Just what is “unsettled” weather? Many people assume it means stormy or wet – but unsettled often just means “changeable”.

We have a very large low moving in this Friday – and it will linger over New Zealand, mostly the North Island, until the end of next week. That sounds dramatic but let’s break it down.

Yes the low is “large” – it will cover most of the Tasman Sea this weekend from the Aussie coast to the NZ coast. But large doesn’t always mean aggressive. In this case large means lazy and slow – this system will take several days to cross the country and because it’s so large the energy is being spread out over a much greater distance.

The greater the distance the more ‘gaps’ there are in the weather. So we have bands of rain, areas of showers, areas of cloud and some sunny spells in there too. Just take a look at the latest satellite map of the Tasman Sea and you can already see how messy it looks.

Over the weekend there will be a mix of rain or showers. Saying “rain or showers” isn’t a cop out when most forecasters use it. A shower is defined as precipitation that lasts a short time – less than 30 minutes – and generally doesn’t cover a huge area. This weekend we will have showers moving in but they will be tracking slowly – that means in some areas they could last longer than 30minutes which makes it go from “showers” to “rain”. You can see why this can be a tricky area to nail down but WeatherWatch.co.nz is making a name for itself by trying to break this down as best as possible each time we have a rain event.

With the low being so large, but not packed with a lot of energy, it means there will be huge gaps in between the areas of rain/showers. So long dry spells too, especially in the south and east.

So a messy forecast that will likely deliver more rain where it’s needed the least, the west and north – and less rain where it’s needed the most, the east and south.

– Column by head weather analyst Philip Duncan – @PhilipDuncan on Twitter