In its latest seasonal outlook summary,the National Institute of Water and Atmospheric Research (NIWA) has noted the agency is now on a “La Nina watch” status, along with other international agencies.
La Nina (“The Girl”) conditions tend to bring wetter, subtropical nor’easterly conditions to eastern and northern parts of New Zealand.
The last significant La Nina event was over the 2017-18 summer when water temperatures resulted in a marine heatwave in the eastern Pacific. This was also accompanied by three powerful cyclones that brushed across NZ – Fehi, Gita and Hola.
A “La Nina watch” is issued when tropical winds are stronger than normal and equatorial Pacific Ocean waters become unusually cool.
NIWA anticipates these conditions to hold for several months heading through spring and early summer.
During a La Nina period nor’easterly winds and sub-tropical low-pressure systems, and even cyclones, can become more likely. They are capable of bringing intense rainfall events similar to those experienced with devastating effect in Northland last month.
However, at this stage forecasters are still taking a ‘wait and see’ approach with prediction of only slightly greater than 50% chance it will happen between August-October.
WeatherWatch meteorologist Philip Duncan also remains ambivalent about the likelihood of a La Nina forming, and he cautions farmers not to stack too much significance on early predictions at this stage.
“There is talk about La Nina but we tend to trust the scientists at Australia’s Bureau of Meteorology,” Duncan said.
“They say the chance of a La Nina forming later this year does increase, but the risks remain fairly low.
“It is not locked in yet and for now our weather remains in something of a “neutral” state, even if it is feeling a little like La Nina.”
But he said a La Nina event later in the spring period would not necessarily be a bad thing for regions like Northland, Bay of Plenty and Hawke’s Bay, where year to date rainfall figures continue to remain well below their historical year to date average.
Bay of Plenty has received 66% of its average year to date rainfall, and Hawke’s Bay only 60%.
“Soil moisture levels are generally pretty good but we do forget how fast things dry out with a couple of days of strong winds over spring time and if we get that over spring, it could be a problem for those areas that are historically low in their year to date rainfall,” he said.
“It would certainly be better than having an El Nino that brings strong drying winds from the west.”
Qualifying forecasts with a 65% accuracy over longer range into early spring, Duncan said modelling suggests a similar pattern heading into spring to what is already being experienced.
NIWA’s forecast has odds on the northern North Island being warmer than average, and rainfall about normal.
Rain-short Hawke’s Bay can only expect “normal” rainfall over the period, while temperatures will be above average for Manawatu-Wellington, with average rainfall.
South Island temperatures are forecast at being above average for Canterbury-Otago, with normal rainfall.
WeatherWatch’s modelling estimates the northern and eastern parts of the North Island can expect to be drier between August-October, while the South Island and western North Island tend to be wetter.
“But I have to caution, you need to blur your eyes a bit to these extended forecasts. We can still expect some weekly rain events. Overall, we are seeing what I would call a healthy, normal weather pattern occurring,” said Duncan.
