Zelia downgraded but NZ still set for gales, rain

File photo / Kenny Rodger

Zelia is no longer a tropical cyclone, but is still expected to whip up a storm as she races over the country today.

MetService has issued a severe weather warning for Marlborough, Nelson, Buller, Westland, Taranaki, Wanganui, Manawatu, Wairarapa, Wellington, Waikato, Waitomo, Bay Of Plenty Rotorua, Taupo, Taumarunui, Gisborne, and Taihape.

A low is forecast to move over the northern South Island today and MetService forecast another - Zelia - would likely pass west of Northland and Auckland this afternoon and evening, move over the southern North Island tonight and pass west of the Chatham Islands late tomorrow.

MetService is warning severe gales can be expected in exposed parts of the North Island and the top of the South Island. It said trees, powerlines and insecure roofs, could be damaged and driving conditions could be hazardous.

WeatherWatch.co.nz weather analyst Philip Duncan said the air pressure had been "plummeting" in the Far North as the outer-bands of Zelia reached New Zealand in the past hour, with the "compact" storm centred about 300kms to the north west and closing in fast.

Gale force winds have also arrived at Cape Reinga, with gusts to almost 90km/h, he said.

Mr Duncan said the "air pressure gradient" between the cyclone and land was quite significant earlier this morning.

"The air pressure was around 1000hPa at Cape Reinga and just 980hPa a couple hundred kilometres to the north. To put that into perspective in a normal storm we would see that pressure difference stretched out across the entire length of the country, not the length of just one region."

Mr Duncan said just like the lines on a topographical map the air pressure lines (isobars) are tightly packed around the centre of this low. A steep drop in air pressure means severe winds.

However Zelia's air pressure is "filling in", with the barometer rising sharply inside the depression quickly.

Current air pressure is around at 990hPa with winds averaging around 90km/h and gusting to around 120km/h. If Zelia was still a cyclone she would have a category one status, Mr Duncan said.

WeatherWatch.co.nz predicted heavy rain would spread down Northland this morning with winds becoming very strong by late morning or early afternoon as the centre of the low moves by.

"Tropical cyclones are very different to other lows that we get in New Zealand. With other lows the strong winds are usually created in a squash zone between a deep low and a large high. With a tropical cyclone the severe winds are only near the very centre so we can have destructive winds for a hundreds kilometres or so from the centre but then a few hundred kilometres away it's mostly calm."

Meanwhile, very humid conditions left by ex-tropical cyclone Vania made for an uncomfortable sleeping night for many. In Auckland humidity reached 100 per cent with foggy conditions reported across the region. Auckland currently has 98 per cent humidity, Mr Duncan said.

Several flights have been cancelled at Tauranga Airport because of misty rain.

Flights to Auckland, Wellington and Christchurch scheduled to depart before midday, have been cancelled. Those scheduled to leave from 1pm are on time.

Zelia is merging with the remnants of Tropical Cyclone Vania and another low in the south that contributed to the flooding in Victoria, which has triggered several MetService weather warnings and watches.

MetService forecaster Paul Mallinson said the path of Zelia is difficult to determine, but the lows should heavy rain and northerly gales over much of northern and central New Zealand today and early tomorrow.

The heaviest falls of 200 to 250mm are expected near the ranges of Westland, Buller and Nelson north of about Franz Josef and 70 to 150mm falls can be expected about the Marlborough Sounds, in the southwest of the North Island, and in a belt from Taranaki to southern Waikato across the central North Island to eastern Bay of Plenty and the north of the Gisborne region.

MetService warned streams and rivers may rise quickly and there are likely to be slips and surface flooding on some roads.

The strongest winds are likely to be from Taranaki and Taumarunui to Taihape and inland Wanganui, with gusts around 130 km/h in exposed places this evening.