Key Points:

Will we reach 40 degrees on Sunday? It's a question I was asked all day yesterday and my answer was this - we have all the right ingredients for it, now they just need to come together at the same time.

From one end of New Zealand to the other, Sunday is shaping up to be a 30-degree-plus day for many of us - whether that's the air temperature or air temperature plus humidity.

So what are the ingredients for New Zealand to climb to 40 degrees Celsius and above? Well before I explain that, let me explain why southern Australia can easily reach 45 degrees. One word: Desert. When there's a high in the Tasman Sea it feeds hot northerlies straight off the desert and into Adelaide and Melbourne. Nothing clever happens - just a hot northerly and voila, it's stinking hot.

New Zealand isn't quite as simple. We have 3 ingredients that we need before we can attempt to hit 40 degrees.

1) The air needs to be hot in the first place. The air moving towards New Zealand has travelled from Aussie so it's already at a good temperature.

2) We need some nor'westers! The north'westerly flow is a very hot, dry wind flow for eastern New Zealand and gives us our best shot at hitting 40. As that air arrives along our west coast it needs to climb over the ranges - such as the Southern Alps, the Tararua Ranges and the Hawkes Bay ranges. As it rushes down the eastern side of those ranges the air starts to heat up (friction) and of course as it heats up it burns off any moisture (humidity). The drier it gets the more it heats up...so by the time it reaches those east coast towns and cities it's bone dry and scorching hot.

3) The skies need to be clear. Thick high cloud can sometimes shave several degrees off the temperature so we need the skies to be pretty much blue.

The forecast for Sunday, especially the Canterbury region, favours all 3 ingredients but they need to all be perfectly in time - all meeting up during the peak of the heat which is mid afternoon. If we can get that today then we'll be climbing towards 40. If the cloud rolls in early, or the winds aren't quite right, you could take a few degrees off that high.

It's a hard thing to forecast - and there's no guarantee 40 will be hit today - but we do have the right ingredients for it and that's quite rare.

WeatherWatch.co.nz will be closely monitoring the temperatures throughout today with updates on the highest temperatures every hour from 8:30am until 6:30pm - it's all at this special link here www.weatherwatch.co.nz. We also have tips on how you can avoid the extreme heat and stay a bit cooler.

Elsewhere - across northern New Zealand - the heat is definitely back. Yesterday in Auckland it was officially 28 but many would've recorded temperatures in the lower 30s. Combined with 50 per cent humidity yesterday felt more like the mid 30s. Today it may be even hotter as humidity levels rise ahead of a low coming down from the sub-tropics. If Auckland reaches 30 degrees, combined with 70 humidity, it will feel more like 40 degrees. Another day to just blob and do nothing...I like that advice! Wish I heard that more often!

By the way - tomorrow night is going to be a hot and humid one in the north. The overnight low on Monday night/Tuesday morning is around the lower 20s. It's very clear that we're now in the hottest month of the year!

- Philip Duncan
Image: Summer at Mission Bay, Auckland. Photo / Doug Sherring