A warm end to summer likely
A strong La Niña in the tropical Pacific is expected to persist into early autumn 2011, says the NIWA National Climate Centre. La Niña conditions are likely to ease during the autumn.
Late summer (January to March) temperatures are very likely to be above average for the time of year, for most districts of the country, apart from eastern districts of both islands where average or above average temperatures are likely.
Seasonal rainfall is likely to be near normal or below normal in the western South Island, normal or above normal in the eastern North Island, and near normal elsewhere. Soil moisture levels and river flows during January–March are likely to be near normal in all regions, according to the National Climate Centre’s latest seasonal outlook.
The seasonal outlook states that mean sea level pressures are likely to be above normal over New Zealand and to the east of the country, for the three months as a whole.
Tropical cyclone activity is likely to be near- or above-normal this season (through to May 2011). The risk of an ex-tropical cyclone passing close to New Zealand is slightly above the long-term average. On average, at least one ex-tropical cyclone passes within 500km of New Zealand in 9 out of 10 cyclone seasons.
Overall Picture
Temperature:
On average for late summer (January-March), temperatures are likely to be average or above average in eastern regions and are very likely to be above average elsewhere. Sea surface temperatures are presently near normal around New Zealand, and are expected to become warmer than normal over the coming three months.
Rainfall, soil moisture, and river flows:
Seasonal rainfall is likely to be near normal in most districts, but normal or above normal in the eastern North Island and normal or below normal in the west and south of the South Island. Late summer soil moisture levels and river flows are likely to be near normal in all regions.
Regional predictions for the next three months:
Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are very likely to be above average. Seasonal rainfalls, 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 | 60% | 30% | 20% | 20% |
| Near average | 30% | 50% | 50% | 45% |
| Below average | 10% | 20% | 30% | 35% |
Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Temperatures are very likely to be above average during January-March. Seasonal rainfall totals, soil moisture levels and river flows are all 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 | 60% | 20% | 20% | 20% |
| Near average | 30% | 50% | 50% | 50% |
| Below average | 10% | 30% | 30% | 30% |
Gisborne, Hawke’s Bay, Wairarapa:
Temperatures are equally likely to be near average or above average. Seasonal rainfall totals are equally likely to be in the near normal or above normal range, while soil moisture levels and 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 | 40% | 40% | 25% | 25% |
| Near average | 40% | 40% | 50% | 50% |
| Below average | 20% | 20% | 25% | 25% |
Nelson, Marlborough, Buller:
Temperatures are very likely to be in the above average range. Seasonal rainfall, 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 | 60% | 30% | 30% | 30% |
| Near average | 30% | 50% | 50% | 50% |
| Below average | 10% | 20% | 20% | 20% |
West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures are very likely to be above average, for the three months as a whole. Seasonal rainfall is likely to be near normal or below normal. Soil moisture levels and river flows 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 | 60% | 20% | 25% | 20% |
| Near average | 30% | 40% | 45% | 45% |
| Below average | 10% | 40% | 30% | 35% |
Coastal Canterbury, East Otago:
Temperatures are equally likely to be in the near average or above average range. Seasonal rainfalls, soil moisture levels and river flows are all 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 | 45% | 30% | 20% | 20% |
| Near average | 45% | 50% | 50% | 50% |
| Below average | 10% | 20% | 30% | 30% |
Background
The tropical Pacific is in a strong La Niña state, which is likely to persist into early autumn 2011, and then to ease.
Notes to reporters & editors
- 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.
- 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.
- 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:
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- Above average: 60%
- Near average: 30%
- Below average: 10%
We therefore concluded that above average temperatures were very likely.
- 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.
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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).