Seasonal Climate Outlook: June - August 2015

29 May 2015

An El Niño event is now under way in the tropical Pacific. In the second half of May, the Pacific trade winds weakened substantially and the Southern Oscillation Index (SOI) dropped below -1, indicating coupling had been achieved between the warmer sea surface temperatures and the overlying atmospheric circulation.

International guidance indicates that El Niño conditions are very likely (90% chance) to continue over the next three months period (June – August 2015). The likelihood of El Niño persisting or strengthening as we reach into spring is also very high (above 80%).

During June – August 2015, above normal pressure is forecast to the west of New Zealand, while below normal pressure is expected to the northeast of the country. This circulation pattern is likely to be accompanied with anomalous westerly to southwesterly wind flows.

Sea surface temperatures for the coming three months are expected to be near average around the coasts of New Zealand.

Download the full Seasonal Climate Outlook June - August 2015 (PDF 614 KB)

Outlook Summary

June – August 2015 temperatures are about equally likely (35 to 45% chance) to be average or above average in all regions of New Zealand except in the north of the North Island, where temperatures for the next three months as a whole are most likely (45 % chance) to be in the near normal range. Note that cold snaps and frosts are to be expected in some parts of the country as we progress into winter.

June – August 2015 rainfall totals are about equally likely (35-40% chance) to be in the near normal or below normal range for all regions of New Zealand except for the west of the South Island, where near normal rainfall is the most likely outcome (50% chance).

Soil moisture and river flow are most likely (50 % chance) to be below normal in the east of the South Island and about equally likely (35-40% chance) to below normal or near normal in the north and east of the North Island. In the west of the North Island and the north of the South Island, soil moisture levels are most likely (40 % chance) to be in the near normal range, while river flows are about equally likely (35-40% chance) to be in the near normal or below normal range.

Soil moisture levels and river flows are about equally likely (35-40% chance) to be in the near normal or above normal range in the west of the South Island.

Regional predictions for the June to August season

Northland, Auckland, Waikato, Bay of Plenty

The table below shows the probabilities (or percent chances) for each of three categories: above average, near average, and below average. In the absence of any forecast guidance there would be an equal likelihood (33% chance) of the outcome being in any one of the three categories. Forecast information from local and global guidance models is used to indicate the deviation from equal chance expected for the coming three month period, with the following outcomes the most likely (but not certain) for this region:

Temperatures are most likely (45% chance) to be near average. Rainfall totals, soil moisture levels and river flows are about equally likely (35-40% chance) to be in the normal or below normal range.

Other outcomes cannot be excluded. The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	35	25	25	25
Near average	45	35	35	35
Below average	20	40	40	40