A dry and settled spring and early summer

NATIONAL CLIMATE CENTRE 30 September 2008

Seasonal Climate Outlook: October–December 2008

A dry and settled spring and early summer

The NIWA National Climate Centre says that the upcoming spring and early summer period is likely to be relatively settled, and drier than normal overall.

The centre’s seasonal climate outlook for October to December indicates below normal or near normal rainfalls are the most likely outcome across the country.

The chances of above normal rainfall anywhere are estimated at 20 percent or less. This applies to the season as a whole, and there will inevitably be wet periods within it. Normal or below normal soil moisture levels and stream flows are likely everywhere in the country.

The outlook predicts that temperatures are likely to be average or above average everywhere. For most areas of New Zealand there is a very low likelihood of below average temperatures – less than 20 percent.

Mean sea-level pressures are likely to be higher than normal over the country, especially the South Island, with lighter winds than normal. This means that the normal westerly gales, typical of October and November, are likely to be less frequent.

There is presently no La Niña or El Niño in the tropical Pacific, and no indication of either La Niña or El Niño developing for the rest of the year. The chance of an ex-tropical cyclone affecting New Zealand over the summer is slightly higher than normal.

Overall picture:

Temperature:

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

Rainfall, soil moisture, and stream flows:

Rainfall is likely to be near or below normal over the entire country. Normal or below normal soil moisture levels and stream flows are also likely in all regions.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Average temperatures are likely. Rainfall, soil moisture, and stream flows are likely to be normal or below normal for the season as a whole.

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:

Average temperatures are likely overall. Below normal rainfall, soil moisture, and stream flows are likely.

Gisborne, Hawkes Bay, Wairarapa:

Average temperatures are likely for the three months October-December. Normal rainfall, soil moisture and stream flows are likely overall.

Nelson, Marlborough, Buller:

Average or above average temperatures are likely. Below normal rainfall is likely, with normal or below normal soil moisture levels and stream flows.

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

Average or above average temperatures are likely. Rainfall is likely to be normal or below normal. Soil moisture and stream flows are likely to be normal.

Coastal Canterbury, East Otago:

Average temperatures are likely for the spring-early summer period. Rainfall, soil moisture, and stream flows are likely to be normal or below normal.

Background:

Climate and Oceans:

In the New Zealand region, mean sea level pressures are likely to be higher than normal, especially over the South Island, resulting in lighter winds than usual over the country and periods of easterlies affecting the North Island.

Neutral conditions (no La Niña or El Niño) now prevail in the tropical Pacific and are expected to continue through to summer. The Southern Oscillation Index is positive and stronger than normal easterly winds are present in the western equatorial Pacific, but these lingering effects of the La Niña earlier this year are expected to ease towards normal through the rest of 2008. All climate forecasting models indicate conditions in the neutral range during October to December and beyond.

The tropical cyclone season for the southwest Pacific begins in November, and with ENSO neutral conditions in the Pacific, there is a 4 out of 5 chance of an ex-tropical cyclone passing within 500 km of the country between November and May, with the districts at highest risk being Northland and Gisborne.

© 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, and 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