Canterbury flooding: What's causing the huge amounts of heavy rain?

Sunday, 30 May 2021

A MetService Red severe weather warning for heavy rain has been maintained in Canterbury, with conditions set to get worse before they get better.

Canterbury is being hit by an extremely significant rain event. Keith Lynch spoke to Metservice meteorologist Alwyn Bakker to explain just why it’s so bad.

It’s a slow-moving and complicated low-pressure tranche of air, and it’s causing chaos.

It’s caused a state of emergency, it’s closed roads throughout Canterbury, and prompted Metservice's second ever Red Warning.

This low-pressure system moved towards New Zealand from off the east coast of Australia last week.

Low-pressure means rain. Why? When air isn't under pressure from above, it's free to move up into the atmosphere where it cools and turns into clouds. Clouds produce rain.

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The flooding is causing major disruptions across Canterbury, with road closures and evacuations.

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These kinds of weather systems develop off Australia's east coast all the time and are often flung towards New Zealand.

Often low-pressure systems cause huge amounts of rain on the West Coast. This happens when the air hits the Alps and is forced upwards. There the air condenses and falls as heavy rain.

This one is a little different. It’s complicated and large. It’s moving over the centre of the country and its clockwise direction is pushing all the cold air into the Canterbury region.

(All low-pressure systems move in a clockwise direction in the Southern Hemisphere).

OK, so imagine a clock face. Now imagine this system arriving at 10pm on that clock face, let's say around where Waikato is.

That air then swoops around New Zealand, and eventually slams into the east coast of the South Island.

These systems can be huge by the way, much larger than New Zealand itself. They are leeching up air from near the tropics, for example, and that air is full of moisture.

Why is all this water being dropped on Canterbury?

Good question. The first batch of low-pressure air that arrived on Saturday afternoon would have passed over Canterbury's plains and reached the Alps. But once it did, it would have risen up and deposited all its moisture.

This would have blocked the next batch of air and subsequently the next. The Southern Alps essentially acted as a roadblock and now all that low-pressure air has nowhere to go.

Why is it lasting so long?

The Metservice warning is extremely long. It began around 3pm on Saturday running through to Monday. This is because the low-pressure system is pretty much trapped. There's two high-pressure systems surrounding it, one near Tasmania and the other in the Pacific Ocean. High-pressure air is heavier and has boxed all that lighter air in.

If those systems weren't present, it would still have dumped a load of water, but it may have moved off by now.

Anything else?

As of 1pm on Sunday, 355mm had been picked up by the Mount Somers weather station. That's an awful lot for Canterbury, but it wouldn't have been extraordinary for the West Coast.

The West Coast’s topography has developed to deal with huge amounts of rain. Canterbury has not, which is why we're seeing so many problems.

Final thing, it hasn’t been too cold. Why?

Two reasons for this. The first is that the air sucked in from the tropics is a little warmer. But primarily as there is so much cloud, residual heat has had nowhere to go. That’s why it was about 12C in Christchurch at 3pm on Sunday. As Bakker puts it, this is the only silver lining.