Warm Start To Early Summer Likely, Drier In South

NATIONAL CLIMATE CENTRE DRAFT FOR 31 October 2007
SEASONAL CLIMATE OUTLOOK: November 2007 – January 2008

WARMER START TO EARLY SUMMER LIKELY; DRIER IN THE SOUTH

Temperatures in the period November – January are likely to be at least average for the time of year according to NIWA’s National Climate Centre. According to the centre’s seasonal climate outlook there is only a 10-20% chance of cool (below average) conditions over the country. In all regions except the far north and the far south, there is a 50% chance of warm (above average) temperatures with a 40% chance of average temperatures.

The centre says La Niña conditions are well established in the central and eastern Pacific. As a result, normal or above normal rainfall is expected in the north of the country, and normal or below normal rainfall in much of the South Island.

The centre is expecting more anticyclones over southern New Zealand and these are likely to bring more settled weather over the South Island. The prevailing westerly winds, which can be particularly strong during the late spring, are likely to be weaker and less frequent than normal for the three months as a whole, with easterly winds at times.

The Centre says there is about an 90% chance of La Niña conditions persisting in the tropical Pacific through to the end of summer 2007/08. For the coming tropical cyclone season (November 2007 – May 2008) there is a slightly reduced risk of an ex-tropical cycle passing within 500 km of the country. Should an ex-tropical cyclone approach New Zealand, the regions most at risk are the north and northeast of the North Island.

[Reporters please note: Probabilities are assigned in THREE categories; above average, average, and below average. See end for more explanation.]

Overall Picture

Temperature:
Air temperatures are likely to be average or above average in all regions. Sea surface temperatures are expected to be above average around the North Island, and near average around the South Island.

Rainfall, soil moisture, and stream flows:
Rainfall, soil moisture and stream flows are likely to be normal or above normal in the north of the North Island. Normal or below normal rainfall, soil moisture and stream flows are likely in the west, south and east of the South Island. Normal rainfall soil moisture and stream flows are likely elsewhere.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Average or above average temperatures are likely with normal or above normal rainfall, soil moisture and stream flows.

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Above average temperatures are likely. Normal rainfall, soil moisture and stream flows are likely.



Gisborne, Hawkes Bay, Wairarapa:
Above average seasonal temperatures are likely. Normal rainfall soil moisture levels and stream flows are likely overall.

Nelson, Marlborough, Buller:
Above average temperatures are likely, with normal rainfall. Soil moisture levels and stream flows are likely to be normal.

West Coast, Alps and Foothills, Inland Otago, Southland:
Average or above average temperatures are likely. Normal or below normal rainfall, stream flows and soil moisture levels are likely.

Coastal Canterbury, East Otago:
Average or above average temperatures are likely. Normal or below normal rainfall is likely, with normal or below normal soil moisture and stream flows.

Background

Climate and Oceans:
Over the November 2007 to January 2008 period, mean sea level pressures are expected to be higher to the south of New Zealand, with weaker than normal westerly winds across New Zealand.

La Niña conditions have become well established in the central and eastern Pacific over the last month, and the majority the climate forecasting models indicate these conditions continuing to the end of summer 2007/08. The chances of El Niño re-emerging are at or below 10%.

More information can be found on NIWA’s Website at: http://www.niwascience.co.nz/ncc
© Copyright NIWA 2007. 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, 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%
• Average: 30%
• Below average: 10%
We therefore conclude 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.

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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