NIWA National Climate Centre Seasonal Climate Outlook

NIWA National Climate Centre 30 June 2011

Seasonal Climate Outlook: July – September 2011

Mild conditions likely to continue through late winter
The recent La Niña event has dissipated, and conditions in the tropical Pacific are expected to remain near neutral over the coming season, according to the NIWA National Climate Centre.

The Centre’s latest outlook for New Zealand, for late winter (July to September), indicates that temperatures are likely to be above average in North Island regions and in Nelson-Marlborough, near or above average in the west and south of the South Island, and near average in the eastern South Island. Cold snaps typical of winter will occur from time to time through the period.

Seasonal rainfall is likely to be near normal or below normal in the north of the North Island and the west and south of the South Island, and near normal in all other regions. Soil moisture levels are likely to be near normal or below normal in western and eastern South Island, and near normal in all other regions. River flows are likely to be near normal in the west and east of the North Island, and normal or below normal in all other regions.

The outlook states that mean sea-level pressures are likely to be above normal across New Zealand, on average, with weaker westerlies over the country and more frequent than normal easterly flow over the North Island.

Overall Picture

Temperature:
For the July-August-September period as a whole, air temperatures are likely to be above average in North Island regions and in Nelson-Marlborough, near or above average in the west and south of the South Island, and near average in the eastern South Island. Sea surface temperatures near New Zealand are expected to be near normal or above normal through the outlook period.

Rainfall, soil moisture, and river flows:
The National Climate Centre says that seasonal rainfall is likely to be near normal or below normal in the north of the North Island and the west and south of the South Island, and near normal in all other regions. Soil moisture levels are likely to be near normal or below normal in western and eastern South Island, and near normal in all other regions. River flows are likely to be near normal in the west and east of the North Island, and normal or below normal in all other regions.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are likely to be above average for the time of year. Rainfall totals and river flows over July-September are all likely to be in the normal or below normal range, while soil moisture levels and 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% 25% 25%
Near average 35% 40% 45% 40%
Below average 15% 40% 30% 35%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
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 average, near average, and below average. The full probability breakdown is:

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

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 55% 20% 25% 20%
Near average 30% 50% 45% 45%
Below average 15% 30% 30% 35%

Nelson, Marlborough, Buller:
Temperatures are likely to be in the above average range. Seasonal rainfall and soil moisture levels are likely to be in the near normal range, while river flows are likely to be in the normal or 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 River flows
Above average 50% 30% 30% 25%
Near average 35% 40% 40% 40%
Below average 15% 30% 30% 35%

West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures are equally likely to be near average or above average, for the three winter months as a whole. Seasonal rainfall, soil moisture levels and river flows are all equally likely to be in the near normal or 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 River flows
Above average 40% 20% 20% 20%
Near average 40% 40% 40% 40%
Below average 20% 40% 40% 40%

Coastal Canterbury, East Otago:
Temperatures are likely to be in the near average range, while 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 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 River flows
Above average 30% 20% 20% 20%
Near average 50% 45% 40% 40%
Below average 20% 35% 40% 40%

Background

The tropical Pacific is now in a neutral state, with the previously strong La Niña event having dissipated. In the longer term through late winter and the rest of 2011, a continuation of the neutral state is considered the most likely eventuality. However, the international consensus is that the development of an El Niño by late in the year cannot be ruled out.

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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