17:16:41 Wednesday 20 August 2025

Weather forecasting explained

When it comes to second-guessing the weather, everyone has an opinion, and NIWA meteorologist Chris Brandolino has heard most of them.

Here he explains the magic, mystery - and science – behind weather forecasting.


NIWA meteorologist Chris Brandolino explains the mystery and science behind forecasting weather.

Chris, sometimes it seems like weather forecasters get it all wrong - just how accurate is your average weather forecast?

Typically a weather forecast is more accurate the closer are to the point in time of the forecast.

What I mean by that is the further you look into the future, the less accurate the forecast. We can be confident one, two or three days out. Up to a week and there's some unknowns.

That is because of precipitation, especially convection or rain that is driven by heat or rapidly rising air.

To forecast that you need really high resolution weather models to simulate that in the atmosphere. If you can't do that, you can't get the best results.

So what do you mean by a weather model?

A weather model is a simulation or projection of the state of the atmosphere over a period of time. It is based, like all scientific models, on a set of observations, data and calculations.

Weather models include observations from weather balloons, ground stations and satellites that are combined with equations that represent the rules of the atmosphere.

When all that is combined into something as high-powered as the NIWA supercomputer, we have an ability to trust its predictions about similar situations and therefore make our forecasts.

Is the supercomputer the first place you look when you start to prepare a weather forecast?

The first thing I want to know when preparing a forecast is what's happening right now.

Understanding ‘the now' through observational data and comparing it to our computer modelling is crucial. If the two are fairly similar then I can feel pretty confident that my forecast for tomorrow or the next day might be right.

The least challenging part of the forecast is temperature projections, while the most challenging is precipitation.

Precipitation - or rainfall, sleet, hail etc - is driven by rising air and you need really high resolution models to simulate that in the atmosphere. If you can't do that, you can't get the best results.

Also small differences have a big impact. I always say it's like a dripping tap. When you see it dripping, the drops don't look like much but over time they add up to litres and litres of water.

If you don't get ‘the now' right, it can make a huge difference in a fairly short space of time.

Are there any other challenges?

There are a couple. The Earth is 70 per cent water so collecting data is challenging which is where satellites help to bridge the gap.

Having the ability to crunch the data at a high resolution is also important.

But there's also the issue of scientists who like to deal in probabilities versus most people who just want to know if it will rain.

A scientist is more likely to ask: 'What is the probability of the temperature going higher or lower than 11C?” Someone else will just say: 'Is it going to be hot today?”

What are the differences between forecasting the weather for a small geographic area compared to the whole of New Zealand?

In smaller areas people expect finer detail and you have to know how the landscape affects places; there are microclimates across the country which makes a more general forecast more difficult.

It really depends on where you are. In Canterbury for instance, the Southern Alps are a key influence on climate, in Wellington it's Cook Strait.

But generally, because New Zealand is an island nation, the ocean tends to temper the really extreme weather other countries experience - don't get me wrong, we're not immune to it but it's not generally as bad.

What's the most common question you get about the weather and how do you answer it?

Sometimes I get asked how come I have a job where it's okay to be half right all the time but mostly I get asked if it's going to rain.

3 comments

Rastus

Posted on 06-01-2015 13:21 | By rastus

Considering he accepts that he is generally half right then give every kiwi a free 50¢ piece and we can all flip our own forecast each day - we mathematically have the same chance of getting it correct. BTW Ken Ring does his forecasting more than a year ahead and never uses the excuse that its too hard to predict into the future. What a load of useless information - a bit like all the highly paid seismologists who are so good at predicting volcanoes and earthquakes - still I suppose it keeps them off the streets.


Supercomputer

Posted on 06-01-2015 13:23 | By KenRing

Most farmers can look out the window and predict tomorrow's weather - they don't require any $40m supercomputer. Also, a herd of cows will give a good indication of coming weather conditions, from the grazing direction, to sounds, moods and milk production levels. The supercomputer missed the fire risks and the Christchurch earthquakes yesterday/today that were assocuated with yesterday's full moon. Since the supercomputer was installed it has also missed all droughts. Most weather comes from and returns to, the oceans, as Mr Brandolino correctly points out. it is well accepted that the moon controls the oceans of the world. If NIWA wishes to do successful extended or longrange forecasting beyond the next few days, they need to factor-in the moon.


Ken Ring

Posted on 07-01-2015 07:03 | By phildan89

You have been proven wrong time and time again. I've seen your January quake prediction and yesterday was not covered by any of your forecasts...the 4th and 5th were along with 11 other days this month alone! Not to mention I tried out your almanac once and it was more wrong than right over a two month period. Niwa last year issued their long term forecast for this summer in NZ (see their seasonal climate outlook for nov 14-jan 15), they have so far hit the nail on the head for TGA (temperatures at or below average, normal to slightly below normal rain and normal river/soil water). Similarly they specifically say that soil water in Canterbury is likely to be below normal (a drought!!) Well done NIWA, keep up the good work


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