Seasonal Climate Outlook: June – August 2011

La Niña gone, but a mild winter still likely

The strong La Niña event which has dominated the tropical Pacific for several months has faded out, according to the NIWA National Climate Centre.

The Centre’s latest outlook, for winter (June to August), indicates that temperatures are likely to continue above average in many regions of the country, but near average in the west and south of the South Island, and average or above average for the northern North Island. Despite the overall pattern of mild conditions, cold snaps typical of winter will still occur from time to time through the period.

Seasonal rainfall and seasonal river flows are likely to be near normal in the north and east of both Islands, and normal or below normal over western regions of both Islands. Soil moisture levels are likely to be near normal in all regions.

The outlook states that mean sea-level pressures are likely to be above normal to the south and east of the country, and below normal north of New Zealand, with weaker westerlies and more frequent than normal easterly flow over the country.

Overall Picture

Temperature:
For the June-July-August period as a whole, temperatures are likely to be average or above average for the northern North Island, near average for the west and south of the South Island, and above average in all other regions. Sea surface temperatures near New Zealand are expected to be average or above average.

Rainfall, soil moisture, and river flows:
The National Climate Centre says that seasonal rainfall is likely to be near normal in the north and east of both Islands, and normal or below normal over western areas of both Islands. Seasonal river flows are likely to follow the same pattern as rainfalls, while soil moisture levels are likely to be near normal in all regions.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are equally likely to be near average or above average for the time of year. Winter rainfall totals, soil moisture levels and river flows are all likely to be in the normal range.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

/ Temperature / Rainfall / Soil moisture / River flows
Above average / 40% / 30% / 35% / 25%
Near average / 40% / 50% / 45% / 50%
Below average / 20% / 20% / 20% / 25%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Temperatures are likely to be above average. Seasonal rainfall totals and river flows are equally likely to be near normal or below normal during the winter, while soil moisture levels are likely to be near normal.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

/ Temperature / Rainfall / Soil moisture / River flows
Above average / 50% / 20% / 30% / 20%
Near average / 30% / 40% / 40% / 40%
Below average / 20% / 40% / 30% / 40%

Gisborne, Hawke’s Bay, Wairarapa:
Temperatures are likely to be above average. Seasonal rainfall totals, soil moisture levels and river flows are all likely to be in the near normal range.
Probabilities are assigned in three categories; above normal, near normal, and below normal. The full probability breakdown is:

/ Temperature / Rainfall / Soil moisture / River flows
Above average / 50% / 30% / 35% / 25%
Near average / 30% / 50% / 45% / 50%
Below average / 20% / 20% / 20% / 25%

Nelson, Marlborough, Buller:
Temperatures are likely to be in the above average range. Seasonal rainfall, soil moisture levels and river flows are all likely to be in the near normal range.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

/ Temperature / Rainfall / Soil moisture / River flows
Above average / 50% / 20% / 25% / 15%
Near average / 30% / 50% / 45% / 50%
Below average / 20% / 30% / 30% / 35%

West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures are likely to be near average, for the three winter months as a whole. Seasonal rainfall and river flows are both equally likely to be in the near normal or below normal range. Soil moisture levels are likely to be near normal.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

/ Temperature / Rainfall / Soil moisture / River flows
Above average / 30% / 20% / 20% / 20%
Near average / 50% / 40% / 45% / 40%
Below average / 20% / 40% / 35% / 40%

Coastal Canterbury, East Otago:
Temperatures are likely to be in the above average range. Seasonal rainfall is likely to be in the normal range, for the 3-month period as a whole. Soil moisture levels and river flows are also likely to be near normal.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

/ Temperature / Rainfall / Soil moisture / River flows
Above average / 50% / 30% / 40% / 35%
Near average / 30% / 50% / 45% / 50%
Below average / 20% / 20% / 15% / 15%

Background

The tropical Pacific is now in a neutral state, with the previously strong La Niña event having dissipated. In the longer-term through winter and the rest of 2011, the majority of the international forecast models maintain neutral conditions in the Pacific, although the development of an El Niño, or another La Niña, cannot be ruled out for later in 2011.

ENDS

© Copyright NIWA 2011. 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 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%
• 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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