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July–September 2008 Climate Outlook


NATIONAL CLIMATE CENTRE 30 June 2008

SEASONAL CLIMATE OUTLOOK: July–September 2008

Overall mild winter, with rainfalls near normal

Warmer than average conditions which prevailed over New Zealand in June look set to continue into late winter, according to the NIWA National Climate Centre.
The centre’s seasonal climate outlook for July to September says above average temperatures are likely over the North Island, with average or above average temperatures likely across all of the South Island. Despite the overall temperature expectation, cold outbreaks typical of winter will nevertheless occur from time to time.

For rainfall, near-normal falls are the most likely outcome for all districts of the country, according to NIWA climate scientists. In all regions, the NIWA outlook puts the probability of normal precipitation [rain & snow combined] at 50% (compared to a long-term average probability of 33%). River flows are likely to be near normal for alpine and western regions of the South Island, including key hydro catchments.

Recent storms have helped bring total hydro storage up to 58% of normal by 29 June, from a low of around 49% of normal earlier in the month. Total hydro inflows are likely to follow a normal pattern for the time of year. Lake inflows typically decrease on average to a minimum in September, as during winter precipitation that falls at higher elevations is in the form of snow rather than rain, and therefore unavailable for flow into the lakes and rivers, until the melt season in late spring.

The La Niña weather pattern, which influenced New Zealand earlier in the year, has now ended. There is no indication of either La Niña or El Niño developing for the rest of the year.


Overall Picture
Temperature:
Air temperatures are likely to be above average in the North Island, and average or above average over the South Island. Sea surface temperatures around New Zealand are expected to remain above normal.

Rainfall, soil moisture, and stream flows:
Rainfall is likely to be near normal in all regions. Normal soil moisture levels are likely in most regions, with normal or below normal levels likely on the South Island east coast. Normal or below normal stream flows are likely in the north and east of both islands, with normal stream flows likely in the west and south of both islands.


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

Regional predictions for the next three months:
Northland, Auckland, Waikato, Bay of Plenty:
Above average temperatures are likely. Rainfall and soil moisture levels are likely to be near normal for the season as a whole, with normal or below normal stream flows likely.

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

Gisborne, Hawkes Bay, Wairarapa:
Above average seasonal temperatures are likely. Normal rainfall and soil moisture levels are likely overall, with stream flows likely to be normal or below normal.

Nelson, Marlborough, Buller:
Average or above average temperatures are likely. Normal rainfall and soil moisture levels are likely, with normal or below normal stream flows.

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

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

Background

Climate and Oceans:
In the New Zealand region, mean sea level pressures are expected to be higher than normal to the south of the South Island and lower than normal to the northwest of New Zealand, with more winds from the northeast than normal over the country.

La Niña has effectively ended, with conditions near neutral in the tropical Pacific. At the ocean surface, temperatures are close to average across much of the Equatorial Pacific. The Southern Oscillation Index is near zero. Most climate forecasting models indicate conditions in the neutral range during July to September.

© Copyright NIWA 2008. 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.

ENDS

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