KEY POINTS:
Let the battle begin! It's time to watch the annual battle between anticyclones (highs) in the north and depressions (lows) in the south as Spring weather really kicks into gear.
Deep lows are churning through the Southern Ocean, full of winter strength and fury, while high pressure systems are making a regular come back to the North Island bringing sunnier, drier, weather. The 'squeeze zone' in between these two main systems creates very warm northerlies or nor westers over most regions south of Manawatu.
We saw that on Tuesday this week when temperatures jumped to 27 degrees in some parts of Christchurch and Dunedin officially reached a summery 25. In the North Island conditions weren't quite as warm despite the sunny weather, still temperatures in the late teens were common place.
We can expect more of this weather in the weeks ahead - and as we creep into October we're quite likely to see a return to violent weather. I find September to be a transition month as winter ends and spring starts. In October we often see plenty of thunderstorms, severe gales and quite a contrast in temperatures as the wind flicks like a light switch from north to south, north to south and winter air mixes with warmer air from the north.
Across the coming weekend it appears as though the North Island/South island contrast will continue, with a high over the North and those strong or strengthening nor'westers in the South. It's perfect weather for farmers, skiers, boaties in the north and for many kiwis nationwide it'll spell more good news as we tackle our lawns, gardens and dust off our bbqs, T-shirts and sunglasses!
Meanwhile the South Island's hydro lakes should again see more rain Wednesday as a front moves in. It's great news for our power woes and combined with snow melt the lakes should now, or soon, be returning to a very healthy state.
A few people have asked my why Auckland and Northland have barely made it into the late teens while the South Island - which is obviously much closer to Antarctica - has been much hotter. It's quite a simple process - and it's the Southern Alps that cause it. The rain clouds move in to the West Coast, the air dumps all the rain as it moves up and over the Alps, by the time it passes over the peaks, it's almost dropped all it's moisture. The air then races down the eastern slopes and as it picks up speed it heats up and even more moisture is removed. Being a nor'wester those in the east and south are most affected - which is why it was so dry and hot in places like Dunedin, Timaru and Christhchurch on Monday and Tuesday.
Strong westerlies have a similar affect from Wairarapa to Gisborne and also the Bay of Plenty. It's the lack of mountains around Auckland that keep the temperatures fairly consistent throughout the seasons. However regions from Waikato northwards get something many of the South Island places don't - and that's high humidity.
In Summer the Weather Watch Centre uses a tool called the "Humidex" which combines air temperature with humidity and calculates what it "feels like" on your skin (basically the Summer version of the "wind chill factor"). So in January Christchurch and Auckland may share the nation's high of 26 degrees but in Auckland it felt more like 32 due to the humidity. Here's an example of how the humidity affected temperatures in December last year, click here.
We have had a year of extremes - so how will Spring shape up? Well as you know I'm not a huge fan of long range forecasts but with no La Nina or El Nino (we're in a neutral period) maybe we'll see more settled weather. Of course two tiny islands in the South Pacific tend to have their own weather patterns, bucking the trend this past Summer when La Nina should've spelt a soggy summer for many, instead of such a dry one. So I guess it really is 'up in the air'.
Philip Duncan
Pictured above: Storm clouds and rainbow over Tauranga harbour. Photo / Stuart Whitaker
For the latest weather news keep up to date with The Radio Network's new Weather Watch Centre or the NZ Herald weather section.
