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The Swiss Cheese Effect – How to read NZ’s Seasonal/Monthly forecasts and get more from them

We all know the generic forecast that often gets it wrong. “A dry weekend” says the weather – then you get a downpour and your gutters overflow. Welcome to New Zealand and why forecasters age here faster than most nations on earth!

NZ’s location – half in the Roaring Forties and parked near the cyclone centre of the South Pacific – means our weather is all over the show.

Head forecaster Philip Duncan raises a great point about long term forecasting in New Zealand. “How on earth do we get it accurate for 3 months ahead when we still struggle for 3 days ahead?”. He says it really all comes down to how you INTERPRET the long range/climate data.

“When we say, for example, the entire upper North Island leans drier than normal that often can’t *possibly* be true for every nook and cranny, for every valley and mountain top. So how do we make sense of this? We grab a piece of Swiss cheese of course!”.

Swiss cheese – “the cheese with holes in it!”.
So apply the long range rainfall map – areas that leaning drier than normal, for example. Now we add a uniquely NZ recipe – a tropical airflow from near Fiji, a cooler airflow from the Southern Ocean, a large high over the Chatham Islands to our east…and perhaps we start getting those afternoon downpours. They love forming inland – Central Plateau, inland Northland, the Waikato basin, the Canterbury plains, the eastern Otago ranges, the Hawke’s Bay and Gisborne ranges. Some classic thunderstorm breeding grounds. Then, because NZ is so narrow, those downpours drift one way or the other in a straight line quite often. 

Scatter a few downpours – throw in a low from the tropics or Southern Ocean we didn’t expect – and now our long range 3 month rainfall map has holes in it!

It’s important to note that the overall trends and data are a great guide – but the localised daily weather events make for localised changes.

The local weather changes that pop in NZ are critical. It’s what we love also calling the “wild card effect” as it gives farmers, growers and others in desperate hope of more rain a silver lining to feel positive about when things look depressing, stressful and dire. There is always a silver lining in NZ – no matter how depressed you are about the weather. Please remember that if you’re in stress right now. (Also, reach out to us using the Contact Us button if you’re in stress or depression due to the weather and climate – we can help).

But – NZ will do what NZ does…we will continue to be unique, think outside the box and challenge authority. In other words – the Swiss Cheese Effect. There may be holes in our long range data – and the more you know and understand why (and the less we try to sell you a ‘perfect’ nonsense forecast) the more you’ll trust what we say and see how Mother Nature works. – wishing you a wonderful summer ahead. 


Guest on 30/12/2019 8:23pm

Brilliant article once again WW, thank you for the explanation of our unpredictable weather.
I can understand why forecasting the weather in NZ drives you nuts. But you do it well as usual.
Happy New Year and I look forward to more of your great weather articles.

Guest on 4/12/2019 7:56am

Been very, very impressed with WeatherWatch for a long time now. You’re leaving the others in your dust…

Guest on 4/12/2019 5:55am

you can by delivering a decent hot sunny summer thats long.

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