Weather Watch: An Indian summer for New Zealand

While the weather here may be sunny, hot and calm, a violent storm is churning up the sea. Photo / David White.

Monday is the last official day of summer and while many regions are in for extended summer weather this year there are signs this week that the weather pattern is turning Autumnal (one of my favourite words!).

There are a few of ways of measuring the start of Autumn - one is the increase in deep lows in the Southern Ocean moving closer towards us and the other is the increase of winds over the South Island going from northerly quarter to southerly quarter every few days. Another is the drop in overnight temperatures.

Very slowly this north/south wind change is developing and this week it starts to look more classic. Over the coming weeks we'll see more rain warnings for the West Coast, more nor'westers in Canterbury and more extreme temperature ranges along the east coast of the South Island with highs in the late 20s and early 30s one day then low teens the next.

But the powerful La Nina we have this year will also mean more of the warmth in the north. Certainly the sea temperatures are at their warmest now (best time of the year to swim in the sea) and while the tropics look quiet this week, next week may be a different story.

Speaking of swimming in the sea, I noticed in the Herald on Sunday today a story about a group of students being caught in a vicious rip south of Napier. This is why WeatherWatch.co.nz makes a big deal about tropical cyclones that pass us well offshore. While other forecasters almost ignore these lows it's very important for those on land to realise what is happening out to sea. While the weather here on land may be sunny, hot and calm, a violent storm is churning up the sea a few hundred kilometres away and you can bet that those big waves and dangerous, hidden, rips will be pushing along our coastline. In fact I'd go as far to say that it's the rough seas that are the deadliest aspect of tropical cyclones here in New Zealand. So they don't have to hit land to be deadly - which is why WeatherWatch.co.nz will continue to talk about them even if they pose no threat to land at all.

Speaking of the tropics, some long range models are picking a sub-tropical low forming in the Tasman Sea next weekend and bringing rain - possibly heavy - to the upper North Island anywhere between the weekend and Wednesday of next week. We'll need to see a few more computer sweeps before we're certain of it, but even though the peak of the cyclone season has passed we are not out of the woods yet. Sub-tropical lows can be just as nasty too - the storm that brought the major coastal flooding to Auckland last month was only a sub-tropical low.

The long hot days - for both islands - are likely to remain for some time yet, with what people call an "Indian summer". An Indian summer is pretty much warm to hot days continuing despite the leaves changing colour. Here in NZ it's still a bit early to say we're having one, but in a few weeks it will be apt.

As I look outside my office window all the trees I can see are still deep green, although some leaves do look a little "old". So even though technically summer ends on Monday the summer weather does not. At least not this year. This is also the reason why so many people - especially in the northern hemisphere - go by the astronomical seasons (based around the equinox and solstice). If you live by that rule then summer still has a few more weeks left. Sounds good right? Well if you live by that rule you have to also except that winter lasts until the third week of September...I'm an inpatient man...I like spring starting on September 1st and summer starting on December 1st...even if it does mean Summer technically ends earlier.

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