KEY POINTS:
It's been talked about for a couple of days now - the approaching low pressure system in the Tasman Sea. It has a two prong attack and although it's not as severe as the previous three lows (the air pressure should be much higher and that usually limits the severity of these systems) it does have a bit of a bite to it.
During Wednesday and Thursday it will slowly drift eastwards towards New Zealand from the Australian coast. It'll deepen a little and should move over the country late on Friday, passing over central New Zealand.
Of course this means western parts of the North Island could get some good rainfalls (I use the word good as in "heavy" not as in a positive thing!). Luckily areas such as Taranaki have missed most of the severe weather lately so some heavy rain on Mt Taranaki shouldn't disrupt too many people.
Heavy rain falls could move in to other western areas from Nelson up Northland, so I guess after the heavy rain lately it's something to keep a close eye on. The delayed flood can often be the most devastating. This is what we're seeing along the Waikato River. The storm surge is slowly making its way downstream and silently flooding farms and properties under blue skies.
This next low pressure system - and I'm reluctant to call it a storm at this point - is going to bring further rain to the catchments of the Waikato and Waipa rivers but it's too early to know if it's something to be seriously concerned about. Upstream the river should be showing signs of lowering so fingers crossed the additional rain will only limit the lowering - and not reverse it.
But it's probably the South Island's east coast that will see something more along the lines of a "storm". The low, once it moves across central New Zealand and out into the Pacific, is expected to deepen on Saturday driving in a very cold south easterly right up the South Island's east coast and a strong southerly into Wellington, Wairarapa and Hawkes Bay.
Snow is looking very likely right down to sea level during the weekend. The models we use at the Weather Watch Centre are showing highs of just 6 degrees in Dunedin on Friday spreading northwards over the weekend to include Canterbury and Kaikoura. For drenched Wellington this could drive in more rain so properties vulnerable to slips should definitely be watching this low.
For those in Auckland this also means another battle of the warm and cold air masses - which creates thunderstorms. There was a lot of hype about severe thunderstorms last Saturday - but we simply got it wrong.
The low had all the right ingredients but the cake didn't rise - at least not really over Auckland. Personally I think the air was too cold and dry for good thunderstorms over us last weekend.
This Friday and Saturday looks a little better but thunderstorms can be very tricky to pinpoint. Meteorologists all over the world can predict, quite accurately really, when we have the right weather ingredients for these storms to develop, but zooming in and saying "this is exactly where they'll develop" is still something that needs working on.
As someone who loves thunder and lightning I know how incredibly disappointing it can be when a thunderstorm prediction is wrong. If you're like me, you'll probably be glued to the lightning radar at the Weather Watch Centre later this week (and of course the site will update you on the chances of thunderstorms - as well as here during my Friday weather blog).
Philip Duncan
For the latest weather news keep up to date with The Radio Network's new Weather Watch Centre or the NZ Herald weather section.
Pictured above: Sheep cling onto dry land during heavy flooding at Ahuriri Station between Napier and Bay View. Photo / Paul Taylor
