Peak of West Coast downpour likely still to come

Monday, 25 March 2019

The peak of the West Coast deluge forecast to bring up to 700 millimetres of rain by the end of Tuesday is yet to come.

Rain has been falling all day on the Coast, with the predicted heavy falls likely to become even more widespread overnight on Monday and on Tuesday morning.

Rainfall rates will be boosted by thunderstorms, developing off the coast late on Monday afternoon, with as much as 30mm to 40mm of rain falling an hour in some places on Tuesday.

Dumpling Hut, on the Arthur River southwest of Milford Sound, has already recorded 300mm of rain, making it one of the wettest places with a raingauge in the 24 hours to 4pm Monday.

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According to MetService, Milford Sound received 174.8mm of rain during the same period, with Franz Josef 89mm, Haast 41mm and Hokitika 20mm.

Raingauges in the Otago lakes catchments had measured up to 130mm in the 24 hours to 4pm Monday.

On the eastern side of the island, hot foehn nor'wester winds lifted temperatures, with Christchurch peaking about 28 degrees Celsius.

MetService senior communications meteorologist Lisa Murray told Stuff the quantity of rain expected to fall on the western slopes of the Alps by the end of Tuesday was 'significant even by West Coast standards'.

'It's the accumulated totals in this which are significant. The up to 700mm we're warning about is around the ranges for Westland south of Otira. But even around the actual coast, we're looking at 2-300mm by the end of Tuesday.'

The intensity of the rainfall meant there would be considerable rain spilling across the main divide into the headwaters of the Canterbury and Otago lakes and rivers.

'Within 20km of the main divide there could be 200-300mm of rain, so we'll see that in the rivers and streams, and that will help the farmers in terms of irrigation.

'But the rivers could rise quite swiftly and that could catch people off guard,' Murray said.

MetService had a severe weather warning out for gale or severe-gale force downslope northwesterlies across the Canterbury high country, with gusts possible to 130kmh from 3am until 11pm on Tuesday.

There were also severe weather watches posted for winds approaching severe-gale strength in exposed places for the Wairarapa, Wellington, Marlborough, the Canterbury Plains and Banks Peninsula, Otago, Southland, Fiordland and Stewart Island.

While most of the rain will fall in the South Island. MetService forecasts show a chance of some reaching the driest areas of the North Island later in the week.

MetService is expecting the front associated with the heaviest rain to move over the south of the country during Tuesday.

'The front gradually moves up the South Island on Wednesday, reaching around Nelson by early afternoon,' MetService meteorologist Andy Best said.

'As it goes onto the North Island on Thursday it starts to fizzle out and slows down and stalls.'

MetService meteorologist Stephen Glassey said a slow-moving trough was responsible for the exceptional rain.

'It's a humid northerly flow, lots of moisture coming down from the north, just continuously feeding into that, and also the rain's getting enhanced by the uplift over the mountains.'

The Otago Regional Council says it is closely monitoring river and lake levels.

Natural hazards manager Jean-Luc Payan​ said the last time a swollen Lake Wakatipu caused problems was in May 2010, when the lake level reached 311.48m (above sea level) and caused localised flooding in Queenstown through the stormwater system.

Lake Wakatipu was expected to rise to between 310.6m and 311.4m, depending on rainfall, which could lead to 'minor ponding of low-lying areas around Queenstown' once the lake reached 311.3m.

Low-lying parts of the lakeside communities of Glenorchy and Kingston could also be affected.

Lake Wanaka was expected to rise to between 276.9m and 278.2m, with surface flooding possible there at 280m, he said.

Up to 500mm of rain was possible on the main divide between 5am Monday and 5pm Tuesday, with 180mm to 280mm spilling over up to 20km east.

The heaviest rain was predicted between 6am and 2pm on Tuesday.

'We advise people camping or tramping in low-lying areas to take care and be mindful of rising lake and river levels. Boaties should also be wary of floating debris in Lakes Wakatipu, Wanaka and Hawea that may be washed in from swollen tributaries.

'The lakes are expected to be high from early tomorrow and remain high for the rest of the week,' Payan said.

Local flooding was also possible in parts of Central Otago and the lower Clutha region after heavy rain around the middle of Tuesday.