The puzzling science of weather forecasts

Did forecasters get it right on Saturday? Or did they cry wolf? Was the storm as bad as predicted? Or a complete fizzer?

These are the questions I've been asked by reporters, colleagues and friends over the weekend as our sub-tropical low moved in. Despite warnings from MetService, Civil Defence, and the Weather Watch Centre damage was relatively little.

So, were the warnings needed?

I'll go back a couple of years to Canterbury to answer this question. A snow storm blew up and took some forecasters by surprise. The snow storm isolated communities for weeks and left people without power for just as long.

Farmers, businesses and locals demanded to know why the storm wasn't predicted. The appropriate warnings weren't issued.

Last winter a similar rain storm to this weekend was forecast for Auckland...warnings were put out, Civil Defence in Auckland advised people to stay indoors...and...well...not a lot happened.

I had just started writing for nzherald.co.nz back then and one of my first blogs was titled "We were lucky". The storm that was predicted to hit skirted around Auckland and left the city mostly unscathed. The storm happened - but Auckland missed it. That's usually considered a good thing.

So we can either not warn...or we can warn. It's really as black and white as that.

With all the data I had at the Weather Watch Centre...and with all the data MetService meteorologists had in Wellington...everything indicated Auckland was in for a drenching...but it never happened. Even on Saturday morning watching the live rain radars I still thought the rain was coming.

Instead, it eased early in the morning and the rest of Saturday was mostly dry. All day it looked like at any second it could move back in...but it didn't.

I've been trying to think of an analogy to explain this but am struggling. The best example I can think of is to compare Auckland and Northland to a game of Tetris.

Imagine the sea on either side are blocks...and the land is the gap that we have to line the thin blocks up lengthwise to fit (in this case, that's the rainband moving in from the north). When a low comes from the subtropics the upper North Island has a thin area of land jutting up from the Waikato (Auckland and Northland). The systems from the tropics usually drop south or south-eastwards.

In the case of Saturday's rain the band itself was really only as wide as Northland. Everything was lined up...but as Saturday progressed that rain band (the Tetris block) went too far east....and not by very far...infact, less than 100kms off the Northland and Auckland coast for much of the day. It brought torrential rain to the Coromandel, eastern Waikato and western Bay of Plenty...and to be fair to Northland in the early hours...but the rest of the day was mostly dry.

Were forecasters wrong? Yes and no. Yes, we didn't expect the main rain band to be so far east...and I thought it would be much more widespread and not so narrow. But no, forecasters definitely weren't wrong in warning of this.

I'm guilty of enjoying severe weather...I love a good storm. So do journalists. It makes for a news story and weather news was the biggest news story of 2008...and actually has been each year for years.

Most people love a good storm - or at least reading about them. Terms like "that was a fizzer" usually come out of disappointment of wanting a good blast...not from disappointment at "Oh they said my house would be flooded and it wasn't!".

I am both a storm watcher and a news writer...and so I thought it was a fizzer in some sense. But to be annoyed at the forecasts is another thing. All the ingredients were there...but the cake didn't rise.

The storm was dangerous and, like last year, we were lucky. If it had been a little bit further west I have no doubt there would've been major flooding in Northland and more damage in Auckland.

Despite the populated places avoiding the worst, the storm did blast rural areas - and there was damage. Flooding spread across parts of Northland - especially the town of Kaeo.

Rain totals over Northland and Coromandel matched the warnings issued the day before. Wind blew down trees in Auckland, Te Aroha and Levin. Windows were blown out and power was cut to thousands of people from the southern North Island to Northland.

The warnings may have saved lives. Last year dozens of people were killed by the weather. Maybe if the warnings hadn't been issued more people may have gone outside...maybe we'd have a story this morning about someone swept away in a swollen stream. Thank god we don't have that story.

It's better to warn of an event than not to warn. But it's important we know more about how the weather works...everyday kiwis should watch systems form...learn their personalities (are they a rain storm, wind storm, or both). Is confidence high or low?

Does this rain warning mean my house may flood or does it simply mean it's a wet day, so just stay away from streams.

MetService and other forecasters have a huge job predicting the weather on these two islands smack bang in the roaring 40s, covered in mountainous terrain and surrounded by huge seas.

The more people understand how the weather works and how rain bands shift and change shape - especially tropical storms - the better they can understand warnings and forecasts.

But I do appreciate the fact that events were cancelled on Saturday...that people changed plans. Forecasters need to learn from these events to ensure we are even more accurate for next time.

So what do you think? Do you think forecasters went overboard? Or do you think it's vital that we're told what might happen...to be prepared for the worst? Post a comment below.

Top photo: What do Tetris and the North Island's weather patterns have in common? More than you'd think, says Philip Duncan. Photo / Supplied