Seasonal Climate Outlook: May - July 2015

1 May 2015

Overview

Warming of the sea surface across the equatorial Pacific Ocean continued in April 2015, building upon the warmer than normal waters observed in previous months. These patterns, in combination with weaker trade winds and increasing cloudiness near, and to the east of, the International Date Line are consistent with weak El Niño conditions. However, further coupling between the ocean and atmosphere is needed for El Niño to become established.

The international guidance monitored by NIWA indicates the probability of El Niño formally commencing over the next three months (May – July 2015) is close to 80% (up from 60% last month). Nonetheless, it is important to recognize that the skill of ENSO forecasts issued at this time of year during the northern hemisphere spring tends to be lower than at other times of the year (the so-called “spring predictability barrier”). NIWA’s National Climate Centre will maintain a close watch on the developing conditions.

During May - July 2015, higher than normal mean sea level pressures are expected over the New Zealand region, extending across the Tasman Sea. Lower than normal mean sea level pressures are expected well south of the country. This circulation pattern is likely to produce more south-westerly quarter wind flows than normal.

Outlook Summary

May - July 2015 temperatures are most likely (35-50% chance) to be average or above average for all regions of New Zealand, although cold snaps and frosts can be expected in some parts of the country as autumn advances into winter. Coastal waters are forecast to be in the near average to above average temperature range.

May - July 2015 rainfall is equally likely (40% chance) to be normal or below normal for the east of the South Island, and most likely (40-50% chance) to be near normal for all remaining regions of the country.

May - July 2015 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 North Island and the west of the South Island. Soil moisture levels and river flows are most likely (45-50% chance) to be in the near normal range in the east of the North Island and the north of the South Island. In the north of the North Island, soil moisture levels and river flows are about equally likely (35-40% chance) to be in the near normal or below normal range. Finally, in the east of the South Island, soil moisture levels are about equally likely (40-45% chance) to be in the below normal or near normal range, and river flows are most likely (50% chance) to be in the below normal range.

Regional predictions for the May to July 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 about equally likely (40-45% chance) to be in the above average or near average range.
Rainfall totals are about most likely (45% chance) to be in the near normal range.
Soil moisture levels and river flows are about equally likely (35-40% chance) to be in the near normal or below normal range.

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

	Temperature	Rainfall	Soil moisture	River flows
Above average	45	25	25	20
Near average	40	45	40	40
Below average	15	30	35	40