Warning of rapidly rising swells and dangerous surf conditions through weekend

Friday, 7 June 2019

MetService is warning of rapidly rising swells and dangerous surf conditions, as a cold southwest flow builds over the country.

Southwesterlies could get to severe gale strength in coastal Southland, Clutha and the Coromandel Peninsula from mid-afternoon on Friday. On Saturday, southwesterlies could get to gale strength along the east coast from Gisborne to Otago.

The winds are forecast to become strong in Auckland during Friday, and there's a possibility of heavy, thundery showers with hail around lunchtime.Gales could gust to more than 80kmh in the Hauraki Gulf and to more than 70kmh in the Waitemata and Manukau harbours during the afternoon and evening.

Wellington is expected to have some showers and strong southerlies through to mid-afternoon, while strong southwesterlies at first in Christchurch should ease and showers should clear.

**READ MORE:

* Temperatures plummet in eastern, inland South Island

* Motorists left stranded in freezing temperatures as winter batters NZ

* Caravan flips and building damage after tornado hits Far North**

Combined wave heights - swell plus the effect of wind - could get up to 8 metres offshore from the west coast, and possibly up to 6m high at the coast from about Raglan north.

Swell, which was generated by wind elsewhere, would account for about 6m of the 8m maximum, MetService meteorologist Gerrit Keyser said.

In this case, the swell was generated between Tasmania, the South Island and the Auckland Islands.

'The South Island west coast is feeling it already. Later today it's reaching the North Island,' Keyser said on Friday morning.

The swell height would be rising rapidly - from perhaps 1.5-2m in the morning to 6m by the evening.

'That's what's dangerous. You're out there. It's very windy already … You can see the wind waves, it looks choppy but the waves aren't that big. But within the span of about six hours it rises rapidly to 6m,' Keyser said.

The large swells would be coming through in pulses. 'After this initial pulse it dies down, back to about 1.5-2m. Then it comes back Saturday afternoon to evening, again mostly the west coast.'

Offshore from the east coast a pulse was expected to start overnight Saturday, then spread along the whole of the coastline by the end of Sunday.

With the swell coming from the southwest, the angle of the east coastline meant the swell would be even more offshore in the east than the west.

For most of the west coastline the swell was moving parallel to the coast, so some height was lost when it bent toward the coastline, Keyser said.

But from about Raglan north, the angle of the west coast meant a southwest swell hit it directly, and so waves at the coast would be bigger for that area.

It was hard to say how big waves would be at the coast, but it was possible the largest waves on the west coast north of Raglan could be 6m high, Keyser said. 

'That is large, that is significant. People like boaties and surfers need to be careful out there.'

People at the coast would not see the full 6m height, which was a measure from the trough to the crest of the wave. That's the height that would be measured by a buoy, but people looking at the sea would only see the upper part of the wave.

'It's (the swell) going to be 6m everywhere, but most of the swell will be offshore. It will still be very big on the coastline, but not 6m. That's because of the way the country is angled.

'But that Northland, Kaipara area does feel the direct swell … so it will be a bit higher there.'