Like popping a water balloon, the tropical low north of New Zealand may drag down some more humidity and rain onto the country later this week, says WeatherWatch.

'Long range data has been showing a very large area of low pressure forming north of New Zealand this coming week with the Fiji MetService saying there is moderate risk that the central area of low pressure will spin into a tropical cyclone as it tracks southwards,” says WeatherWatch head forecaster Philip Duncan.

This means after Waitangi Weekend's rain maker the North Island may have another rain event this weekend.

'Even if a tropical cyclone doesn't form a tropical depression looks quite likely to drop southwards towards NZ by this Friday or the weekend and even if that low misses us, it may still connect with other systems in the Tasman Sea to generate heavy rain on the West Coast again and into the North Island.

'It's not yet locked in - but certainly one to watch.”

The setup means the sub-tropical jetstream or the boundary that separates NZ's temperate climate with the more muggy and humid tropical air will drop south over the upper North Island this coming week.

"This is the opposite to winter when the sub-Antarctic airflows come over NZ bringing snow to sea level, in this case instead of Antarctic air bringing Southern Ocean conditions we havetTropical air bringing weather conditions Fiji and Tonga usually have," says Philip.

GFS (American modelling) expects more rain around the North Island and West Coast, but very little for Otago and eastern Southland next two weeks. Image: WeatherWatch.

With the low yet to properly develop, rainfall totals in local forecasts may still move around a bit as better data comes into the mix in the days ahead.

"Basically this low pressure zone is dragging down the tropics with it, like popping a water balloon it's likely to drop a fair bit of moisture in a short period time. The only thing left to work out is precise tracking as that determines if the rain will directly affect NZ, or mainly fall nearby out at sea,” says Philip.

"We're not just watching one storm and the rain with it - we're watching to see how this large pool of tropical low pressure will interact with other systems in the NZ area. It is quite complicated so we need a few more days to be more confident of how this will pan out.”

The rainfall percentage and totals in local WeatherWatch and RuralWeather forecasts are updated hourly and utilise IBM's supercomputer Watson to crunch international modelling as well as local data.

'The closer to next weekend we are the more accuracy and confidence there is in the rainfall data,” says Philip.