Weather Watch: Question mark as twin storms approach

Rain is on its way, although how much is yet to be determined. Photo / Mark Mitchell

I think most of us have probably heard that two tropical systems have their sights set on New Zealand this coming week, but we still don't know exactly what is going to happen.

It's safe to say everyone - and I mean everyone - will get rain at some stage. But certainly a question mark hangs over just how much will fall, especially in places like Northland and Waikato where rain is so desperately needed.

Vania (no longer a tropical cyclone), and Zelia (now upgraded to a severe category 3 storm) are both moving towards us from different origins. The two are going to merge...or at least join forces, with Zelia coming in from the north west and being caught up in Vania's "gravitation pull" if you like. Zelia will then make a direct hit over the North Island while the remnants of Vania push into to the South Island.

That sounds impressive but I wonder if the joining of these two systems will actually spread the energy further - which tends to decrease the winds overall, but spreads the unsettled weather further.

This is going to be one of those systems that causes forecasters to be working right up until the last minute to figure out just what rain will fall and where. I don't envy the work MetService forecasters have coming up with the various severe weather warnings. This system is not text book by any means. Two tropical lows from two origins meeting up at the same time is certainly rare.

So this is what we do know now. Vania was downgraded to an ex-tropical cyclone Saturday. The system doesn't look dramatic (or even that clear) on the satellite map today but it has tapped into some very moist air. Often it's what can potentially happen with these systems that drives the numerous weather updates (and therefore news stories) as opposed to us saying stormy weather is definitely coming.

While Vania was being downgraded Zelia was doing the opposite - bumping up three categories in just 36 hours the storm is now classed as "severe" tropical cyclone. Using the American standard for measuring tropical storms this would now be the equivalent of a "strong Category 1 " hurricane.

Severe Tropical Cyclone Zelia continues to strengthen and I personally think she may even hit category 4 status for a time out over the very warm waters of the Coral Sea (super heated due to La Nina) - although we may never know for sure. Currently the cyclone has winds averaging 150km/h (up from 120km/h earlier on Sunday) and gusts are now over 200km/h. It is currently a dangerous storm but is of no threat to land today.

The tracking of both these systems takes them in to the central and upper Tasman Sea and joining forces on Tuesday.

So what don't we know? Well we still don't clearly know just how rough this system will be for New Zealand however WeatherWatch.co.nz doesn't believe it has the same aggressive hallmarks as other cyclones (think Drena, Fergus, Bola).

In saying that there is still definitely potential for damaging winds and flooding - and personally I think this is going to be a serious rain event for some regions - in particular the Nelson region may see sustained heavy rain.

Be prepared:
WeatherWatch.co.nz is not advising people to change their holiday plans, we simply ask that you keep up to date with the latest weather news and forecasts and have a back up plan should you need to safely move from your location.

We also suggest now is a good time to check on your emergency survival kit (or get one). Go to www.getthru.govt.nz for more information.

Hopefully most of us will just get a normal wet and windy couple of days, but with this very large area of unsettled weather moving in the potential for something more nasty is certainly there, which is why we're educating the public as much possible.