Seasonal climate outlook - March – May 2019

28 February 2019

Outlook summary

• A central Pacific El Niño is now occurring as the ocean and atmosphere have been weakly coupled for a third consecutive month. Traditionally, this occurs farther east toward South America and during the early summer season.

• Mean temperatures are forecast to be above average for all of New Zealand.

• For the autumn season, the regional circulation (flow of air) for New Zealand is likely to be influenced by the tropics to the north and the Tasman Sea to the west. Furthermore, when the tropics become the dominant driver of weather patterns, easterly quarter winds will become more likely. Otherwise, westerly quarter flow anomalies are favoured.

• The continuation of warmer than average sea surface temperatures in the Tasman Sea as well as a central based El Niño may contribute extra warmth, moisture, and increased risk for occasional heavy rainfall events this autumn season, although long dry spells are forecast to begin the season.

• For the current tropical cyclone season (November 2018 to April 2019), NIWA’s Southwest Pacific Tropical Cyclone Outlook indicates that the risk for New Zealand is near normal. On average, at least one ex-tropical cyclone passes within 550 km of New Zealand each year. Significant rainfall, damaging winds, and coastal damage can occur during these events.

March – May 2019 temperatures are forecast to be above average (50-60% chance) for all regions of New Zealand. A particularly warm start is expected for the autumn season, but cold snaps and frosts are likely to occur in colder locations as the season progresses.

March – May 2019 rainfall totals are forecast to be normal or below normal (35-40% chance) for all of the North Island. Normal or above normal rainfall (35-40% chance) is expected for the west of the South Island with near normal rainfall elsewhere (45% chance).

March – May 2019 soil moisture levels and river flows are about equally likely to be below normal or near normal (35-40% chance) for all of the North Island. For the west of South Island, soil moisture and river flows likely to be near the climatological range (i.e. near the long term normal) with near average conditions for the north and east of the island.

Regional predictions for the March – May 2019 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 (60% chance).

• Rainfall totals are about equally likely to be near normal (35% chance) or below normal (40% chance).

• Soil moisture levels and river flows are about equally likely to be below normal (40% chance) or near normal (35% chance).

The full probability breakdown is: TemperatureRainfallSoil moistureRiver flowsAbove average 60252525Near average30353535Below average10404040

Central North Island, Taranaki, Whanganui, Manawatu, Wellington