Two powerful storms bookend New Zealand as we head into the end of the working week, Severe Cyclone Gita to the north and a deep Southern Ocean low to the south west. An area of weak high pressure only just holds on to the country today and tomorrow. 

Both storms to the north and south have similar deep central air pressure, in the 950hPa range, but this set up also clearly demonstrates how tropical storms behave and look differently to storms in colder parts of the globe.

With a tropical cyclone you notice the perfect circular centre and the isobars around it look like the rings of an onion, circular and closely packed in together around the centre. This tight packing of the isobars indicates ferocious winds around the eye of the cyclone – this is where they are most severe, it’s all packed in towards that centre. A bit like when you pull the plug from the bath and water sucks around the draining plug the air pressure is spinning fast around the eye of Cyclone Gita. The faster it spins the more powerful the spinning action becomes.

Tropical cyclones have a warm core which help give them this circular shape but lows that move into New Zealand or form southwards have a cold core and this helps make them larger and operate differently. The energy is spread further afield from the centre so the rough weather reaches further away – the centre of the storm can be very large and the worst winds may not be near it. 

So a tropical cyclone has all the worst winds right near the centre of it but a lot of Southern Ocean storms spread the energy further afield – often the strongest winds are when they move into large high pressure systems forming a “squash zone” which can be hundreds, sometimes thousands of kilometres stretching out from the centre.

As Cyclone Gita moves into New Zealand it will be transitioning out of being a tropical cyclone and into a cold core extra-tropical cyclone. This process changes the centre shape of the low and shifts energy in ways that can be tricky to forecast in advance. This is why we always say cyclones are hard to forecast as they move in to New Zealand – there is a huge structural change happening to the entire system. Once completed the extra-tropical cyclone can suddenly deepen – this is what happened to Ex-Cyclone Fehi recently.

The centre of the storm in the Southern Ocean this weekend won’t come too near to the South Island but it will push strong winds over the Southern Alps and Southland at times – followed by a temperature drop early next week. Some heavy rain in and around Fiordland too.

Southerlies behind it could merge with Cyclone Gita so this is creating more uncertainty about how the tropical cyclone will change into an extra-tropical cyclone next week when it arrives in New Zealand, probably around Tuesday/Wednesday.

– Friday air pressure map shows the storm south west of NZ and Severe Tropical Cyclone Gita to the north west / ECMWF.

– By head forecaster Philip Duncan, WeatherWatch.co.nz