Weather blog: Why was Auckland spared?

A rainy day in Auckland. Photo / Doug Sherring

It had all the ingredients to be a devastating storm for Auckland but it was clear to us at WeatherWatch yesterday morning that the storm wasn't going to make a direct hit on Auckland and rather be a mostly Northland focused event. So why were there so many warnings and how come the severe gales never really blasted Auckland?

The low was a significant one. Appearing on our computer models as a moderate sub-tropical storm that had all the potential to bring severe gales and torrential rain. Whenever a deep low appears on our weather maps from the north west you know we're in for something bad...and something to warn the public about.

But, like most storms, the system jogged a little in one direction. All week both WeatherWatch.co.nz and MetService warned that even a slight change in direction could have a dramatic affect on the weather forecast...a jog to the south or west would mean an increase in wind and rain for all of northern New Zealand...a jog to the east would reduce the severity...and that's exactly what we saw yesterday afternoon...a jog to the east.

Around 3pm the low stopped tracking south east and quite clearly started moving east. It did this for several hours before slowly resuming it's south east track. You often see this with hurricane coverage in America...they have a predicted path, which they call the cone of uncertainty. The more days out you forecast the wider the path becomes. When hurricanes approach land it's these small jogs or 'wobbles' that make forecasting hard. A slight jog with hurricane Katrina could've seen a completely different outcome for New Orleans and it's something forecasts still aren't able to accurately predict. The detail of these jogs is simply too small.

It was this jog to the east that saw the most severe winds remain JUST off Auckland's coast and JUST north of the city. If you looked at all the official weather stations we had winds gusting to 130 even 160km/h from Cape Reinga to the Hauraki Gulf. The weather stations at Whangaparaoa doesn't measure gusts but it showed a sustained wind of over 70km/h last night and I would guess that exposed parts had gusts to at least 110 or 120km/h. Waiheke Island took a battering and so too did the eastern Waikato and Coromandel Peninsula. Auckland city only reached about 30km/h sustained...although that has increased a little today.

You could draw a line between all those weather stations and it was clear that severe gales only just missed Auckland. It was very close.

The warnings from MetService were bang on - and I'd tell you if they weren't. When you consider those damaging winds only missed Auckland by a few dozen kilometres distance-wise you know that warnings were needed. The rain was also heavy and lasted all night.

Northland was a different story...that line of weather stations I was telling you about saw northern and eastern half of the region engulfed in severe winds. Even this afternoon the islands near Whangarei are still recorded gusts to 130km/h and gusts to 90km/h in the city itself.

If that low had continued tracking south east instead of east for those few hours then I'd say Aucklander's would've been waking to more than a few power cuts and phone and internet outages. They would've been waking to trees down and roofs lifted.

Another very near miss for the city. Last July there was a similar storm that brought more damage to Auckland but again really pounded Northland. For Auckland to be hit hard the centre of the low needed to be closer... and that last minute jog to the east saved a lot of people a lot of money, stress and damage this morning...that, we can be very thankful for.

Philip Duncan

Photo: A raindy day in Auckland. Doug Sherring