NIWA Outlook: February – April 2019

Outlook Summary
• The atmospheric circulation around New Zealand is forecast to be characterised by slightly higher than normal pressure to the southwest and southeast of New Zealand and lower pressure than normal to the northeast of the country. Weak easterly-quarter air flows are favoured.
• Summer-like warmth and humidity may continue into autumn as mean temperatures through to April 2019 are most likely to be above average for all regions of New Zealand.
• February through April 2019 rainfall has about equal chances of being near normal or above normal in the north of the North Island and west of the South Island and near normal for all other regions.
• The Tasman Sea and sub-tropics north of New Zealand may become more active heading into late February and/or autumn, which may increase the risk for heavy rainfall events.
• While the ocean retreated from weak El Niño conditions during January 2019, the atmosphere responded to warmer than average waters in the central and western tropical Pacific. For New Zealand, El Nino is not expected to be a significant driver of circulation and climate over the next three months, but atmospheric patterns may occasionally be influenced by the tropics.
• The continuation of much warmer than average sea surface temperatures in the Tasman Sea and in New Zealand coastal waters may contribute extra warmth, moisture, and invigorate low pressure systems as they approach the country heading into autumn.
• 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.
February – April 2019 temperatures are forecast to be above average (50-60% chance) for all regions of New Zealand.
February – April 2019 rainfall is about equally likely to be normal (40%) or above normal (35%) in the north of the North Island and west of the South Island. Near normal rainfall is most likely (45% chance) for all remaining regions of New Zealand.
February – April 2019 soil moisture levels are forecast to be below normal (45% chance) in the north and west of the North Island, about equally likely to be below normal or near normal (35-40% chance) in the east of the North Island, and north and west of the South Island, and most likely to be near normal in the east of the South Island.
February – April 2019 river flows are about equally likely to be normal (40% chance) or below normal (35% chance) in the north of the North Island and most likely to be near normal for all other regions of New Zealand.
Regional predictions for the February – April 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 (40% chance) or above normal (35% chance).
• Soil moisture levels are most likely to be below normal (45% chance).
• River flows are about equally likely to be near normal (40% chance) or below normal (35% chance).

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