KEY POINTS:
It's been a while coming but this week Winter will finally take control of New Zealand thanks to a blast of polar air from Antarctica.
Snow, gales, heavy rain, thunderstorms and possibly tornadoes are in the forecast for the next 7 days as the wild wearther sets in.
The reason for the extra cold weather is because of the polar jet stream shifting over us.
We don't talk a lot about jet streams here in New Zealand - mainly because they don't have a regular impact on our weather - but in large countries such as Australia, America and Canada it's quite common.
Jet streams are 'motorways in the sky'. Lanes of very fast moving air that pretty much draws a line in the sky where warm and cold air is seperated. We have one to the north of New Zealand and one to ths south of us - which is why New Zealand is so temperate.
We're not in the tropics but neither are we polar. In winter the polar jet stream lifts northwards, the bigger and stronger the lows are behind it, the further north it gets pushed northwards.
They are both constantly being pushed north and south depending on the systems surrounding them. This week's winter storm will give the jet stream a good shove northwards - click here to get an idea as to what I'm talking about:
Everything south of that jet stream is bitterly cold. as it slides eastwards it will eventually cover the entire country. Hello winter!
So what's in store for us this week. Well here's a quick look:
Monday: A band of heavy rain and isolated thunderstorms will spread over the North Island this morning but all eyes are now on a cold front moving into the South Island. The front will arrive late today.
Tuesday: Snow continues, but ir'll be moving north very quickly. Snow is expected in Invercargill, Queenstown and Cromwell with a snow flurry possible in Dunedin, Timaru and Christchurch. Through Tuesday the cold front will march northwards bringing a bitterly cold change to Wellington and heavy snow to the Central Plateau. The Desert Road and other roads around Mt Ruapehu may well be closed during Tuesday or Tuesday night with a high of just 3 degrees forecast for Waiouru!
Wednesday: A very strong wind will blast the country with gales across central New Zealand. There will be some squally showers along the west coast of both islands with isolated thunder, hail and possibly small tornadoes. On Sunday the Radio Network's Weather Watch Centre predicted the risk of tornadoes over the North Island and reports came in of a small one in Papamoa, according to One News. The mix of warm humid air and cold polar air creates an incredible amount of instability in the atmosphere - this is the breeding ground for dangerous squally showers.
Thursday/Friday: Conditions will ease everywhere with showers becoming less intense and winds shifting to relatively warmer westerlies.
Saturday: Another cold blast will move up the South Island - again snow to low lying areas.
So yes - it's hello to winter! For those of you who hate winter, look on the bright side - literally: With the winter solstice behind us the days will now start to get a little longer!
Philip Duncan
For the latest on this winter storm keep up to date with The Radio Network's new Weather Watch Centre, the Newstalk ZB weather website or the NZ Herald weather section.