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Blog: Today’s showers & thunder can be a nightmare to forecast

By head weather analyst Philip Duncan — You know there’s something up when the data, which is normally highly reliable, says there is an 80% chance of rain yet as I read that earlier this morning I had to close the blinds due to the bright sun shining onto my work desk.

I knew yesterday the clouds would gradually move in – and that timing would be tricky… but it’s days like this that I probably dislike the most – because it’s high risk forecasting. 

When it comes to running a business, many people advise staying away from high risk ventures, but some advise of it – many business or investment junkies love the rush of taking a high risk and, hopefully, making a lot of money in the short term.

But when it comes to forecasting the weather, high risk forecasting can be a gut churning, sleep reducing, very worrying time of work.  In the same way that many of you who rely on the weather to run your business may find this weather does the same thing to you too.  Perhaps the incoming showers are what you desperately need – or desperately don’t need – and nothing seems to feel close to 100% correct.

Many farmers I work with are in-tune with the various models and data available out there – and they will be relying on gut instinct a lot today because even the computers are struggling.

So why is today so tricky?  Well it’s a fairly typical set up that we as forecasters don’t especially love.

Problem 1: The low to our west is fairly large – but fairly weak.  That means the energy has been spread too far and so big gaps of settled weather form in between the nasty bits.  A bit like using one bucket of water to either water a few potplants – or in todays case, the entire garden.  Some will get it, other big areas will not.

Problem 2: It’s slow moving.  When something is fast we have high confidence of saying, the rain will arrive this afternoon then clear tonight.  The models we use show this front stalling over parts of NZ for a day or two… that means heavy rain for some while nothing for others just down the road.  In other words, in can increase the risk of a wildly inaccurate forecast.

I remember all forecasters predicting a big thunderstorm for Auckland a years or so ago…. the weather for the city remained hot and dry and cloud free almost all day.  Meanwhile just 20 to 30kms offshore stood one of the biggest thunder clouds I’d seen in a long time.  The forecast was so wrong – but at the same time, so right.  That storm was big and nasty… but due to the small size of NZ it missed land, despite coming within viewing distance of land.  It sat there all afternoon – had it been slightly blown to the west Auckland would’ve had major flooding.

Problem 3: The band of rain is narrow.  Like the example I mention above, you can have ALL of the ingredients but with one problem – no oven to bake it in.  The oven today is NZ.  The ingredients are JUST offshore.  As it moves onshore it will create the heavy downpours and thunderstorms we are predicting.  But it’s so narrow – as is much of NZ – that there is a high risk that if it’s a little late arriving it means some who thought they were having thunder, instead have cloudy and dry.

So what’s the solution?  Well there is no silver bullet – it all comes down to 1) educating the public about how some forecasts are much riskier than others.  2) people being aware that today’s forecast may be evolving throughout the day. 3)  If any forecasts are wrong today, working out why – and seeing if next time that can be avoided.

In my world any inaccurate forecast is a blow to – so even though we admit today is a very tricky one to accurately pin-point, you can rest assured that we are watching everything that’s happening very closely and will update as often as we need to.

Take a look at the rain radar at MetService – and you’ll see how broken up the rain clouds are already – plus how narrow that incoming western rain band is in places… but where it’s falling, it’s looking very heavy – and for the most part that is without the inland heating already.

– Homepage image / File, thunder over the Hauraki Plains, Philip Brewer

– By head weather analyst Philip Duncan


Zelda Wynn on 13/02/2012 11:32pm

The admission that the weather today is so hard to interpret is appreciated. I find the confidence % very easy to understand.
Be sure to not beat yourself up over the mixed up weather that hangs over us.

Craig on 13/02/2012 11:09pm

Thanks for the background Philip, it’s great to get this sort of context behind the decisions forcasters make.

Maybe this could open up some debate about how to best get across this ‘variability/risk/uncertainty’ factor to your audience/clients?

How about adding a ‘certainty/confidence factor’ into your forecasts, using something like the UV Index gauge so that we can see e.g. if its in the red its a high confidence forecast?
It may be better than the arbitrary phrasing “Low/High/some risk of…” often used.

Link to the UV Gauge image:—2-SM.jpg


Robert on 14/02/2012 12:17am

This is a GREAT idea! If there was an election, I’d vote for this.
Hope I see someone use it someday.

Robert on 13/02/2012 9:55pm

I don’t really mind when forecasters get it wrong. I understand it’s a difficult science – especially in this country. What bugs me is the air of papal infallibility the met service seem to exude. Many times they’ve forecast bad weather for Christchurch, and it’s been a beautiful day, and I’ve tuned in to see Toni Marsh or Jim Hickey ask me if I’d managed to stay dry with all our rain! Or the exact opposite. And I have yet to see an apology (or even an explanation) on the met service website when they’ve got it sooo wrong it ain’t funny.
As a boatie you understand I need some confidence in the forecasts – and I’d actually like to see them done as %ages. E.g. 60% chance of rain – 80% chance of high winds, 20% chance of thunderstorms, etc. Then I could make up my own mind about whether it’s safe to go out.
Keep up the good work 🙂

WW Forecast Team on 13/02/2012 10:35pm

Very interesting points Robert – we’ve actually debated for some time the possibility of introducing something along those lines – to help make people better informed decisions.

One of the things today is throwing at us is the slow movement of the rain band – and the area that is approaching Auckland, for example, has narrowed to the point that it may bring only showers – but slightly north and south it’s a widespread area of frequent showers or even rain.

It’s something we’ll serious discuss internally over the next couple of months. 

Thanks for the food for thought!

– Philip Duncan

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