WeatherWatch meteorologist and director Philip Duncan says the late stages of spring still offer the hope of some La Nina influence, but cautioned the event may not have the horsepower behind it many in dry East Coast regions wish for.
A La Nina event typically accompanies increasing low-pressure activity out of the northern tropical regions, delivering heavy and sometimes cyclonic north easterly conditions to New Zealand.
Duncan has long contended the extent of the latest La Nina event remains to be seen.
So far, Australia Bureau of Meteorology modelling has December-January as La Nina’s peak, with the best chance of bringing northern rainmakers.
“But we need to look at it as the silver lining for rainmaking, not the silver bullet,” he said.
At present, much of the East Coast from Northland to South Canterbury has experienced well below average rainfall for the year-to-date.
Duncan’s rainfall predictions for the coming two weeks do little to change this, with large high-pressure systems continuing to butt against the East Coast in coming days.
“Over the next 14 days to mid-November, the eastern regions may only experience 10-30mm of cumulative rainfall. Meantime, the West Coast can expect up to 200mm over that period,” he said.
The low-pressure systems that are bringing so much rainfall to the West Coast may, however, signal a shift in weather patterns that could play out better for eastern regions for the second part of November.
“That rain on the West Coast is coming from the rainmaker low-pressure systems we are now seeing over south-eastern Australia,” he said.
Last year Australia’s bushfires were exacerbated by high-pressure systems completely covering the country. However, this year those same high-pressure systems were sitting lower.
This enabled the rainmaking low-pressure systems to deliver good rains to much of coastal and inland Australia, and across to NZ.
“We are starting to also see an increase in lows to the north, it is not that solid line of blocking highs like it was a month or so ago. There are some breaks there, including lows to the north-east,” he said.
These could prove invaluable in delivering heavy but isolated afternoon downpours in inland North Island regions closer to the ranges from early November onwards.
“This is what we have seen in previous La Nina events. But at present it is a bit of a battle between the La Nina and the high-pressure systems,” he said.
NZ’s weather has been dominated by extremely intense and unseasonal high-pressure systems recording 1030-plus hectopascals, levels typically found earlier in the year.
“The downside is also NZ’s small size. We are so small compared to high-pressure systems that are often as big as Australia,” he said.
“And, we are only just on the edge of the sub-tropics, it means we can get a lot of close calls for rainfall, which can be very frustrating if you are in a drought.”
WeatherWatch’s IBM modelling has all the country warmer by about 1C on average over November.
“It is these wind-flows from the north that are pushing our temperatures to above normal, a lot of northerlies and sub-tropical conditions,” he said.
Meantime, soil moisture levels remained very dry throughout most of NZ.
“This does not surprise me much. Some places have had a rainfall deficit for the past two years now. Usually when you get one-metre down at this time of year the soil should be quite wet,” he said.
NIWA’s seasonal climate outlook has confirmed the arrival of La Nina conditions, and expects rainfall to be “near normal” for the north of the North Island.
NIWA expects the likelihood of sub-tropical rain events to increase over December-January.
