Second tropical cyclone - Zazu - brews in Pacific, but it's Yasa likely to hit NZ

A second tropical cyclone has formed in the Pacific leaving the islands with many wet and windy days in the lead-up to Christmas.

Cyclone Zazu is currently tracking at sea between Tonga and the Cook Islands and as of 7am today, is a category 1 carrying gale force winds.

Tropical cyclone Yasa has since been upgraded to a category 2 with winds now reaching up to be between 90km/h and 120km/h.

Philip Duncan of Weatherwatch.co.nz says Zazu will likely also increase to a category 2 cyclone today.

However, fortunately for New Zealand, Zazu didn't pose a risk, whereas there was a 40 per cent chance that Yasa's remnants would reach our shores.

There was a 20 per cent chase that Yasa would prompt severe warnings if it did land.

The Global Forecast System (GFS) currently shows risks of rain or showers for many western areas and most of the South Island for Christmas Eve. Image / weatherwatch.co.nz

Given Zazu was so close to the islands, it was likely to cause issues to both Tonga and parts of the southern Cook Islands.

That would see dangerous seas, heavy and sustained gales brushing the island nations, with the worst hopefully remaining at sea.

While Yasa was likely to cause damage and possible evacuations in Fiji, by the time it reached New Zealand, Duncan said it might not even be a storm.

ECMWF's current estimated tracking for Christmas Day in NZ shows Yasa having weakened due to large highs in the area. Image / weatherwatch.co.nz

"Every single tropical cyclone loses its warm core just as it approaches New Zealand. Some, like Cyclone Gita a couple years ago, can retain the look and power of a cyclone during this process. Others fall apart.

"Modelling certainly is hinting that any possible New Zealand hit may not be from a storm at all - but simply the remnants of an ex-cyclone, in the form of a low-pressure system and rain."

Given Yasa's volatility, Duncan said there were currently a few different scenarios for Kiwis on Christmas Day.

"High pressure dominates, Yasa's remnants fail to arrive, dry for most. The remnants of Yasa fall apart over New Zealand bringing rain and showers. A combination of the above with high pressure bringing dry areas to some regions, but Yasa's remnants bringing some cloud and wet weather too."

Cyclons Yasa and Zazu pictured today in the Pacific. Image / weatherwatch.co.nz

By the time Yasa reached New Zealand, there was only "a low risk this may be a 'storm' still by then in our part of the world".

However, its impact on Fiji looked concerning.

It was expected to "have a decent impact on parts of Fiji", possibly bringing some wind, rain and dangerous seas around Vanuatu for a time from the east.

Data indicated Yasa could grow into a severe category 4 cyclone or stronger.

Tropical cyclone Zazu's tracking and category estimates by Fiji Met. Image / weatherwatch.co.nz

"Currently it's spinning in roughly the same spot out at sea. It's likely to remain in this similar location across Tuesday and Wednesday.

"However, from late Wednesday and across Thursday and maybe Friday, Cyclone Yasa could have a major impact on Fiji as it starts to move southwards."

Fiji Met's predicted tracking and categories of Cyclone Yasa through to Friday. Image / weatherwatch.co.nz

That could see serious flooding, slips and structural damage and could see Fijians having to evacuate to higher ground.

"This now more direct path into Fiji is of serious concern, especially with Yasa projected to be a major cyclone strength-wise by then ... potentially category 4 which can have sustained winds of almost 200km/h."