Chance of 'weather event' ahead, but dryness expected to dominate

Thursday, 2 May 2019

After some remarkable downpours already this year, the west of the South Island is the only part of the country thought to be at risk of above average rainfall in the next three months.

Some decent rain could show up for many areas late next week but overall, much of the country could be drier and warmer than normal for the next three months - although June could toss some active weather into the mix.

Both of the state meteorological agencies have just published longer term outlooks - MetService for May and Niwa for the three months from May to July.

Niwa expects factors dominating the weather during the next three months to include more westerly winds than normal, and continuing warmer than average seas around the country.

It is forecasting temperatures to be above average in the north and east of the North Island, and east of the South Island during the next three months. Elsewhere, the temperature's expected to be average to above average, with little chance it will be below average.

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Rainfall is forecast to be normal to below normal for all the North Island and the east of the South Island, normal in the top of the South Island, and normal to above normal in the west and south of the South Island.

Speaking to Stuff, Niwa meteorologist Ben Noll advised Kiwis to enjoy the sunshine in early May now, because gloomy weather was on its way.

There's a possibility of a 'weather event' sometime between May 10 and 15, as weather system in the Tasman Sea could possibly move it's way towards New Zealand, and bring rain and wind with it.

A cold front could then follow, and bring cooler temperatures to country.

Noll presented the outlook with Niwa principal scientist Chris Brandolino. In it, he said there was plenty of atmospheric moisture in the Tropics, but it was not yet clear exactly what the country could expect.

'So we're watching basically around the start of the second week of May for potential for a rain event that could kind of break this dry start to the month,' Brandolino said.

There was also a chance of some more weather activity - possible wind and rain - in June.

Despite that, dryness was the theme, Noll said.

The more frequent westerly winds expected would be blowing over a pretty warm Tasman Sea, which could invigorate low pressure systems as they came toward New Zealand. But those active periods of weather might not reach the North Island that frequently.

'As we look at the outlook as a whole, it certainly looks like a settled start overall; a couple of opportunities in May for some active weather. But maybe as we go into the month of June things do turn a little bit more active … and then July may edge back in a more settled direction. So out of the three months there could be one month that is a standout,' Noll said.

'If I had to pick one month in the coming three-month period that had quite a bit of active weather potentially around New Zealand, that month is June. So, as we go through late May into early June, there could be a chance for some more active weather,' he said.

'Certainly for farmers in the North Island that are looking for the rain, this outlook is certainly on the drier side. And ground water recharge an important thing to monitor this time of year in Canterbury, and that region could see normal or below normal rainfall,' Noll said.

There could be a slower start to the ski season. 'Overall this kind of settled flavour may not bode well, but I guess if you're looking to head outdoors, enjoy the outside, maybe some late surf, it's still kind of warm out there, not out of the question.'

Brandolino said higher air pressure than normal was expected to be a dominant feature during the next three months. 'That'll be a real key, I guess, player as to how far north the fronts get and how far north the rain gets.'

As for temperatures, Noll said the chance of temperatures being below average for the three-month period was 'very low'.

'What that might mean is that mild periods outnumber those colder periods. It doesn't mean that you won't get frosts and cold snaps through the three-month period… But it means those will be quick, fleeting perhaps, and they'll be replaced with more milder temperatures.'

MetService said the odds for rain in most northern and eastern areas were better next week, but the rest of the month looked rather quiet in those areas.

'So, overall, an unusually dry May is forecast north of Taupō. Below normal May rainfall is also predicted for the rest of the North Island (excluding Wellington and Horowhenua), as well as Nelson, Marlborough and Canterbury. Expect near normal May rainfall elsewhere.'

MetService also said sea surface temperatures in the Tasman Sea and around the country had cooled significantly during the past four weeks. Around the North Island coastline, sea temperatures were within 0.5 degrees Celsius of normal, while waters around Cook Strait were well below average. Around the South Island, seas were 1-2C above average.