The Niwa Outlook for September to November shows occasional air masses from Australia will contribute to "unusually warm conditions" with dry spells in coastal Otago at times - but at the same time, some flooding may be on the cards in western parts of Otago and in Southland during September.
Adding weight to the long-term forecast is MetService’s prediction for much of Otago to reach 21°C today, while Southland will reach the high teens.
Niwa National Climate Centre forecasting principal scientist Chris Brandolino said the effect of a minor Sudden Stratospheric Warming (SSW) event in July and August was expected to contribute to lower-than-normal air pressure south of New Zealand during September, leading to unsettled and windy weather patterns, particularly in the west.
On the West Coast, the Southern Alps and foothills, inland Otago, and Southland, temperatures were about equally likely to be above average or near average, he said.
However, the second week of September may come with a chance of frosts.
Rainfall totals were equally likely to be near normal or above normal - particularly in the west - and soil moisture levels and river flows were most likely to be near normal, he said.
The higher spring rainfall was consistent with more westerly quarter winds, fronts, and low pressure systems - especially early in spring.
"Frequent fronts and lows are expected to deliver regular rainfall during September and may sometimes be heavy enough to cause flooding."
In coastal Otago, temperatures were most likely to be above average.
"Occasional air masses from Australia will contribute to unusually warm conditions at times during spring.
"Rainfall totals are about equally likely to be near normal or below normal.
"Westerly flows during September will likely contribute to dry spells."
Soil moisture levels are most likely to be near normal, while river flows are equally likely to be near normal or below normal.
"Winter snowfall, catchment-level river flows, and groundwater recharge were below normal in several regions.
"These factors could present challenging conditions for water-reliant sectors heading into spring."
Mr Brandolino said La Nina-like patterns might develop in mid to late spring, favouring the return of higher-than-normal air pressure to New Zealand.
As a result, a La Nina Watch continued and there was a 50% chance that a La Nina event would officially develop by the end of spring.
