Seasonal Climate Outlook: May – July 2011

Niwa National Climate Centre

29 April 2011

A mild early winter shaping up for many regions; La Niña bowing out
The La Niña event in the tropical Pacific continues to weaken and should be gone by the end of June, says the NIWA National Climate Centre.

The Centre’s latest outlook, for early winter (May to July), indicates that temperatures are likely to be above average across all the North Island and in Buller/Nelson/Marlborough, and average or above average over the rest of the South Island. Despite the overall pattern of above average temperatures, cold snaps typical of winter will still occur from time to time through the period.

Seasonal rainfall is likely to be normal or above normal in the north and east of the North Island, near normal in the southwest North Island and northern South Island, and normal or below normal over the rest of the South Island. Soil moisture levels are likely to be normal or above normal in the North Island and near normal in the South Island. River flows are likely to be above normal in the northern North Island, normal or below normal in the western South Island, and near normal elsewhere.

The outlook also states that mean sea level pressures are likely to be below normal to the north of New Zealand and above normal to the east and south of the country, with more frequent than normal northerly or northeasterly winds over the country.

Overall Picture

Temperature:
For the May-June-July period as a whole, temperatures are likely to be above average across all the North Island and in Buller/Nelson/Marlborough, and average or above average over the rest of the South Island. Sea surface temperatures near New Zealand are expected to be average or above average, with above normal conditions more likely around the northern North Island.

Rainfall, soil moisture, and river flows:
The National Climate Centre says that seasonal rainfall is likely to be normal or above normal in the north and east of the North Island, near normal in the southwest North Island and northern South Island, and normal or below normal over the rest of the South Island. Soil moisture levels are likely to be normal or above normal in the North Island and near normal in the South Island. River flows are likely to be above normal in the northern North Island, normal or below normal in the western South Island, and near normal elsewhere.

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. Early winter rainfall totals are equally likely to be in the normal or above normal range. Soil moisture levels and river flows are likely to be above 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% 40% 45% 45%
Near average 30% 40% 40% 40%
Below average 20% 20% 15% 15%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Temperatures are likely to be above average. Seasonal rainfall totals and river flows are likely to be near normal during May-July, while soil moisture levels are equally 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 River flows
Above average 50% 30% 40% 35%
Near average 30% 50% 40% 45%
Below average 20% 20% 20% 20%

Gisborne, Hawke’s Bay, Wairarapa:
Temperatures are likely to be above average. Seasonal rainfall totals and soil moisture levels are equally likely to be in the near normal or above normal range. River flows are likely to be near normal.
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% 40% 40% 35%
Near average 30% 40% 40% 45%
Below average 20% 20% 20% 20%

Nelson, Marlborough, Buller:
Temperatures are likely to be in the above average range. Seasonal rainfall, soil moisture levels and river flows are 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% 30% 35% 30%
Near average 30% 50% 45% 45%
Below average 20% 20% 20% 25%

West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures are equally likely to be near average or above average, for the three 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 40% 20% 20% 20%
Near average 40% 40% 45% 40%
Below average 20% 40% 35% 40%

Coastal Canterbury, East Otago:
Temperatures are equally likely to be in the near average or above average range. Seasonal rainfall is likely to be in the normal or below normal range, for the 3-month period as a whole. Soil moisture levels and river flows are 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% 20% 20% 20%
Near average 40% 40% 50% 50%
Below average 20% 40% 30% 30%

Background

The tropical Pacific is in a weakening La Niña state, which is expected to have dissipated by the middle of the three-month outlook period. In the longer-term beyond July, the majority of the international forecast models maintain neutral conditions in the Pacific, although a few models suggest the development of an El Niño event late in 2011.

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.

ENDS

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