Weather Watch: Prepare for the storm

A satellite image dated September 15 shows a storm gathering in the Tasman Sea due to hit New Zealand Thursday September 16. Photo / Japan Meteorological Agency

It's not too often one of the largest storms on the planet roars by our little ol' country - but that's certainly the case over the next few days.

From west to east the strong winds circling the large centre cover an area about the size of Australia. I can't recall many lows that large which have passed by us.

Click here to see the latest maps of this very large low.

The maps give you a clear understanding as to just how large this low is - but it also shows that the centre of the storm is well south of the country.

In fact, for Spring, this is fairly normal Southern Ocean weather. This is, of course, where the name "roaring 40s" comes from.

The difference with this low is that it's surging northwards and between high pressure north of New Zealand (around 1020hPa) and the very deep low south of us (around 950hPa) there is a massive air pressure gradient that is bigger than the length of our country. (Also worth noting that Hurricane Igor, currently in the Atlantic at Category 4 status - 5 is the highest - had a central air pressure of 942hPa on Thursday morning NZT).

What does that mean? It means strong winds - gale force at times - sweeping across many regions. The strong winds out at sea will start to push the water creating monster waves along the west coast of New Zealand. This is NOT a weekend to take the boat out or go fishing.

On the South Island's West Coast front after front will slam into the Southern Alps creating torrential rain that will hang around for the next few days. Heavy rain will also affect the ranges of the North Island from the Tararuas to Mt Taranaki and Whanganui catchment areas.

To top it all off the storm - which hits New Zealand in the north and touches Antarctica in the south - will spread a southerly change into Southland, Otago and Fiordland later on Friday and into Saturday. This bone chilling air will be coming from above the Antarctic Ice Shelf so it is super chilled. Not the best news for farmers in the middle of lambing in Southland and parts of Otago. There is some good news - the cold blast will be short lived with some forecasts already indicating that Sunday will bring in warmer westerlies. Here's hoping.

If you're reading this and thinking New Zealand is about to get wiped off the map - don't panic. While the low is certainly large and very aggressive the warnings are mostly for people who are planning on being outdoors. The true danger will lie in areas easily flooded in the west, large seas up the west coast, falling branches/trees around central New Zealand and snow in the deep south. Most of us won't see weather bad enough to cause damage.

Either way, the weekend will be a good excuse to blob and do nothing if you can.

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