NZ in for wintry, stormy, weekend and week ahead

In a press release I sent out last night I wrote: "It probably sounds more dramatic than it will be but New Zealand is certainly about to have a wintry, stormy, weekend and week ahead". I think that clearly sums up what is on the way. It's not a massive storm - it's a massive low. The difference? One is destructive, the other is probably described by most as a 'nuisance'. A deep and large area of low pressure will form over the country today and across the weekend and last right through until the end of next week. Today we see a high chance of thunderstorms across western and northern parts of the country. Be sure to track the thunderstorms at the free and live lightning detector here. As a big fan of thunderstorms I hope today is a busy day for that detector.

Squally conditions will feature right across western and southern coastlines of both islands today with colder air moving in. Tonight will definitely be a 'light the fire and pour a glass of red wine' night for me.

By the way, if you hate thunderstorms I do have some good news for you. While I think there's a pretty good chance of thunderstorms today they will be fast moving. I don't see any evidence of thunderstorms hanging around for hours and hours. Most likely scenario one or two booms then it's carried away in the strong to gale force squally wind that's mostly likely to accompany it. I also don't think they'll be hugely widespread - more like isolated pockets.

When you have a low as large as this one it makes it hard to forecast specific potential rain, wind and snow warnings. Smaller systems are quite clear and you can usually say "Southland will get snow" and be relatively confident of that 24 hours out. However with this system every "small" change the giant low makes will be a "big" change to the localised weather patterns. So a slight shift in where it's centred could make all the difference. The low may well have several centres all spinning around each other. Each one, like a planet, seems to have a gravitational pull and eventually one centre will become dominant and either suck them all together - or 'fill in' and fall apart.

Our most long range weather maps show this large low still centred firmly over New Zealand on Tuesday next week which indicates the low may affect our weather as late as next Friday. This is why people are talking about it. The low isn't especially severe looking - at least not at the time I wrote this - but it is going to bring what most of us would describe as 'nasty weather'. Gusty winds, low temperatures, single digit highs in the south, thunderstorms and hail in the west and north. There'll be sunny and calm periods too...quite possibly across both islands... but they'll be minority not the majority.

Each low usually has a feature that forecasters need to focus on - sometimes it's heavy rain, sometimes it's gales, sometimes it's snow and sometimes...well sometimes there's simply nothing to really focus on. Unless the models change in the next day or so I think the two main features from this storm will be cold air and instability. Cold air especially for southern and eastern parts of the South Island bringing snow to low levels...maybe even sea level however current models still show the coldest air just missing the South Island and possibly remain out at sea. The instability in the atmosphere means thunderstorms, heavy showers and hail will pop up in western areas over the next day or so - and maybe again next week on the back end of this system.

However - there's literally so much ground to cover with this large low pressure system it's probably prudent to take things one day at a time. I'm focusing on today's weather at this stage and as the day progresses I'll shift to Saturday. With this particular low I feel anything more than 48 hours is pretty much guess work. The computer models, while consistent at predicting this low, aren't consistent at predicting precisely its exact shape and location.

We have a special news link set up for this low which will be updated frequently right across the weekend - you'll find it here.

Of course I love this weather...we've had a pretty quiet couple of months so this is nice to blow the old cobwebs out and also a good excuse for watching TV all wrapped up warmly. I do, however, have sympathy for those parents who have to stand on the side lines tomorrow morning at their kids sports game. Some places may see the sun - fingers crossed it's where you are.

Philip Duncan

The strom rolling in over Titirangi. Photo / WeatherWatch.co.nz