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NIWA Outlook: July-September 2015

Overview

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Gisborne, Hawke’s Bay, Wairarapa
Nelson, Marlborough, Buller
West Coast, Alps and foothills, inland Otago and Southland
Coastal Canterbury, east Otago

NIWA Outlook: July-September 2015

Overview

An El Niño event is under way in the tropical Pacific. Sea surface temperatures have continued to warm across many areas of the eastern and central Tropical Pacific. Cloudiness, rainfall and convection activity near, and to the east of, the International Date Line has also intensified. These oceanic and atmospheric features are indicative of consolidating El Niño conditions.

International guidance indicates that El Niño conditions will continue through winter and spring, and into summer 2015/16.

During July – September 2015, above normal pressures are forecast to the west of New Zealand, with below normal pressures expected well south, and northeast, of the country. This mixed pressure pattern is likely to be accompanied by anomalous southerly-quarter wind flows, which is typical of El Niño conditions during the winter season in New Zealand.

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Sea surface temperatures around the coasts of New Zealand are expected to be near average for the coming three months.

Outlook Summary

July – September 2015 temperatures are most likely (45 to 50% chance) to be near average in all regions of New Zealand.

July – September 2015 rainfall totals are most likely (45-50% chance) to be in the below normal range for the north of both Islands and the west of the North Island, and about equally likely (35-45% chance) to be in in the near normal or below normal range for the east of both Islands. Near normal rainfall is most likely (45% chance) for the west of the South Island.

July – September 2015 soil moisture levels and river flow are most likely (50-60 % chance) to be below normal in the north of the North Island and the east of the South Island and about equally likely (35-40% chance) to be in the below normal or near normal range in the east of the North Island and the north of the South Island. Below normal or near normal river flows are also equally likely (35% chance) for the west of the North Island. In the west of the South Island, river flows are about equally likely (35-40% chance) to be in the near normal or above normal range. Near normal soil moisture levels are likely (40-45% chance) are forecast for the west of both Islands.

Regional predictions for the July to September 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 (50% chance) to be near average.

• Rainfall totals are most likely (50% chance) to be in the below normal range.

• Soil moisture levels and river flows are most likely (50% chance) to be in the below normal range.

Other outcomes cannot be excluded. The full probability breakdown is:

TemperatureRainfallSoil moistureRiver flows
Above average15151515
Near average50353535
Below average35505050

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are most likely (50% chance) to be near average.

• Rainfall totals are most likely (45% chance) to be in the below normal range.

• Soil moisture levels are most likely (40% chance) to be near normal.

• River flows are equally likely (35% chance) to be in the near normal or below normal range.

The full probability breakdown is:

TemperatureRainfallSoil moistureRiver flows
Above average20203030
Near average50354035
Below average30453035

Gisborne, Hawke’s Bay, Wairarapa

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are most likely (50% chance) to be near average.

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

• Soil moisture levels are about equally likely (35-40% chance) to be in the near normal or below normal range.

• River flows are equally likely (40% chance) to be in the near normal or below normal range.

The full probability breakdown is:

TemperatureRainfallSoil moistureRiver flows
Above average20252520
Near average50403540
Below average30354040

Nelson, Marlborough, Buller

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are most likely (45% chance) to be near average.

• Rainfall totals are most likely (50% chance) to be in the below normal range.

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

The full probability breakdown is:

TemperatureRainfallSoil moistureRiver flows
Above average30152525
Near average45354040
40Below average25503535

West Coast, Alps and foothills, inland Otago, Southland

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are most likely (45% chance) to be near average.

• Rainfall totals are most likely (45% chance) to be in the normal range.

• Soil moisture levels are most likely (45% chance) to be in the normal range.

• River flows are about equally likely (35-40% chance) to be in the near normal or above normal range.

The full probability breakdown is:

TemperatureRainfallSoil moistureRiver flows
Above average30253035
Near average45454540
Below average25302525

Coastal Canterbury, east Otago

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are most likely (45% chance) to be near average.

• Rainfall totals are about equally likely (40-45% chance) to be in the near normal or below normal range.

• Soil moisture levels and river flows are most likely (60% chance) to be in the below normal range.

The full probability breakdown is:

TemperatureRainfallSoil moistureRiver flows
Above average30151010
Near average45403030
Below average25456060

ENDS

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