Each Saturday morning head weather analyst Philip Duncan writes a column about the weather, environment, space and politics

Big tropical low(s)

What a truly weird week of weather over the North Island.  Sorry South islanders – your weather has been stunning for the most part, but it’s been the North Island that’s had just about every weather prediction you can think of, except snow and hail, across this week.

It all started two weeks ago when we first noticed an expected tropical cyclone somewhere near Vanuatu and Fiji.  Sure enough the cyclone did form as predicted and in the right location a week later.  However by then a number of other lows had popped up closer to home.  In the end a giant pool of low pressure developed, covering an area that stretched from the upper North Island all the way to Fiji.

This big pool of low pressure removed much needed resources to help Daphne stay as a cyclone – but instead it cannibalised itself.

Cyclone Daphne – which only made Cat 1 – got caught up in this large low and fell apart quite quickly.  The whole system became so big much of the centre (which was at least as wide as NZ is tall) was filled with – get this – blue sky and light winds.  Not the first thing you think of when you talk about a large tropical low.

But the system was nasty – and the potential it carried was enormous.  Only a few small tweaks would’ve seen more wind and rain warnings – so in this case, we were lucky.

Auckland

And it was a few small tweaks that saw a dramatic change in Auckland’s weather this week.  The very slight shift from an easterly wind to a south easterly saw the heavy rain that was predicted for the city instead fall just a few kilometres east over the Hauraki Gulf for most of the day.

As the low grew in size so too did this south east wind.  It was no longer a local sou’easter, it was not an entire south east flow across the North Island.  For those in Auckland, Waikato and Bay of Plenty this meant little rain and instead very warm and often sunny weather. 

This would not have happened had the winds stayed easterly.  It would’ve been cloudier, cooler and wetter.

Sou’east flows aren’t that common in the upper North Island – so when we get them the weather can be fantastic.  But it’s always a fine line between a sunny and warm sou’easter and a cloudy and cool easterly.

But while these regions enjoyed some surprisingly sunny and warm weather, Gisborne, northern Hawkes Bay, Northland and East Cape all took it in the shoulder – with the predicted tropical low dumping flooding rains and causing yet more slips and road closures for the Gisborne region.

Accuracy

I was called by a reporter this week asking about accuracy in forecasts.  This person had originally called MetService but ended up calling WeatherWatch.co.nz for a direct answer – rather than being directed to the MetService annual report.

We won’t comment on the accuracy of MetService day to day forecasts – we feel that’s your call to work out who is more accurate in your region.  I said to this reporter that WeatherWatch.co.nz has much stricter rules than some others for ticking off accuracy.  Our basic policy is this – if the majority didn’t get what was generally predicted, then our forecast was wrong. Simple as that.

For example – if we predict showers across Auckland, this would only be deemed accurate if at least 60% of the suburbs had them.   If there’s a shower in the Waitakere Ranges and another in Albany – we deem our forecast to be incorrect. 

Whenever we get it wrong, no matter how small, we always look to see why – and to improve on it for next time.  Sometimes this is as petty as the wrong icon used (I hate nothing more than seeing a cloudy icon up when it’s beautifully sunny, for example).

If we get it spectacularly wrong – as all forecasters did for Auckland this week – we’ll front up about it.  Not from public pressure – but because it’s the right thing to do.

We sent a rare press release to the media on Wednesday morning which was picked up by Stuff.co.nz.

So this week has been a learning curve for us.  While we never promised a direct hit from a storm, we did predict a lot more rain and with high confidence too.  And while I know that the rain fell very close to Auckland city that doesn’t make a wrong forecast any less wrong.  Whether that rain was 50kms offshore or 500 – it was incorrect, so we fronted up. 

But just remember – without this large tropical low hovering around the North Island we wouldn’t have had the stunning weather over the past few days in the central and western part of the North Island – so it has certainly played a huge role in the North Island’s weather this week – just not a negative role in most people’s view.

And South Islanders – we haven’t forgotten about you – you’ll be back in the weather news headlines next week as the Southern Ocean starts to wake up a bit more – and a cold, wet, change kicks in on Tuesday for the Deep South.
 

I wish you all a Happy Easter – or simply just a few relaxed days off work  

– Follow Philip Duncan on Twitter by clicking here

 

– Image / File, Simon Williams

– WeatherWatch.co.nz