As you know, a cold southerly is about to hit the country – bringing snow to low levels, isolated thunderstorms and hail, and strong to gale force winds. There will also be plenty of dry areas and sunny spells, as is the nature of a southerly in New Zealand.
The cold change is mostly being created by a very large high pressure system out to our west, near Tasmania.
In fact, whenever we get a nationwide southerly there is quite often a high stretching from north to south parked out near Tasmania. There’s also some low pressure well south east of Southland – and it’s these two systems that work in tandem to bring in the cold southerly.
But the main reason for the cold change is this very large high to our west – it reaches down into the sub-Antarctic region and scoops up cold air, then pulls it back north over New Zealand.
This same high will bring us warm weather by the weekend, though.
Over the weekend the high starts to track north of New Zealand and this gradually pulls in warmer air from the west and north. By Monday of next week, this same high will lie north east of New Zealand and the anticyclonic wind flow will be bringing both sub-tropical air and mild Tasman Sea air to the country as the high changes shape from north/south to west/east.
Dunedin has snow in the forecast tonight but a high of 16 or 17 on Sunday. Northerners, who see highs of just 12 or 13 on Thursday this week, warm up to 17 or 18 early next week.
And it’s all thanks to one large high pressure system.
This is not a storm hitting the country – just a typical spring blast that will only last around 24 to 48 hours for most areas, which is good news for farmers and newborn stock.
6pm Wednesday shows the large high in the Tasman Sea reaching down over the Southern Ocean and pulling up cold southerlies across New Zealand.
By Monday next week the high has moved from near Tasmania to north east of the North Island. It still retains the anticyclonic airflow so instead of dredging up Southerlies over the country it will start to pull down sub-tropical winds for the north and westerlies elsewhere.
– Maps / Weathermap