‘This is a ka-boom weekend’ – expert

Will Tropical Cyclone Oma hit New Zealand or won’t it?

Computer models are battling it out over TC Oma’s possible severe impact on the country this weekend. Two of the most trusted models – the United States’ GFS and the European ECMWF – are taking opposite views of where Oma may end up, whether that is as a tropical cyclone or a former tropical cyclone.

But they agree a large low-pressure area may deepen east of central New Zealand on Sunday, accompanied by heavy rain and high winds in places, and dragging up very cold southerly air, bringing high country snow, from well south of the country.

Unusually high tides are also expected in Christchurch on Wednesday, Thursday and Friday, which could bring surface flooding.

The latest information from the Fiji Meteorological Service showed Oma at Category 2 strength early yesterday, west of Vanuatu and north of New Caledonia. It was expected to move southwest over the next few days, increasing to category 3 strength by tomorrow.

MetService meteorologist Mark Bowe said there was ‘‘still a lot of uncertainty in the models, not agreeing where Oma should go’’.

‘‘Regardless, there is going to be quite a lot of rain across New Zealand by this Friday. It’ll actually be rainy for most.’’

Blue Skies Weather forecaster Tony Trewinnard said after six weeks of settled, dry and often very warm conditions due to blocking high-pressure, ‘‘this is the ‘kaboom’ weekend’’.

‘‘We need to give a heads-up that there is severe weather on the way, from Sunday through to Tuesday [next week], and to think about how we might cope if there is disruption.

‘‘There’s a lot of potential for a lot of stuff to happen. We are looking at several days of severe weather around the country – some people will see gales or worse, heavy rain, and very cold temperatures with snow on higher ground.

‘‘Those things are probably going to happen, but it’s too early to flesh out the detail of what is going to happen exactly where and when.

‘‘We’ve got a week to get our heads around it. We don’t always have that luxury.’’

A year ago this week, ex-tropical cyclone Gita carved across New Zealand.

Flooding and wind damage was especially bad in Taranaki, Tasman and along the West Coast. States of emergency were declared in Taranaki, NelsonTasman, the Selwyn district, Grey district, Buller district, Westland district and Christchurch city.

The GFS computer model predicts a hit on central New Zealand on Sunday afternoon.

However, the ECMWF model forecasts that Oma will remain near the Queensland coast, but a deep depression will develop rapidly over New Zealand anyway.

‘‘The Tasman Sea is kicking off,’’ Trewinnard said. ‘‘Even if you take that tropical cyclone out of the picture, the models are still giving us a severe weather weekend.’’

WeatherWatch head forecaster Philip Duncan said high pressure – ‘‘like an invisible barrier in the sky’’ – near Tasmania was ‘‘stumping’’ the computer models. He estimated a ‘‘50-50 chance’’ of severe weather from ex-TC Oma.

‘‘A large cooler southerly flow should be moving in to the New Zealand area by this Friday and the weekend, but if Oma directly comes in that will pull down very warm tropical air with it from the north.

‘‘If the two meet, then even bigger rain clouds may be generated over New Zealand. There are a lot of moving parts to this.

‘‘The GFS is showing a direct hit while ECMWF shows Oma doing a 180-degree turn and backtracking north again. Many other models show the low sliding into the New Zealand area while becoming extratropical, the technical change within a storm when a tropical cyclone drops far enough south.

‘‘It is very much a waiting game for the two models to align,’’ Duncan said.

‘‘There is going to be quite a lot of rain across New Zealand by this Friday. It’ll actually be rainy for most.’’

Meteorologist Mark Bowe