Seasonal climate outlook: April 2017 - June 2017

31 March 2017

Overview

The tropical Pacific is currently in an ENSO (El Niño – Southern Oscillation) neutral state overall, but with very mixed signals.

In the western and central Pacific, there is a La Niña-like set-up, with stronger than normal easterly trade winds, wetter conditions over the Indonesian region and drier conditions near the Dateline, and sea surface temperatures (SSTs) near or slightly below average. In the eastern equatorial Pacific, SSTs are above average, and indeed are close to record warmth near the South American coast, associated with heavy rains and flooding in Peru during March 2017.

In the subsurface ocean, warmer than average temperatures (+2^oC anomalies) persist in the western equatorial Pacific, but have shown no sign of progressing further eastwards. The Southern Oscillation Index (SOI) was weakly positive during March 2017.

Over the next three months, the atmospheric circulation around New Zealand is expected to favour higher pressure than normal to the west of the country, with more anticyclonic south-westerly wind flows than normal, a pattern suggestive of El Niño. However, in April 2017 as the season starts, it is expected that New Zealand will be influenced by active sub-tropical disturbances, potentially producing heavy rainfall, and frequent wind flows from the north.

Outlook summary

April – June 2017 temperatures are most likely to be above average (50% chance) in the north of the North Island, and about equally likely to be above average (45% chance) or average (40% chance) for all other regions of New Zealand. There is little chance (only 10-15%) of temperatures being below average over the late autumn season as a whole, but nevertheless frosts will occur from time to time in cooler locations later in the season.

April – June 2017 rainfall totals are about equally likely to be above normal (40% chance) or near normal (35% chance) in the north of both Islands. Conversely, rainfall is about equally likely to be below normal (40% chance) or near normal (35% chance) in the west of the South Island. For other regions, near normal rainfall is the most likely outcome (45% chance).

April – June 2017 soil moisture levels and river flows are about equally likely to be above normal (40-45% chance) or near normal (35-40% chance) in the north and west of the North Island, but below normal (45% chance) in the west of the South Island. In the east of the North Island, soil moisture levels are likely to be near normal (40% chance), with river flows equally likely to be normal or below (35% chance). In the north of the South Island, soil moisture is likely to be normal (40% chance) or above (35% chance), with river flows near normal (40% chance). Finally, in the east of the South Island, both soil moisture levels and river flows are most likely to be near normal (45% chance).

Regional predictions for the April – June 2017 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 to be above average (50% chance).
Rainfall totals are about equally likely to be in the near normal (40% chance) or above normal range (35% chance).
Soil moisture levels are about equally likely to be above normal (40% chance) or near normal (35% chance).
River flows are most likely to be above normal (45% chance).