Outlook Summary
• A weak, central Pacific El Niño continued during April, as patterns of enhanced rainfall persisted in the vicinity of the International
Dateline. Although El Niño is forecast to continue during the upcoming three-month period, it may weaken later in 2019.
• Air pressure is expected to be higher than normal to the west and north of New Zealand and below normal to the
south of the country, resulting in more westerly quarter winds than normal.
• Temperatures are forecast to be above average in the north and east of the North Island and east of the South
Island and about equally likely to be above average or near average in all remaining regions. Warmer than average
coastal and Tasman Sea surface temperatures may influence several spells of unseasonable warmth through the season,
particularly in eastern areas, contributed to by frequent westerly air flows.
• Rainfall is about equally likely to be below normal or near normal in the North Island and in the east of the
South Island, most likely to be near normal in the north of the South Island, and about equally likely to be above
normal or near normal in the west of the South Island. Influenced by El Niño and a progressively more active Tasman Sea,
frequent fronts are possible for western areas through the season.
• During April 2019, marine heatwave conditions continued in the Tasman Sea. Warmer than average seas may lead to
a reduction in the intensity and duration of cold spells during the coming season, however frosts are still to be
expected.
May – July 2019 temperatures are forecast to be above average in the north and east of the North Island and east of the
South Island (50-55% chance) and about equally likely to be above average (45% chance) or near average (40% chance) in
all remaining regions of New Zealand. However, cold snaps and frosts are likely to occur, particularly in colder
locations, as the season progresses.
May – July 2019 rainfall is about equally likely to be above normal (40% chance) or near normal (35% chance) in the west
of the South Island, most likely to be near normal (45% chance) in the north of the South Island, and about equally
likely to be below normal (35-40% chance) or near normal (35-40% chance) for all remaining regions of New Zealand.
May – July 2019 soil moisture levels and river flows are most likely to be below normal (50% chance) in the north of the
North Island. For the west of the South Island, soil moisture levels are about equally likely to be above normal or near
normal (35-40% chance) while river flows are most likely to be above normal (45% chance). For all other regions of New
Zealand, soil moisture levels and river flows are about equally likely to be normal or below normal (40-45% chance).
Regional predictions for the May – July 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 most likely to be above average (55% 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 most likely to be below normal (50% chance).
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average 55251510Near average35353540Below average10405050
Central North Island, Taranaki, Whanganui, Manawatu, Wellington
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are about equally likely to be above average (45% chance) or near average (40% chance)
• Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance).
• Soil moisture levels and river flows are equally likely to be below normal (40% chance) or near normal (40%
chance).
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average45252020Near average40404040Below average15354040
Gisborne, Hawke’s Bay, Wairarapa
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are most likely to be above average (55% chance).
• Rainfall totals are about equally likely to be below normal (40% chance) or near normal (35% chance).
• Soil moisture levels and river flows are about equally likely to be below normal (40-45% chance) or near normal
(40% chance).
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average55252015Near average35354040Below average10404045
Tasman, Nelson, Marlborough, Buller
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are about equally likely to be above average (45% chance) or near average (40% chance).
• Rainfall totals are most likely to be near normal (45% chance).
• Soil moisture levels and river flows are about equally likely to be below normal (40% chance) or near normal
(40-45% chance).
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average45252015Near average40454045Below average15304040
West Coast, Alps and foothills, inland Otago, Southland
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are about equally likely to be above average (45% chance) or near average (40% 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 above normal (40% chance) or near normal (35% chance) and
river flows are most likely to be above normal (45% chance).
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average45404045Near average40353535Below average15252520
Coastal Canterbury, east Otago
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are most likely to be above average (50% chance).
• Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance).
• Soil moisture levels and river flows are about equally to be below normal (45% chance) or near normal (40-45%
chance).
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average50251510Near average35404045Below average15354545
Background
During April 2019, the atmosphere once again responded to a warm pool of water in the central and western Pacific, with
anomalous atmospheric lift, rainfall, and convection centred along and just west of the International Dateline. The
NINO3.4 Index anomaly for April (to the 28th) was +0.76C, consistent with what occurred during March. The NINO1+2 Index
(in the far eastern Pacific) had an anomaly of +0.57C.
Rainfall and SST patterns remained consistent with a weak, central Pacific El Niño.
At the end of April, the Madden-Julian Oscillation (MJO) became active and was expected to constructively interfere with
the El Niño base-state during the month of May. As this occurs, the atmospheric response may become more aligned with an
east-based El Niño (e.g. westerly quarter winds becoming more common for New Zealand).
The Southern Oscillation Index (SOI) was near neutral during April (preliminary value -0.1), an increase compared to
March (-0.6). The rising branch of the Walker Circulation focused along and just west of the Dateline and a sinking
branch over the central and eastern Pacific. This supports the idea that the ongoing El Niño event is of the
central-based type, nudging the synoptic pressure patterns near Darwin and Tahiti away from what might be considered
their typical positions.
The probability for oceanic El Niño conditions, according to the consensus from international models, is 83% for the May
– July period. For August – October, the probability is 64%. For November 2019 – January 2020, El Niño remains the most
likely outcome at 58%. This continues to suggest the potential for a ‘protracted’ event (multi-year duration).
New Zealand’s coastal water temperatures for April 2019 continued above or well above average in all areas. Marine
heatwave conditions continued in the Tasman Sea.
New Zealand Coastal Sea Surface Temperatures during April 2019 North NI West NI East NI North SI West SI East SI +0.9C +0.8C +0.9C +1.2C +2.1C +1.0C
Warmer than average sea surface temperatures are forecast to persist during the coming three-month period. These warm
seas may contribute to spells of unseasonable warmth and invigorate low pressure systems as they approach New Zealand
from the west.
The Southern Annular Mode (SAM) was variable during the month of April. Continued variability is forecast during May. As
the upcoming season continues, the SAM may have a tendency to become more negative (e.g. 2nd half of May and June) as
the Tasman Sea begins to turn more active.