Large depression swamps entire nation

Our big friendly giant remains to churn around the country - a large depression swamps the entire nation with three low pressure centres embedded within it. The most active one of these centres is just off the Taranaki Coast. Now as I said last week the main thing about this large depression is the fact that it's massive and not severe. The energy is being spread across three individual low pressure centres which are ultimately all wrapped up together. The most significant part of a large pool of low air pressure is the lack of wind near the centre. I've said it before, the isobars you see are like topographical maps. When they're bunched tightly together it has a big impact on the gradient (whether you're talking air pressure or terrain). But no matter how closely packed those isobars are, the very centre of a low is usually calm.

Just like the centre of a cyclone or hurricane - the strongest winds surround the eye, but aren't actually inside it. While we don't have an "eye" with a low like this, we do still have an area (or areas in this case) of light winds. Just like a topographic map, no matter how steep the mountains are around you, you'll eventually reach the flat valley floor (or flat plateau). Well that's what we have over much of New Zealand today - a big valley floor with pretty big - but not huge - mountains (winds) surrounding around us. Mountains of wind that aren't especially nasty but are many times stronger than the relatively light winds over the country today. Only southern parts of both islands are getting a bit of a blast today, but the worst of the wind remains mostly offshore.

So our main low - the one off the Taranaki coast. It's churning away and flicking out "waves" of heavy showers...which are followed by long dry spells and even some sun. This pattern will continue all day - so nothing major, just a few heavy showers and some will be laced with hail and isolated thunderstorms from New Plymouth to Auckland and some parts of Northland. The closer you are to the Tasman Sea the more likely you are to see lightning or hear the odd rumble of thunder. There'll be some pockets that get a drenching and may well cause surface flooding.

Over the next 24 hours, as that low moves in from the sea and starts to move in over land (between Taranaki and Waikato), we may see a brief increase in showers and thunderstorms along the coastline from New Plymouth to Auckland. Nothing severe though.

This low will take until at least Tuesday night or Wednesday to clear the area. Very slow moving. So for those in the north and west of the North Island expect similar weather to Sunday afternoon and night lasting through today and across Tuesday.

Low number two lies off the Wairarapa coast and isn't achieving very much at the moment. It does get hooked up with a frontal band tonight so that will bring in wetter weather to the low North Island tonight and tomorrow.

The third low is off the Otago coastline. This low, at the bottom of the overall system, is where the strong, cold, southerlies are. This low will help guide in bitterly cold weather to Invercargill and Dunedin for the next two or three days. Showers, sleet, hail and snow to low levels will continue. As will the low afternoon highs.

So we're on day 4 of our week long low and as we thought it looks like it will clear the country on Thursday - but only to be replaced by another massive low! The difference with this next one is it should remain in the Southern Ocean with its centre below New Zealand. However it is possible the centre may brush Southland this weekend so that means heavy rain for the South Island's west coast and maybe strong to gusty (but warmer) nor'westers for the east coast. The North Island may see a little rain south of Taranaki and strong winds also south of Taranaki...but it's too far out to know if this weekend will be wet or not. Confidence is low that it will be wet in Auckland this weekend, but confidence is moderate northerlies will be gusty in Wellington, possibly in Christchurch too, and potentially wet and cold again in Dunedin.

Useful links on this low

We're posting regular updates during the day and evening on this big low which you'll find here.  Also check out the long list of amazing photos sent in by everyday kiwis from the past few days - some are simply incredible.

Keep up to date with where the thunderstorms are - check out the free and live Lightning Detector here. Of particular interest the coastline from New Plymouth to Auckland.

Philip Duncan

Pictured above: Wintry Auckland weather. Photo / Brett phibbs