Seasonal Climate Outlook: April - June 2014

The equatorial Pacific Ocean continued in a neutral state (neither El Niño nor La Niña) in March 2014. International guidance indicates that ENSO-neutral conditions are the most likely outcome for the next three months (April – June 2014). El Niño development becomes increasingly likely as we progress towards the winter.

For the coming three months as a whole, mean sea level pressures are expected to be higher than normal to the south and east of the country, with lower pressures than normal to the north of New Zealand. This pressure pattern is expected to be accompanied by weak easterly flow anomalies and perturbed conditions from time to time.

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

Outlook Summary

April-June temperatures are likely (35-40% chance) to be average or below average for the north and west of both Islands, while near average temperatures are most likely (40-45%) for the eastern regions of both islands. Cold snaps and frosts can be expected in many parts of the country as autumn progresses.

April-June rainfall totals are equally likely (40% chance) to be normal or below normal in the southwest of the North Island and the north of the South Island, likely (35-40% chance) to be normal or below normal in the north of the North Island and the west of the South Island, and most likely (40%) to be near normal for the remaining regions.

Soil moisture levels and river flows are most likely (45-50 % chance) to be in the below normal range for the north of the North Island and the west of the South Islands. For the west of the North Island and the North of the South Island, soil moisture levels and river flows are likely (40-45 % chance) to be in the below normal or near normal range. Finally soil moisture levels and river flows are equally likely (40% chance) to be near or below normal for the eastern regions of both Islands.

Regional predictions for the April to June 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 likely (35-40% chance) to be in the near average or below average range.
  • Rainfall totals are likely (35-40% chance) to be in the near normal or below normal range.
  • Soil moisture levels and river flows are most likely (45%) to be in the below normal range.

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

 

Temperature

Rainfall

Soil moisture

River flows

Above average

25

25

20

20

Near average

40

40

35

35

Below average

35

35

45

45

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

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

  • Temperatures are likely (35-40% chance) to be in the near average or below average range.
  • Rainfall totals are equally likely (40% chance) to be in the normal or below normal range.
  • Soil moisture levels and river flows are likely (40-45%) to be in the below normal or near normal range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

25

20

15

15

Near average

35

40

40

40

Below average

40

40

45

45

Gisborne, Hawke’s Bay, Wairarapa

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

  • Temperatures are most likely (45% chance) to be in the near average range.
  • Rainfall totals are most likely (40% chance) to be in the near normal range.
  • Soil moisture levels and river flows are equally likely (40%) to be in the near normal or below normal range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

35

30

20

20

Near average

45

40

40

40

Below average

20

30

40

40

 View the historic rainfall and temperature for this region 

Nelson, Marlborough, Buller

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

  • Temperatures are likely (35-40% chance) to be in the near average or below average range. 
  • Rainfall totals are equally likely (40% chance) to be in the near normal or below normal range.
  • Soil moisture levels and river flows are likely (40-45% chance) to be in the below normal or near normal range. 

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

25

20

15

15

Near average

40

40

40

40

Below average

35

40

45

45

West Coast, Alps and foothills, inland Otago, Southland

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

  • Temperatures are likely (35-40% chance) to be in the near average or below average range. 
  • Rainfall totals are likely (35-40% chance) to be in the normal or below normal range. 
  • Soil moisture levels and river flows are most likely (50% chance) to be in the below normal range. 

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

25

25

15

15

Near average

40

40

35

35

Below average

35

35

50

50

Coastal Canterbury, east Otago

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

  • Temperatures are most likely (40% chance) to be in the near average range.
  • Rainfall totals are most likely (40% chance) to be in the near normal range.
  • Soil moisture levels and river flows are equaly likely (40% chance) to be in the near normal or below normal range. 

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

30

30

20

20

Near average

40

40

40

40

Below average

30

30

40

40

Background

The equatorial Pacific Ocean continued in a neutral ENSO-state (neither El Niño nor La Niña) in March 2014. The NIWA Southern Oscillation Index for March is -1.2 (estimated at 29-March). This brings the 3-month January-February-March value to 0. International guidance indicates that the tropical Pacific Ocean is likely to remain neutral over the next three months (April-June) however in the following season the probability of El Niño increases to about 50% chance. The current state of the Ocean – Atmosphere system in the Pacific also points towards a possible El Niño onset taking place in the next few months.

