Huge storm bringing severe gales and big seas

It looks impressive on the future weather maps  and it may well engulf the entire country for several days - a low pressure system (that will affect an area the size of Australia) looks set to develop and then anchor itself off the South Island's east coast starting this Friday.

The storm will bring big seas to parts of the country and severe gales will wrap around the outside of the low - but due to the size of the low the gales may well surround New Zealand but not affect the land like some invisble wall surrounding us.

Many people are already commenting about its predicted huge size - but there's no point in panicking just yet. As usual I've gone through my self written book of analogies and perhaps this might explain it: You know how sometimes when you're at the supermarket and you buy an extra large apple, or extra large tomato but then when you get home and eat it, it tastes really average? Well I'm not a fruit scientist but I'm pretty sure that's because the nutrients are being spread more thinly across a wider area. Well this low may well be the same - a simply massive area of very low air pressure but perhaps so big that much of the energy is spread over a larger area. (how many marks out of 10 for that fruit analogy?).

The computer models predicting this low have been consistent for a number of days and MetService is now watching it with 3 regions currently in their severe weather outlook. They predict heavy rain in the west and snow in the east.

But even they admit this low is big and that predicting smaller scale weather is tough this far out. It's big, it's slow, it's unstable - it's very unpredictable. My confidence is very high that this low will affect us for several days - yep, a whole week. My confidence is much lower when trying to work out who will get what and later today and certainly by Thursday evening we'll have a much clearer idea.

At this stage it appears as though the low will rapidly deepen off the east coast of New Zealand during the weekend - some computer models show it dropping to in to the 960hPa range - for every day folk who don't understand what that means it's a bit like saying the temperature is going to go from 25 degrees to -10. It's a big drop in air pressure - and the lower the air pressure the bigger the clouds can grow and the more unstable things get. Heavy showers, thunderstorms and hail are all likely for western parts of New Zealand. Meanwhile the low will chug away at dragging a bitterly cold sou'easterly flow onto the South Island that may last quite a few days bringing with it cold air and highs in the single digits and of course snow to low levels in Otago and Canterbury - as to how heavy that's still something we're all working out.

The weather maps, to me, show Sunday and Monday as colder days for the South Island south of about Christchurch.

With those strong winds circling New Zealand like hungry sharks around a swimmer it's possible that early next week, as the low drifts away from NZ to the east, that strong to gale south or south west winds will spread across the entire length of the country and take until at least Wednesday or Thursday next week to die out - provided another low doesn't scoot in afterwards.

So like I said, not ready to push any alarm bells just yet. The predicted storm is going to be widespread but that often reduces the severity, not always but in this case that does look like the case. Many places may just have a showery and windy weekend. Just for the record I wrote this blog on Tuesday evening and things may well be changing by Wednesday afternoon. That's why we've set up this special link here. We'll be updating this link frequently (more frequently as the weekend arrives) so add it to your favourites or bookmark it so you can jump back for faster updates.

A large swell hits Piha on Auckand's west coast. File photo / Greg Bowker