Seasonal Climate Outlook: August – October 2010

Press Release – NIWA

The NIWA National Climate Centre outlook says that the equatorial Pacific is now in a La Niña state. La Niña conditions are likely to continue through the remainder of 2010. NIWA NATIONAL CLIMATE CENTRE 30 July 2010
Seasonal Climate Outlook: August – October 2010

La Niña conditions for the rest of 2010

The NIWA National Climate Centre outlook says that the equatorial Pacific is now in a La Niña state. La Niña conditions are likely to continue through the remainder of 2010.

Early spring temperatures are likely to be near average or above average across the country. However, short-term cold snaps and frosty periods typical of early spring will still occur.

Rainfall is likely to be near normal in most regions, for August-October as a whole. The exception is the lower half of the North Island, with normal or below normal rainfalls likely in the west, and normal or above normal rainfalls likely in the east.

Soil moisture levels and stream flows are likely to be in the normal range for many regions August-October as a whole. But the Centre says that normal or below normal soil moisture levels and stream flows are likely in the west of the North Island, and normal or above normal soil moisture levels are likely in the eastern North Island.

The National Climate Centre’s latest outlook states that mean sea level pressures are likely to be above normal near New Zealand, for August-October as a whole.
Overall Picture
Temperature:

Early spring temperatures are likely to be near average or above average in all regions. However, short-term cold snaps and frosty periods typical of early spring will still occur. Sea surface temperatures are expected to be near average or above average around New Zealand over the August to October period.
Rainfall, soil moisture, and stream flows:

Rainfall is likely to be near normal in most places, but normal or below normal in the western North Island, and normal or above in the east of the North Island. Normal soil moisture levels and stream flows for August-October as a whole are likely in many regions, except for normal or below normal conditions in the west of the North Island, and normal or above normal soil moisture levels in the eastern North Island.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Temperatures are equally likely to be in the above average or near average range. Seasonal rainfall, soil moisture levels, and stream flows totals 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 / Stream flows
Above average / 40% / 30% / 30% / 25%
Near average / 40% / 50% / 50% / 50%
Below average / 20% / 20% / 20% / 25%
Central North Island, Taranaki, Wanganui, Manawatu and Wellington:

Temperatures are equally likely to be in the above average or near average range. Seasonal rainfall totals, soil moisture levels, and stream flows are likely to be either normal or below normal.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:
/ Temperature / Rainfall / Soil moisture / Stream flows
Above average / 40% / 20% / 20% / 20%
Near average / 40% / 40% / 40% / 40%
Below average / 20% / 40% / 40% / 40%
Gisborne, Hawke’s Bay, Wairarapa:

Temperatures are equally likely to be in the above average or near average range. Seasonal rainfall totals and soil moisture levels are likely to be either normal or above normal, while normal stream flows are likely.

Probabilities are assigned in three categories; above normal, near normal, and below normal. The full probability breakdown is:
/ Temperature / Rainfall / Soil moisture / Stream flows
Above average / 40% / 40% / 40% / 30%
Near average / 40% / 40% / 40% / 50%
Below average / 20% / 20% / 20% / 20%
Nelson, Marlborough, Buller:

Temperatures are equally likely to be in the above average or near average range. Seasonal rainfall, soil moisture levels, and stream 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 / Stream flows
Above average / 40% / 25% / 25% / 20%
Near average / 40% / 50% / 50% / 50%
Below average / 20% / 25% / 25% / 30%

West Coast, Alps and Foothills, Inland Otago, Southland:

Temperatures are equally likely to be in the above average or near average range. Seasonal rainfall soil moisture levels, and stream 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 / Stream flows
Above average / 40% / 35% / 30% / 25%
Near average / 40% / 45% / 50% / 50%
Below average / 20% / 20% / 20% / 25%
Coastal Canterbury, East Otago:

Temperatures are equally likely to be in the above average or near average range. Seasonal rainfall, soil moisture levels, and stream 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 / Stream flows
Above average / 40% / 25% / 25% / 25%
Near average / 40% / 50% / 50% / 50%
Below average / 20% / 25% / 25% / 25%
Background
The tropical Pacific has moved steadily towards La Niña conditions over the past few months, and is now in a La Niña state. La Niña conditions are likely to continue through the remainder of 2010.

© Copyright NIWA 2010. All rights reserved. Acknowledgement of NIWA as the source is required.
ENDS
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 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%
• / 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.

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