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Niwa National Climate Centre principal scientist Chris Brandolino said ocean temperatures in the central Pacific cooled in June and were slightly below average for this time of year.
"Niwa’s criteria for a La Nina watch have been met. A La Nina watch is issued when ocean waters become unusually cool in the equatorial Pacific, trade winds are reduced, and there is an expectation for these conditions to continue for at least several months."
It meant air pressure, from July to September, would be up from normal, north of New Zealand.
"Although westerly quarter air flow anomalies were expected for the season as a whole, the potential development of La Nina can influence periodic northeasterly-quarter winds throughout the season.
"During periods of northeasterly winds, the chance for moist, sub-tropical low pressure systems (similar to those experienced in late June) is expected to increase."
In Southland and inland Otago, temperatures are equally likely to be near average or above average.
Rainfall totals, soil moisture levels and river flows are about equally likely to be near normal or below normal.
In coastal Otago, temperatures were also equally likely to be near average or above average.
Rainfall totals are most likely to be near normal, and soil moisture levels and river flows are about equally likely to be near normal or below normal.
Frosts are likely in typically colder locations.