km/h and winds gusting up to 120km/h.

Zelia is expected to quickly move across the extra warm waters of the Coral Sea this weekend – the same body of water that has fueled the devastating rainmakers which have hit Queensland over the past few weeks.

By Monday morning Zelia is expected to be a severe tropical cyclone with Category 3 status.  “That means winds are hurricane force, or sustained at 120km/h” says Mr Duncan.  “Gusts could be well over 200km/h”.

Zelia will likely track south of New Caledonia on Monday and quickly catch up to the remnants of Vania by Tuesday.  “It is incredibly rare to see two tropical cyclones merging like this” says Philip Duncan.  “Like two planes landing on two separate runways that meet in the middle”.

Mr Duncan says while it might sound dramatic it doesn’t mean the storms will become twice as dangerous.  “It simply means we might have more widespread rain and wind – more people will receive unsettled weather as opposed to winds and rain being twice as bad”.

WeatherWatch.co.nz says at this stage it appears rain will likely be the main feature however severe gales are still possible.  The weather news authority predicts gusts up to 120km/h will be possible in some main centres with gusts in exposed areas possible reaching 150km/h.  “The 150km/h winds would be a worst case scenario across exposed parts of the eastern Waikato, Hauraki Gulf and some parts of Northland and the Far North”.

“For main centres like Auckland we doubt winds will be damaging but they may reach gale force (65km/h) for a time with gusts over 100km/h in exposed areas”.

WeatherWatch.co.nz says the current models are showing the two systems merging on Tuesday and Wednesday and moving much further west of New Zealand than initially forecast.

“This shift to the west in the tracking could have a serious impact for the Nelson region.  We may see this tropical system feeding very heavy rain into the Nelson region during the week and following the flooding in December we advise locals to be prepared for possible flooding”.

MetService, New Zealand’s Government owned forecaster, predicts a “high chance” of heavy rain in the Nelson region, also the north west side of the South Island and western Taranaki, including Mt Taranaki, on Tuesday and Wednesday.

The crown forecaster also predicts a moderate chance of severe gales and heavy rain across a large portion of the North Island.

Severe Weather Watch / MetService

MetService says former Tropical Cyclone Vania is expected to pass near Norfolk Island on Sunday. “Although the exact future track is not certain, present guidance indicates that Vania will pass west of the North Island on Tuesday night and move southeast over the northern South Island on Wednesday morning followed by a trough and another smaller low. This active weather system is likely to bring outbreaks of rain and a spell of north or northwest gales over northern and central New Zealand on Tuesday and Wednesday”

MetService predicts the heaviest falls will probably be in northern Westland, Buller, Nelson, Marlborough and north Taranaki where confidence of warning amounts is rising to high. Confidence of warning amounts is lower in a surrounding area including parts of Northland, across the central North Island to eastern Bay of Plenty and the north of the Gisborne region, about the Tararua Ranges and further south in Westland (see chart above).

They says there is a moderate risk of severe north or northwest gales in exposed places across much of central New Zealand also parts of Gisborne from Tuesday afternoon to Wednesday but the risk is a little lower further north.

“As there is the potential for stormy conditions people are advised to remain up to date with the latest forecasts and plan accordingly”.

– WeatherWatch.co.nz, MetService