Climate Outlook: Stormy at times, getting drier
NATIONAL CLIMATE CENTRE 30 July 2008
SEASONAL CLIMATE OUTLOOK: August–October 2008
Stormy at times, but getting drier as spring progresses
NIWA scientists are warning that we could see further episodes of strong winds (both from the north and the south), as storms pass across the country through the remainder of winter.
NIWA’s overall seasonal climate outlook for August to October indicates weaker than normal westerlies. NIWA’s National Climate Centre says near-normal rainfalls are likely in most regions except the west where rainfall may be normal or below normal.
The centre’s outlook also says average or above average temperatures are likely in most areas, with the eastern South Island likely to experience average or below average temperatures, while the west and south of the South Island experiences higher than average seasonal temperatures.
The La Niña weather pattern, which influenced New Zealand earlier in the year and brought dry, settled weather over summer, has now ended. There is no indication of either La Niña or El Niño developing for the rest of the year.
Air temperatures are likely to be average or above average in most regions, but average or below average in the eastern South Island. Sea surface temperatures around New Zealand are likely to be near normal.
moisture, and stream flows:
Rainfall is likely to be near normal in most regions, but is likely to be normal or below normal in the southwest of both Islands. Soil moisture levels and streamflows are likely to be normal or above normal in the northern North Island and normal or below normal in the southwest North Island. In most other regions, near normal soil moisture levels and streamflows are likely, apart from normal or below normal streamflows in the southwest South Island.
[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:
Average or above average temperatures are likely. Rainfall is likely to be near normal for the season as a whole, with normal or above normal soil moisture and stream flows.
Central North Island, Taranaki, Wanganui,
Manawatu and Wellington:
Average or above average temperatures are likely overall. Normal or below normal rainfalls, soil moisture levels and stream flows are likely.
Gisborne, Hawkes Bay, Wairarapa:
Near average temperatures are likely for the three months August-October. Normal rainfall is likely overall, with near normal soil moisture and stream flows.
Average or above average temperatures are likely. Normal rainfall, soil moisture levels, and stream flows are likely.
West Coast, Alps
and Foothills, Inland Otago, Southland:
Above average temperatures are likely, with normal or below normal rainfall and stream flows. Normal soil moisture levels are likely.
Coastal Canterbury, East Otago:
Average or below average temperatures are likely. Normal rainfall is likely overall, with near normal soil moisture levels and stream flows likely.
In the New Zealand region, mean sea level pressures are likely to be higher than normal to the south-west of the South Island. Westerly winds are likely to be weaker than normal over the country, but episodes of strong northerly or southerly winds are likely, as storms pass across New Zealand.
The La Niña has ended, and the tropical Pacific is in a neutral state. Sea surface, temperatures are close to average across much of the Equatorial Pacific, but have warmed near the South American coast. The Southern Oscillation Index is near zero. Most climate forecasting models indicate conditions in the neutral range during August to October.
© 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.
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.
3. 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.
4. 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).
5. 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
6. 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.
7. 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.