Seasonal climate outlook December 2020 - February 2021

30 November 2020

Outlook summary

Consistent with moderate La Niña conditions, December 2020 – February 2021 air pressure is forecast to be higher than normal over and to the southeast of the South Island and lower than normal to the northwest of New Zealand. This is expected to be associated with northeasterly air flow anomalies. For more information, see the background section.
Although patterns of higher than normal pressure and drier conditions are expected to start summer, moist, sub-tropical disturbances are expected to reach the country occasionally, particularly the North Island and northern South Island. These systems can cause areas of heavy rainfall and flooding.
Marine heatwave conditions formed near the northern North Island and northern Tasman Sea during November. Unusually warm seas in other coastal regions may develop into a marine heatwave over summer.
Air temperatures are very likely to be above average in all regions of the country with elevated humidity levels from time to time, particularly in the North Island. (A marine heatwave is defined as periods of extremely warm sea surface temperatures that persist for a prolonged period of time and can extend up to thousands of kilometres.)
Rainfall is about equally likely to be near normal or below normal in the east and west of the South Island and about equally likely to be near normal or above normal in all remaining regions of the country. Extended dry spells may be interspersed with the expected unsettled periods.
For the tropical cyclone season (November 2020-April 2021), NIWA’s Southwest Pacific Tropical Cyclone Outlook indicates that the risk for New Zealand is elevated. On average, one ex-tropical cyclone passes near the country each year. Significant rainfall, damaging winds, and coastal inundation can occur during these events.
Soil moisture levels are about equally likely to be near normal or below normal in the west of both islands and the east of the South Island and most likely to be near normal in the north of both islands. There is no strong guidance for the east of the North Island.
River flows are about equally likely to be near normal or below normal in the west and east of the South Island, about equally likely to be near normal or above normal in the north of the North Island, and most likely to be near normal in the remaining regions of the country.

Regional predictions for December 2020 – February 2021

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 (80% chance).
Rainfall totals are about equally likely to be above normal (40% chance) or near normal (35% chance).
Episodes of heavy rainfall are possible through the season, which could increase the chance for flooding events.
Soil moisture levels are most likely to be near normal (45% chance) while river flows are about equally likely to be near normal (40% chance) or above normal (35% chance).