Why didn't Chch get any snow?

PAUL GORMAN
Last updated 10:29 28/05/2013

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It's called a "critical sou'wester" and it is one of the biggest challenges for forecasters when it comes to Christchurch's weather.

If you are wondering why Dunedin got snow and Christchurch didn't - at least not yet - it's all down to this tricky little phenomenon.

Today's air is bitterly cold. It is so cold you might get only one such outbreak in a winter. It's definitely cold enough for snow.

But for snow, the clouds, their energy and their moisture need to combine at the same time with the coldest areas of the atmosphere.

This morning we have a critical sou'wester at play. The lines on the weather map - the isobars - are just about parallel with the South Island.

As a result, the worst weather hits Southland and Otago head-on, but then gets pushed out to sea as it moves further north, just clipping Banks Peninsula.

Canterbury is therefore sheltered from the worst by the south of the island.

Think of how a rock sticking out of a fast-running creek creates a pocket of calm water just behind it.

All it takes, though, for Christchurch to get snowy weather is for the angle of that southwesterly flow to change subtly more around to the south.

Even 10 degrees of change will make a difference, allowing the showers to attack the Canterbury coast directly.

This is where it can be a nightmare for forecasters to get it right.

That slight change in wind direction is expected as today progresses, which means we could still see snow, sleet and hail from this afternoon into tonight.

Snow could settle to about 100 metres, but by dawn tomorrow the worst of this first cold snap of winter should be over.

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- The Press

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