Christmas weather outlook unsettled - but it's a bit hard to tell

Wednesday, 28 November 2018

Climate guidance late in October didn't do a great job of picking up the drenching rains that hit the South Island later in November. With that in mind, Niwa forecasters are treading carefully when it comes to their predictions for summer.

The chances of a lengthy fine spell during the Christmas break aren't looking great, but the lack of a clear el nino weather pattern is leading to caution from forecasters.

Niwa principal scientist - forecasting Chris Brandolino said indications were for perhaps unsettled weather in the week around Christmas, but there could be some good weather during that time.

Weak el nino conditions were showing up in the tropical Pacific Ocean but the atmosphere had failed to follow suit. That meant the ocean and atmosphere were not coupled, Niwa said in its seasonal climate outlook for December to February - traditionally the months of summer.

El nino wasn't locked in, which meant weather patterns might have a bit more of a tendency to go up and down, with 'episodes of settled weather, episodes of unsettled weather', Niwa meteorologist Ben Noll said presenting the outlook on Wednesday. 'That's something that could be a theme as we go through the month of December.'

* Weather eases after widespread flooding in Dunedin and Otago

* Tornado hits Mid Canterbury, after day of wild weather**

Indications were for unsettled weather across the country in the first days in December as a couple of low pressure systems brought rain, Noll said.

But into the second week of December, indications were for above normal atmospheric pressure. 'So potentially a settled period of weather as we enter say the middle portion of December.' But the high pressure system bringing that weather might not be long lived.

'As we go into the later stages of the month, some of the sub-seasonal guidance - guidance we look at on a weekly timescale - suggest that things could get a little bit more unsettled, potentially toward the end of the month.'

But similar guidance - in the three to four week timescale - had a 'tough time' in November. It 'really didn't talk about some of those heavy rain rains that occurred', Noll said. The failure of the guidance to pick up the extent of the rains later in November was in the back of forecasters' minds as they looked ahead to December.

Forecasting was 'a bit challenging' due to the lack of a locked in el nino acting as a climate driver.

If the lack of coupling between the atmosphere and ocean in a manner typically associated with an el nino event continued, every month may have a bit of a different flavour through the summer, in contrast to last summer with its persistent warmth.

'We see variability coming forward in the next two weeks during the early part of December. That's expected to persist in the second half of the month,' Noll said.

Looking at past years with similar climate characteristics to this year 'it shows that we could be on a more settled track for January'. Some guidance suggested January could be more settled with below average rainfall for maybe large parts of the country, but his confidence in that prediction was 'relatively low', 'out of 10, maybe it's a five or a six'.

'There's some uncertainty built in - again, weak climate drivers that kind of increases the uncertainty,' he said.

The Niwa outlook shows the whole country having about equal chance for average, or above average, temperatures overall during the three months of summer, A cooler than average summer is the least likely outcome.

The upper half of the North Island is thought to have about equal chances of near normal or below normal rainfall. The west of the South Island is seen as having about equal chances of normal or above normal rainfall. The rest of the country is most likely to have rainfall around normal.