Blog, by head weather analyst Philip Duncan — You’d think a low that takes up much of the Tasman Sea would make life much easier for weather forecasters, instead, it does the opposite.

Today’s low is one of the few weather features I actually dislike (along with anticyclonic gloom).  These lows can throw the computer models out and means forecasters have to rely more on good old fashioned gut instinct. I don’t mind doing that normally, but when the data is going the opposite of your gut instinct it can create a fair bit of tension.

The data has been telling us all day that it should be raining in northern New Zealand right now.

In fact, I was chatting to a colleague today at the NZ Herald and he was proudly showing off a new weather App on his iPhone…when he clicked on Auckland it said “Right now – Showers”.  For anyone who lives in Auckland, today has been a beautiful day with warm winds.  The clouds have only recently started to thicken up.

The App clearly was computer generated data (at least you’d hope it was!).  Many of the models we use have been forecasting rain, with our latest sweep of data at 3:30pm predicting a 90% chance of rain right now in Auckland and surrounding regions.   There are a handful of light showers in Northland but nothing that backs up 90%.

The reason for the confidence?  Well, according to the computers we have a huge low near us so of course it must be raining.  But big lows are complicated.

Energy from this low is being spread over such a great distance it’s not really doing its job properly.  A bit like trying to paint an entire house with one bucket of paint.  So it becomes patchy…and some areas remain dry.  That’s the case at the moment – and will be the case for the next few days.

Forecasters use models to help predict long range systems – and often they are very useful for short sharp events.  But models and data can be utterly hopeless, in my view, when it comes to big, slow, lows like this. 

We went against the data this morning and instead said today would be mostly dry with clouds building later. 

That prediction was also made with the help of Weathermap.co.nz who has started to produce some excellent modelling…they were spot on with today’s forecast, predicting the rain band wouldn’t be huge and would mostly lie to the west of Northland by late afternoon…that is exactly what we have right now.

So don’t be surprised over the next few days if you see forecasts chopping and changing a bit, especially over the North Island.  Lows like this are a pain in the proverbial.  It’s times like this a freely accessible rain radar would be useful….one can only dream (or lobby your local MP).