Weather forecasters challenged by recent bad weather

Tuesday, 27 November 2018

Weather forecasting is a tricky enough job without the kind of weather we've had lately.

Even the forecasters have been puzzling over the weather lately.

Snow, hail, heavy rain, a tornado and a waterspout, gales, fog, drizzle, low cloud – you name it, we've had it in the past week or so.

If what's just happened at your place hasn't been exactly what was predicted a few days ago, spare a thought for forecasters.

Snow in mid-November? The chaotic weather - shown here in the Lindis Pass - is about as difficult as it gets to forecast.

Not many jobs involve predicting the future. It's a tricky enough undertaking in New Zealand at the best of times.

**READ MORE:

Periods of heavy rain over central New Zealand on Monday, seen in this MetService rain radar image.

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MetService forecasters in their Kelburn, Wellington, forecast room discuss the weather situation.

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* Why did Christchurch miss the snow?**

But the weather situation of the past 10 days has been among the hardest our forecasters face.

Not only has it been challenging, but such severe weather needs to be predicted precisely, given life and livelihood can depend on being prepared for these events.

Extreme weather also, by its very nature, doesn't occur often, and the usually accurate computer models that provide guidance for human forecasters can often fail to pick the nuances of what is happening.

MetService meteorologist Lisa Murray said the current situation – in which a large area of low pressure spawns lots of mini-lows moving in different directions and simultaneously developing and decaying – was among the most difficult to forecast.

'If we were to pick the hardest-to-forecast, tropical cyclones would be up there, and thunderstorms in terms of the exact position where the biggest cell may be.

'If we are talking broad-scale, something like this situation is very tricky, where we get the low coming out of the Tasman [Sea] and all the smaller lows sitting within that. That can be very hard.'

The small low centres inside the main low moved around like pieces of washing in a washing machine. It was critical for forecasting to know where each low centre would end up, and its intensity, as that would determine the location of the most severe weather.

MetService used three main computer models. In such complex situations, each could suggest different weather for the same place and model runs six hours apart might be inconsistent. Forecasters had to marry up the pros and cons of each model, she said.

'If we are in an environment with more and more extreme events, that is why we will still need the human forecaster.'

Blue Skies Weather forecaster Tony Trewinnard said people now had far higher expectations about the precision of forecasts.

'Twenty years ago, the forecast for Canterbury may have been 'periods of rain, heavy in places' and that was as much as you got.

'This type of pattern is as challenging as it gets for a forecaster.'

Forecasting models were less 'robust' in the volatile conditions of recent weeks – when their guidance was most needed – than when a large anticyclone moved over the country with fine weather.

'We don't have a perfect model of the atmosphere and probably never will. In these models, these small low centres can fall through the cracks in the net.

'Relatively small changes in these lows can make a significant difference in the weather people are experiencing.

'If they move 100km, that can make the difference between South Canterbury soaking and North Canterbury being dry,' Trewinnard said.