A mild start to winter, and the end of the El Niño

NIWA NATIONAL CLIMATE CENTRE 29 April 2010

SEASONAL CLIMATE OUTLOOK: May – July 2010

A mild start to winter, and the end of the El Niño

The latest seasonal outlook from the NIWA National Climate Centre says that mean temperatures are likely to be above average in the North Island and either average or above average in the South Island, for the three months May–July. However, short-term cold snaps typical of winter will still occur.

The El Niño in the tropical Pacific is fading fast and is likely to be gone by June. Neutral conditions (no El Niño or La Niña) are expected over the winter.

Near normal seasonal rainfalls are likely in many places, but the Centre says that eastern regions of both Islands are likely to experience normal or below normal rainfalls. In the driest parts of the country, low soil moistures and stream flows are likely to continue into winter. In contrast, normal or above normal rainfall is likely the west and south of the South Island.

The centre’s latest outlook states that mean sea level pressures are likely to be lower than normal south of the country, associated with stronger than normal westerlies over New Zealand, on average for May–July.

For May–July, normal or below normal soil moisture levels and stream flows are likely over most of the country, but near normal or above normal soil moisture levels and stream flows are likely in the west and south of the South Island.

Overall Picture

Temperature:

Temperatures are likely to be above average in the North Island, and average or above average in the South Island. Sea surface temperatures are expected to remain above average in the Tasman Sea, and below average to the south and southeast of New Zealand.

Rainfall, soil moisture, and stream flows:

Rainfall is likely to be near normal over much of the country, but normal or below normal in eastern regions and normal or above normal in the west and south of the South Island. Normal or below normal soil moisture levels and stream flows are likely over most of the country, but near normal or above normal soil moisture levels and stream flows are likely in the west and south of the South Island.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Temperatures are likely to be above average. Seasonal rainfall totals are likely to be near normal, while below normal soil moisture levels are likely. Stream flows are very likely to be in the below normal range.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 50% 20% 20% 10%
Near average 30% 50% 30% 30%
Below average 20% 30% 50% 60%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Above average seasonal temperatures are likely. Rainfall totals are likely to be near normal, while stream flows and soil moisture levels are equally likely to be in the near normal or below normal range, for the three months as a whole.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 50% 25% 20% 20%
Near average 30% 50% 40% 40%
Below average 20% 25% 40% 40%

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures averaged over the three months are very likely to be in the above average category. Seasonal rainfall totals are equally likely to be near normal or below normal, but stream flows and soil moisture levels are very likely to be in the below normal range.

Probabilities are assigned in three categories; above normal, near normal, and below normal. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 60% 10% 10% 10%
Near average 30% 45% 30% 30%
Below average 10% 45% 60% 60%

Nelson, Marlborough, Buller:

Seasonal temperatures are likely to be in the average or above average range. Rainfalls are likely to be near normal overall, while stream flows and soil moisture levels are equally likely to be near normal or below normal.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 40% 25% 20% 20%
Near average 40% 50% 40% 40%
Below average 20% 25% 40% 40%

West Coast, Alps and Foothills, Inland Otago, Southland:

Temperatures are equally likely to be in the average or the above average category. Seasonal rainfall, stream flows and soil moisture levels are likely to be near normal or above normal.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 40% 40% 40% 40%
Near average 40% 40% 40% 40%
Below average 20% 20% 20% 20%

Coastal Canterbury, East Otago:

Temperatures are equally likely to be in the average or the above average category, on the whole during May–July. Seasonal rainfall totals are equally likely to be in the normal or the below normal range, while soil moisture levels and stream flows are very likely to be below normal.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 40% 20% 10% 10%
Near average 40% 40% 30% 30%
Below average 20% 40% 60% 60%

Background
The El Niño in the tropical Pacific is fading fast and is likely to be gone by June. Neutral conditions (no El Niño or La Niña) are expected over the winter.

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© Copyright NIWA 2010. All rights reserved. Acknowledgement of NIWA as the source is required.


Notes to reporters & 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 stream flows. For example, for winter (June-July-August) 2007, for all the North Island, we assigned the following probabilities for temperature:
• Above average: 60%
• Near average: 30%
• Below average: 10%
We therefore concluded that above average temperatures were very likely.

4. This three-way probability means that a random choice would only be correct 33% (or one-third) of the time. It would be like randomly throwing a dart at a board divided into 3 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% ‘hit rate’ is substantially better than guess-work, 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 U.S. (http://iri.ldeo.columbia.edu/) 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 on-line and is sent to about 3,500 recipients of NIWA’s newsletters, including many farmers. See The Climate Update: www.niwascience.co.nz/ncc

7. All outlooks are for the three months as a whole. There will inevitably be wet and dry days, 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 from the long-term mean, whereas for rainfall the “near normal” category lies approximately between 80% and 115% 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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