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Wahine Weather 50 years on - April 10 1968

Weather Eye
with John Maunder

The storm that grew out of tropical cyclone Gisele caused the strongest wind ever recorded in New Zealand - 181 km/h (98 knots) gusting 269 km/h (145 knots) at Oteranga Bay west of Wellington. Although it wrought havoc to thousands of properties in Wellington (leading, eventually, to a change in building codes), it is primarily remembered for having sunk the inter-island ferry Wahine in Wellington harbour with the loss of 51 lives.

Gisele formed near the Solomon Islands on 6 April 1968, then moved south-southeast towards New Zealand. On the evening of 9 April the low centre was east of Cape Reinga and it crossed the North Island during the night to pass close to Napier at 6.00am. It was about 150 km east of Cook Strait around midday on 10 April.

Conditions in Wellington deteriorated rapidly during the morning of 10 April. As the Wahine crossed Cook Strait, the wind rose to 93 km/h (50 knots), the waves increased, and a large drop in pressure was recorded with the barometer falling 5.6 hPa in just over an hour. At this time, some difficulty was experienced in keeping the ship on course, although it was still in deep water and 45 minutes away from the harbour mouth.

Most of the members of the subsequent Court of Inquiry were critical that no consideration was given to abandoning the attempt to enter the harbour at this point. There was a strong likelihood that the waves would be much worse in the shallow waters of the harbour mouth and the steep fall in pressure clearly indicated the possibility of a further increase in the wind strength.

For reasons never properly understood, though possibly connected with the rain or blowing spray, the Wahine's radar broke down just as she was approaching the harbour mouth. A few minutes later a wave broke over her stern and she sheered off course. After rolling badly off another big wave, the Captain became disoriented in the poor visibility, and manoeuvred the ship in the harbour mouth for almost half an hour before striking Barrett Reef. The ship immediately lost power and both anchors were dropped.

The force of the wind continued to increase and drove the ship upthe harbour dragging both anchors. The extreme winds were also forcing extra water into the harbour which, soon after 1.00pm when the wind began to drop, began to flow back out, helping to turn the ship side on to the wind.

This contributed to the list that had developed due to water reaching the car deck where it was able to move freely from side to side. However, it also provided the first opportunity for lifeboats to be lowered on the side relatively sheltered from the wind and waves. The order was given to abandon ship. Not long after this was completed, the ship capsized.

There is a common misconception that the Wahine received inadequate warning about the weather conditions to be expected because of the storm. In fact, this is not true. About the time the ship left Lyttleton on Tuesday night 9 April 1968, she received a forecast for New Zealand coastal waters. The forecast for the central area, which included Cook Strait, was "strong northerlies changing to southerly after midnight tonight, southerlies gradually increasing to gale or storm from tomorrow morning".

Storm force on the Beaufort scale is a mean wind speed of 88-117 km/h (48-63 knots) with gusts 50% stronger, that is gusts 133-176 km/h (72-95 knots). The forecast also included a storm warning describing winds of over 111 km/h (60 knots) around a deep depression of tropical origin that, at that time, was about 96 km east of North Cape and moving south-southeast at 37 km/h (20 knots).

The assessment of the position and movement of the depression were only given as "fair" - meaning accurate to within 1 degree of latitude and longitude. If the forecaster had been more confident about the position and movement he would have described them as "good" - meaning accurate to within half a degree. Had the speed and direction of movement been exact and unchanging, then the area of winds greater than 111 km/h (60 knots) would not have passed over Cook Strait at the time the Wahine was due to enter Wellington harbour. In the event, however, the depression did speed up slightly and deepen a little during the night, then change to a more southerly movement about the time the ship hit Barrett Reef.

The fact that the forecast for Cook Strait included the possibility of storm force southerlies of up to 117 km/h (63 knots) gusting 176 km/h (95 knots) was, in part, a measure of the forecaster's uncertainty over the speed and direction of the depression.

At the time the Wahine got into difficulty, the wind at Wellington airport had risen to 111 km/h (60 knots) gusting 148 km/h (80 knots) - well within what was predicted. Although the wind continued to rise and peaked at 144 km/h (78 knots) gusting 187 km/h (101 knots) a couple of hours later, the ship was already in serious trouble. Paradoxically, the wind increase may actually have saved lives by blowing the ship back up the harbour: had the Wahine rolled over at Barrett Reef rather than within the harbour mouth, the death toll is likely to have been far higher.

You can see a timeline of the sinking of the Wahine on the Interactives website here: http://interactives.co.nz/2013/Apr/wahine/

The above text is from The New Zealand Weather Book by Erick Brenstrum, pp 95-96.

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