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Upcoming Tasman Sea low “one to watch” forecasters are closely monitoring regular data updates regarding a rapidly forming low pressure system this Friday and Saturday in the northern Tasman Sea and northern New Zealand area.

The low is expected to deepen quickly – in scientific speak by around 18hPA (hectopascals) in 24 hours (a ‘weather bomb’ deepens even faster, at 24hPa in 24 hours, or 1hPa per hour). The air pressure, according to some models, will drop from 1000hPa at 3pm on Friday over the Tasman Sea to 982hPa at 3pm on Saturday nearer to East Cape.

This low will track very close to, or directly over, the upper North Island this weekend – mainly north of Waikato, BOP and Hawkes Bay.

The low will be the central point where sub-tropical air flows down from near Northland into it and further south around Waikato colder southerly air will be sucked into it.

It’s the combination of these two air flows that could spark heavy rain on Saturday as the low crosses northern NZ…but it does look patchy/hit and miss.

Therefore has issued is a LOW risk for gales, a LOW to MEDIUM risk for rain heavy enough to cause localised flooding and/or slips in some of these northern regions.
It’s certainly one to watch – but hopefully it will zip by without too many issues, followed by a colder south to south west flow for most of New Zealand on Sunday and Monday.

Latest modelling today shows the bulk of the severe weather out at sea over Friday and Saturday – but a day ago the bulk of the modelling showed more rain over land. In other words – we need to see more computer model updates…a downside to a low that rapidly forms means we have to place a lot more faith in the computers, which are already quite confident about this rapidly deepening low in a couple days…a low which is yet to even form.



Andrew on 20/05/2015 3:06am

Oh no, you mentioned ‘Weather Bomb’, one of the most over-used and misunderstood descriptions in meteorology.  Once TV news gets hold of it it will be mentioned everywhere.

BTW, another over-used term is ‘mini-tornado’.  It either is a tornado or isn’t.  When a roof comes off a garage at night, four times out of five it’s probably a squall.  Doesn’t make such a dramatic headline though!

WW Forecast Team on 20/05/2015 3:27am

Yes however nnlike the others we put in this story the technical explanation of what it is – to make it clear they have specific criteria when that use is termed.  With regards to Mini Tornado – I said exactly what you said to a Herald reporter the other day – it was either a tornado or it wasn’t!  However in NZ the term “mini-tornado” is used because our Government forecaster for warnings, MetService, does not use the correct terminology when referring to tornados = they refer to them as “small tornadoes” instead of EFO or EF1.  

Philip D

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