Weather: La Niña means you need to look at the forecast differently to get an accurate picture

La Niña has been declared in the Pacific, which Weatherwatch says means more overnight and early morning drizzle and an increased chance of large, but isolated, downpours in the afternoon.

It also means Kiwis watching the weather forecasts should pay closer attention to rainfall totals. 

The Australian Bureau of Meteorology on Tuesday said La Niña is underway in tropical Pacific. This means more north-easterly winds bringing moist, rainy conditions from the Pacific to eastern parts of the North Island. That's because sea temperatures north of New Zealand and around the country are warmer than average.

During La Niña summers, Weatherwatch says drizzle can be expected in the morning in northern and eastern parts of both the North and South Island. Then, as it gets warmer in the afternoon, the moisture is rounded up into "one, or a few, big tall afternoon downpours, which can also turn into severe thunderstorms". 

The forecaster says the conditions over the last 12 hours are an example of this. 

"High pressure is covering much of New Zealand right now and for those in places like Auckland and Waikato we saw isolated heavy afternoon downpours yesterday and then as it eased we saw more widespread areas of drizzle last night.

"For many the drizzle barely registered in the rain gauge while others had a top up from the isolated downpours."

In large, dry areas over summer, Weatherwatch says there can be the "swiss cheese effect", where a region may have "holes in it from where the afternoon downpours developed and then drifted".

How to check?

Weatherwatch says there are two things people should focus on when looking at weather forecasts over the coming weeks: "A) The daily rainfall total, as if it’s below 5mm you may understand it’s some afternoon isolated downpours (and they can also miss you, despite being forecast). B) The hourly rainfall forecasts to make more sense of it all."

Despite La Niña's presence, Weatherwatch says there won't be non-stop tropical rain. New Zealand sits in the middle of two weather patterns, with La Niña's moisture coming from our north-east and westerly weather coming from the Southern Ocean and southern Australia.

That's evidenced by the large amount of rainfall forecast to move from the Southern Ocean towards the West Coast and Fiordland over coming days. 

"This rain will also be helpful spilling over into Southland and Otago which have had plenty of rain events lately. This isn’t always the case in a stronger La Niña as that can stop these westerly and southerly rain events from coming in and increase the chances of southern droughts."

This means more variety in weather for New Zealand in December, and "that does help somewhat reduce the chances of droughts forming". 

" La Niña may be short lived again this season – peaking over summer and easing in Feb/March – and while it does increase the risk of some localised floods and droughts, it’s worth balancing that with other weather patterns too.

"This summer NZ needs to be prepared for whatever is thrown at us – it might be coming in from any direction, not just out of the tropics."