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If we have computer models, why do we need forecasters?

Top Debate – Have your say — With the surge in tropical lows lately we’ve been talking more and more about the computer models we use.  Here in New Zealand you don’t often hear forecasters talk about them – but for those who have ever watched US television as a tropical storm approaches it’s all you hear about.

All weather forecasters, including MetService, rely heavily on global computer models.   The two main ones are GFS (American) and ECMWF (European).  There are many others out there too.  Forecasters heavily rely on these models to give us long range predictions – but for more shorter term we use more of our own forecasting skills, tied in with the models/data.

Someone asked me the other day, “Why bother with forecasters at all?  Especially when all of your stories about this tropical trouble at the moment have mentioned computer models”.  A fair question – so let me explain it.

For those who have relied on long range weather forecasts that other websites offer (10 days) you’ll notice something – the forecasts chop and change wildly.  That’s because they 100% using computer models and not human input.

For example, if we had simply reported what the models were predicting seven days ago for the North Island we’d be saying “sunny” one day “torrential rain” the next, then “sunny” again, then “windy but dry” etc.  This is because the models are working through the various possibilities – sometimes they can be way off the mark and so it takes a human forecaster to look at it and work out the most logical forecast.

No one is perfect – but a blend of computer guidance, a forecaster and an understanding of local topography is the best formula for accuracy.  Remove just one of those things and accuracy starts to drop significantly.

The other issue is which model to trust.  The two main models both have strengths – but they both have significant downsides too.  One model may pick 9 out of 10 storms right, then spectacularly get it wrong the 10th time.  We need to be aware of that every time we make a prediction – we never make a prediction based on one model or data update.

So yes, 10 days ago, mostly based on computer models, I first tweeted about the risk of a tropical storm cyclone north of NZ for this week. tried to keep the message as consistent as possible all last week, despite the models wildly chopping and changing each day – and both disagreeing with each other up until the weekend (making it especially tricky to work it out).

In the end all models were actually off the mark – but you can see why.  A very large low has now formed that stretches from the North Island to Fiji.  None of the models picked the size – but when you combine the three we use, they actually painted an area of low pressure similar to what we have now.  In other words – they all had the right ingredients but the cake only baked when you combined them.  And you know what?  Next time it won’t work that way – which is why every single time we need to look at more than just what the models are saying.

They are simply there to help guide us, but not tell us.

The current low is so big It has now sucked cyclone Daphne and a couple of other lows (or soon to be lows) in to the equation.  Now we’re seeing a low so large it is filled with blue sky and light winds in the middle – and the wind and rain stretches around the outside for a few thousand kilometres.  The central part of the low is roughly 1000kms wide with no rain and limited wind and cloud within it.  Not the usual sort of low we get.

But either way – we’d be lost without the models in this day and age, but without human forecasters you’d find a far more frustrating forecast to deal with, that changes wildly from update to update.

And even in the future, as the models become more accurate with improved technology, you still need human eyes to check it and add local knowledge.  After all, a plane can land on autopilot, but most of us would prefer the pilot doing it and looking out his cockpit window at the same time – the same goes with weather forecasting and the use of automated data.

– By head weather analyst Philip Duncan


deepsouthweather on 4/04/2012 2:23am

I have to agree with some of the comments above about 5-10day metservice forecasts. I just had a look at these on metservice for Invercargill & Southland rural both say rain day after day with West or SW winds (days 6-10). Yet if you look at thier own models or anyone elses it appears only a little rain or showers appear likely. As mentioned in the other comments it must be a generic computerised forecast when West or Southwest conditions prevail.
Looks like a change in weather patterns anyway next week, Weather has been majic in the deep south over the last week, A little fog in the mornings and stunning calm clear afternoons and quite mild (high teens/low 20’s), Summer just keeps rolling on!

Claudie on 4/04/2012 12:04am

Well put.
Thank you very much for your clear and honest forecasts every day. I love watching the weather and I look at lots of different sites and models. Since I discovered you through the Herald’s videos, you became my favorite, but I still go to metservice for the rain radar picture.
Thank you for your great job, especially for that last one so capricious!…

Sunshine on 3/04/2012 11:10pm

Please advise how the weather is looking in Paihia on Saturday 7th . Can we go on a cruise around the Bay, will it be choppy ride.

hitadmin on 3/04/2012 11:49pm

Bay of Plenty – like Auckland, Coromandel, East Cape and Northland – remain in a grey area this weekend, on the cusp of two lows with tropical connections.  Your best bet is to keep up to date with our wind and swell maps here:

That way you can zoom into Paihia or the upper NI and work out the best window of opportunity for you – it will continue to change daily due to the complex nmature of these lows!

All the best – we hope you manage to get out on the water!
Cheers WW

Dave on 3/04/2012 9:33pm

Thanks for the explanation Phil, I agree we can’t do without human input. The models sure have been all over the place and still are really. I guess the weather is the weather & will decide for itself.

I have seen these scenarios before where collapsed lows actually provide excellent weather. Most bizzare.

Does next week look like a more traditional SW or W wind pattern in the upper North Island?

robnz on 3/04/2012 8:39pm

I think forecasters have become too dependent on computer models and concentrate less on learning how to read weather observations and data for themselves, particularly the newer forecasters brought up with the technology and the fact that all of the models were “off the mark” illustrates why this is not a good thing.

Guest on 3/04/2012 8:23pm

Hi guys
At the end of the day, it’s great the sun is shining in auckland and that we don’t have a massive cyclone bearing down on us. I personally would like to know the risk of cyclonic activity and have it a bit off than not know about it at all, we could have had a direct hit, always 50 50 with the weather. You guys do a great service. A free service at which some of us are greatful for. Keep up the good work.

sw on 4/04/2012 12:03am

The sun is shining all day here,but at the end of the day the moon will shine:-)
Anyway nice warm wind here and unfortunately yes be back to traditional SW next week starting over easter.

Guest on 3/04/2012 8:08pm

Which is why from day 5 of an extended forecast Nelson will often get ‘Rain and Westerlies’ forecasted several days in a row. But rain from the west is very rare here. Unfortunately, these Day 5-10 forecasts are not corrected by forecasters so the average Joe always thinks there’s lots of rain around the corner. When it doesn’t happen, the forecasters are wrong.
I prefer to think of Day 5-10 forecasts as Situations rather than forecasts – so when we get these dodgy ‘Rain and Westerlies’ forecasted it’s just a westerly situation. ie: Rain in the western ranges with a few spills elsewhere.

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