New Zealand is in for a spring of long dry spells, warmer than average temperatures, and a couple of heavy rainfall events, NIWA says.
The research institute released its seasonal climate outlook for September to November today.
It said La Niña conditions are "very likely" - 75-80% chance - to continue, so would be an important climate driver during spring.
NIWA also said a "triple-dip" La Niña - three consecutive La Niña events from 2020-2022 - is "very likely" and would be the first since 1998-2000.
It remarked this spring may feature a "battleground" of feature pressures, with occasional sub-tropical lows and strong high pressure belts. Seasonal rainfall predictions are as a result "particularly challenging".
NIWA said a sub-tropical low in the first week of September may elevate the risk for heavy rainfall in the northern North Island. Another sub-tropical low is possible in the third week of September and may bring more rain, possibly heavy, to parts of the country.
However, temperatures are "very likely" to be warmer than average for much of the country and there will be long dry spells after a short-lived cold spell in early September.
Regional spring predictions
For Northland, Auckland, Waikato and Bay of Plenty, temperatures are "very likely" - 70% chance - to be above average. More northeasterly winds and marine heatwave conditions will result in unseasonable warmth and humidity at times.
Sub-tropical low pressure systems may occasionally track towards these regions, NIWA said, such as during the first and third week of September, increasing the risk for heavy rain.
The northeasterly winds could also lead to dry spells in south-west places such as Waikato.
Looking at the central and lower North Island, Taranaki, Whanganui, Manawatū and Wellington, temperatures are also "very likely" - 70% chance - to be above average. Here, more northeasterly winds and marine heatwave conditions may cause increased sunshine and warmer temperatures.
For Gisborne, Hawke's Bay and Wairarapa, temperatures are also "very likely" - 65% chance - to be above average. More northeasterly winds may lead to higher humidity levels, more cloud cover, and warmer overnight temperatures.
NIWA said the potential for more frequent onshore winds in these regions could lead to more wet days. Sub-tropical lows may also occasionally track toward these regions, again increasing the risk for heavy rain, particularly for northern parts such as northern Hawke's Bay and Gisborne.
Looking at Tasman, Nelson, Marlborough and Buller, temperatures are also "very likely" - 60% chance - to be above average. Here, more northeasterly winds and marine heatwave conditions will result in unseasonable warmth at times.
Sub-tropical lows may also occasionally track towards these regions, such as during the third week of September, once again increasing the risk for heavy rain. Also, fewer westerly winds may lead to dry spells, particularly in western places like Tasman and Buller.
For West Coast, alps and foothills, inland Otago and Southland, temperatures are also "very likely" - 65% chance - to be above average. More northeasterly winds may cause periods of increased sunshine and warmer temperatures.
Fewer westerly winds may lead to lengthy dry spells, NIWA said. However, this may be interspersed with occasionally heavy, moisture-laden fronts.
Looking at coastal Canterbury and east Otago, temperatures are "most likely" - 55% chance - to be above average. Fewer westerly winds may lead to fewer warm days, although more frequent onshore winds may contribute to more cloud cover and warmer overnight temperatures.
NIWA said frequent anti-cyclones may contribute to longer dry spells, although these may be interspersed with occasional sub-tropical lows that transport moisture from the north.
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