The cold blast has already started to work its way up the lower South Island and will today head north.  The data we rely on has been doing what it usually does just before a storm – flip flopping – as computers struggle to work out exactly what is going on.

So at times like this it’s best to take a step back and just look at the entire set up.  Polar air, directly from Antarctica, is moving up New Zealand – with snow and severe gales, hail and showers, sunny spells in the mix too.

The flip flopping by the models has been fairly minimal but still very frustrating – why?  Because it’s the temperatures that are fluctuating the most.  With most storms this is irrelevant – but when snow is possible in our main centres we have to narrow in on temperatures.

Think of this way – for water to freeze the air must be zero or below.  But what if the air is hovering between -2 and +2?  Then it comes down to how much time is spent below zero.  The snow forecasts for coastal NZ are similar – we have high confidence of snow down to 100m – but down to sea level will need perfect conditions.  If the temperature scales tip down a couple degrees – we’ll have snow, heavy and settling.  If it tips up a couple degrees, it may only be a rain/snow mix.    It’s rare in weather forecasting in NZ that we have to be this specific – so focused on a 1 or 2 degree margin of error – but that’s the nature of snow forecasting to sea level here, especially when so many New Zealanders tell us they want it to snow in their town or city.

The storm is running about 24 hours behind schedule from our earlier predictions on Monday.  We now expect the bulk of the severe weather to be across Friday and now pushing into Saturday for the North Island, but clearing as the day goes on.

We’ll do our best to keep you up to date with the very latest over the next few days.

If you’re a farmer inland across Canterbury, Otago and Southland – snow amounts may be heavy across Thursday PM and Friday AM.  One sector that will be hoping we are wrong with our predictions will be the farming one – wind chills and heavy snow could lead to stock losses, but as farmers have pointed out – at least it’s not late winter in the middle of lambing.

– WeatherWatch.co.nz