Rain, hail or shine: Forecasting the weather still 'very tricky'

Saturday, 24 August 2019

When unexpected downpours, snow, or extreme winds hit, the reliability of our weather forecasts can be called into question.

Extreme weather events around New Zealand - including snow in Southland, flooding on the West Coast, and wildfires in Nelson - made checking the daily weather forecast all the more important.

But how reliable were these weather forecasts?

The process of forecasting the weather was still 'very tricky', according to MetService meteorologist Andy Best.

* A warmer, wetter more unpredictable weather future for Taranaki

Forecasters rely on multiple models and verify the data before sharing an official forecast.

* At last, a weekend in August with some ok weather - but there may be gales

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'Obviously, models have a certain degree of resolution … we can use a model, but it depends on the initial conditions, as well as how accurate the data [is that] goes and feeds into that model.'

Which is why forecasters relied on multiple models and verified the data gathered before sharing an official forecast.

Best said they mostly relied on three international models from the US, UK and Europe, and analysed that data three to four times a day.

He said they 'drill down the detail' provided by each and applied them to New Zealand's 'very complex system'.

Whether a sudden stratospheric warming is or isn't already occurring, computer models are showing a much more significant winter stratospheric warming could begin in about 10 days or a fortnight.

But 'no model is perfect,' Best told Stuff.

'Just like anything, really, can miss a mark, no matter how much thought process anyone puts into it. Sometimes little things can slip through.'

Each model's behaviour over the previous days - such as its accuracy and consistency - was monitored. Best said if it had a 'great deal of consistency' they had a lot of confidence in its predictions.

Forecasts for the next three days were 'quite easy', even five to 10 days were possible and accurate with longer term models, but looking further than that was difficult, he said.

'Looking further ahead, we're into some sort of climatological process so we have to give a trend as to what is going to happen.'

The identified trend differed from a forecast as it used decades-old data to outline expectations for the coming month.

'That's based on [the] climatological norm and looking at … up to 20 or 30 years of weather data in the past, maybe even more.'

Best said they would 'look at the averages and see what the trends [were]', as opposed to providing a detailed forecast.

Over the years, forecasting models had improved and resulted in more reliable data, Best said.

Gathering information from satellites and data around the world was easy thanks to the World Meteorological Organisation, a specialised agency of the United Nations.

'It's one of the most organised global networks that there is because it's independent of politics really, everyone contributes,' he said.

'It's amazing how it all works so smoothly.'