The monthly sea surface temperature (SST) anomaly for New Zealand was approximately +0.2°C in March, close to the anomaly recorded in February. SSTs have been warmer than normal in average around New Zealand for 15 consecutive months, reaching a peak in December 2013. Sea surface temperatures are expected to be in the near normal range for the coming three months around New Zealand.

The remnants of Tropical Cyclone Lusi affected the country around mid-March, bringing localized intense rain and gusty wind to the New Zealand region as it moved from its genesis region close to Vanuatu to the west of the North Island, before crossing the upper South Island to finish its life in the open waters of the South Pacific Ocean. For the remainder of the tropical cyclone season (through the end of April), the risk of an Ex-Tropical Cyclone (ETC) approaching New Zealand is expected to be close to normal. Based on the long-term record, ETCs come within 550km of New Zealand for 9 out of every 10 years. While typically these storm systems approach New Zealand during February-March, late season ETCs have occurred in the past and cannot be ruled out.

Finally, at the end of March 2014 soil moisture levels were well below normal across western areas of the Northland, the Waikato, inland Wanganui-Manawatu and eastern Bay of Plenty Regions. 

For comment, please contact

Dr Brett Mullan, Principal Scientist, NIWA National Climate Centre
Tel (04) 386 0508, Mobile (027) 294 1169

Mr Darren King, Environmental Scientist, NIWA National Climate Centre
Tel (09) 375 2086, Mobile (021) 399 637

Notes to reporters and editors

  1. NIWA’s outlooks indicate the likelihood of climate conditions being at, above, or below average for the season as a whole. They are not ‘weather forecasts’. It is not possible to forecast precise weather conditions three months ahead of time.
  2. The outlooks are the result of the expert judgment of NIWA’s climate scientists. They take into account observations of atmospheric and ocean conditions and output from global and local climate models. The presence of El Niño or La Niña conditions and the sea surface temperatures around New Zealand can be a useful indicator of likely overall climate conditions for a season.
  3. The outlooks state the probability for above average conditions, near average conditions, and below average conditions for rainfall, temperature, soil moisture, and river flows. For example, for winter (June–July–August) 2007, for all the North Island, we assigned the following probabilities for temperature:
    • Above average: 60 per cent
    • Near average: 30 per cent
    • Below average: 10 per cent
    We therefore concluded that above average temperatures were very likely.
  4. This three-way probability means that a random choice would be correct only 33 per cent (or one-third) of the time. It would be like randomly throwing a dart at a board divided into three equal parts, or throwing a dice with three numbers on it. An analogy with coin tossing (a two-way probability) is not correct.
  5. A 50 per cent ‘hit rate’ is substantially better than guesswork, and comparable with the skill level of the best overseas climate outlooks. See, for example, analysis of global outlooks issued by the International Research Institute for Climate and Society based in the US published in the Bulletin of the American Meteorological Society (Goddard, L., A. G. Barnston, and S. J. Mason, 2003: Evaluation of the IRI’s “net assessment” seasonal climate forecasts 1997–2001. Bull. Amer. Meteor. Soc., 84, 1761–1781).
  6. Each month, NIWA publishes an analysis of how well its outlooks perform. This is available online and is sent to about 3500 recipients of NIWA’s newsletters, including many farmers. www.niwa.co.nz/our-science/climate/publications/all/cu
  7. All outlooks are for the three months as a whole. There will inevitably be wet and dry days, and hot and cold days, within a season. The exact range in temperature and rainfall within each of the three categories varies with location and season. However, as a guide, the “near average” or middle category for the temperature predictions includes deviations up to ±0.5°C for the long-term mean, whereas for rainfall the “near normal” category lies between approximately 80 per cent and 115 per cent of the long-term mean.
  8. The seasonal climate outlooks are an output of a scientific research programme, supplemented by NIWA’s Capability Funding. NIWA does not have a government contract to produce these outlooks.

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Outlook - April - June 2014