Many people are today asking why such a large storm is predicted to only bring minimal damage. 

Because the air pressure is similar to that of a major hurricane or cyclone we’ve explained below why this storm is different and why damage isn’t expected to be widespread.

Rain:
Rather than one big rain band moving in (like we see with a tropical storm) these Southern Ocean storms tend to fire up a number of aggressive but fairly short lived fronts.

The air is much colder in the Southern Ocean which mean’s it’s drier – so less moisture means less rain.  Unlike a system that comes out of the Tasman Sea or sub-tropics which can tap in to the rich, moist, warm air north of us. 

As MetService says in their Severe Weather Outlook today “Periods of heavy rain or showers are expected in the west of the South Island throughout the period from Saturday to Monday. However rainfall totals on a day by day basis are not extreme and are marginal for warning criteria”.

Wind:
The wind still appears to be the main feature with strong to gale force winds affecting most of New Zealand this weekend.  Severe Gales are also likely across a number of exposed regions for a time on Friday or Saturday.

Severe gales are also expected to affect inland Canterbury, near the quake’s epicentre, however Christchurch still remains too far east to see the strongest winds.

The energy of the low is being spread evenly and over a large area – as opposed to a tropical cyclone that tightly wraps it around one small centre creating fierce winds. 

So while the air pressure of this low is similar to that of a major hurricane, the two systems are built very differently – and therefore have a different affect on our trees, buildings and power lines.

Finally – the wind direction is mainly from the west.  This is our prevailing wind and most trees lean away from the wind, decreasing the chance of damage.

In a tropical storm the winds usually come from the east which greatly increases the risk of tree and power line damage.