Clean off the barbecues and dust off the outdoor furniture — next month is shaping up to be "unusually warm", and the last two months of the year are expected to have an increased number of days with temperatures rising over 25°C.
Niwa National Climate Centre forecasting principal scientist Chris Brandolino said a La Nina watch remained in effect and there was a 60%-70% chance that a La Nina event would officially develop by December.
"La Nina-like patterns will favour more high pressure south of New Zealand over the next three months, coming with a gradual reduction in the typical westerly winds.
"However, during La Nina-like conditions, blocking high pressure to the south of New Zealand may enable low pressure systems to become stalled near the country.
"This introduces the chance for heavy rainfall events, which could occur during October.
"But the chance for more high pressure near the South Island and easterly quarter air flows later in the three-month period, may see dry spells develop, especially in the west of both islands."
Coastal Otago temperatures were most likely to be above average for the three-month period, Mr Brandolino said.
"Occasional air masses from Australia will contribute to unusually warm conditions at times during October.
"More onshore winds later in the three-month period could lead to warmer nights, but cooler days."
Rainfall totals, soil moisture levels and river flows were most likely to be near normal over the next three months.
"Consistent with La Nina-like conditions, slow-moving and moisture-laden low pressure systems may occasionally approach the region from the north, bringing a chance for heavy rain."
On the West Coast, the Southern Alps and foothills, inland Otago, and Southland, temperatures were also likely to be above average.
"More northeasterly winds later in the three-month period may contribute to more days above 25°C than normal."
Rainfall totals, soil moisture levels and river flows in the area were most likely to be near normal.
Although heavy rainfall events were possible during October, more northeasterly winds may cause dry spells later in the three-month period.
"Despite near-surface soil moisture increases in the last month, soil moisture levels at depth continue to be below normal in the east of both islands."
Mr Brandolino also warned of a return of marine heatwaves around the country.
Coastal sea surface temperatures ranged from 0.54°C to 1.02°C above average during September.
At the end of the month, marine heatwave conditions were occurring near the northern and western North Island and northern South Island.
"It suggests marine heatwave conditions may intensify in the months ahead."
