Seasonal Climate Outlook: July - September 2016

01 July 2016

The latest remnants of the strong El Niño which peaked late in 2015 have now vanished in the tropical Pacific Ocean, and the Pacific is ENSO-neutral. Ocean temperatures along the equatorial Pacific are now near or slightly below normal, and recent developments in the ocean – atmosphere system are pointing to a possible transition to La Niña over the coming three months.

Atmospheric circulation along the equatorial Pacific has returned to near-normal, and the Southern Oscillation Index (SOI) is slightly positive. Cooler than normal sub-surface ocean waters are present in the central equatorial Pacific.

International guidance indicates that La Niña conditions are likely (57% chance) to appear over the next three month period (July – September 2016). The likelihood of La Niña conditions becoming established in the Pacific increases slightly later in the year, and reaches 65% in the first three months of 2017. However, models forecasts indicate that if La Niña indeed develops, it is likely to remain in the weak or moderate category.

For July – September 2016, weak anomalously high pressure is forecast to the north of New Zealand and expected to bring weak northerly quarter flow anomalies. Anomalously high ocean temperatures around the country mean warmer and more humid air masses are likely to affect most of New Zealand.

Seasonal Climate Outlook Video

In the video below NIWA Forecaster Chris Brandolino describes what New Zealanders can expect, in broad terms, in the way of rainfall and temperature over the coming winter season:

NIWA Seasonal Climate Outlook July - Sept 2016: Warm Theme Continues

Watch Video

Summary

July – September 2016 temperatures are very likely (60-70% chance) to be above average in all regions of the country. Nevertheless, frosts and cold snaps will occur from time to time. Sea surface temperatures are forecast to remain above normal over the next three months, especially to the west of New Zealand.

July – September 2016 seasonal rainfall totals are about equally likely to be near normal (35-40% chance) or above normal (35-40% chance) for all regions of New Zealand. Warmer sea surface temperatures around the country will remain a major driving force for the New Zealand’s climate over the coming season. Consequently, there remains an elevated risk for significant rainfall events and severe storms.

July – September 2016 soil moisture levels and river flows are about equally likely to be in the near normal (35-40% chance) or above normal (35-40% chance) range in the north and west of both Islands. In the east of the North Island, soil moisture levels are most likely to be near normal (40% chance) and river flows are equally likely (35% chance) to be near or below normal. In the east of the South Island, soil moisture levels are about equally likely to be in the below normal (40% chance) or normal range (35% chance), while below normal river flows are most likely (50% chance).

Regional predictions for the July – September 2016 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 very likely to be above average (70% chance).
Rainfall totals are about equally likely to be above normal (40% chance) or near normal (35% chance).
Soil moisture levels are about equally likely to be in the above normal range (40%) or near normal range (35% chance).
River flows are equally likely (35% chance) to be near or above normal.