Seasonal Climate Outlook:April – June 2009

NATIONAL CLIMATE CENTRE 30 March 2009

SEASONAL CLIMATE OUTLOOK: April – June 2009

La Niña weakening, with a mild end to autumn

La Niña is on the wane, and the end of autumn and start of winter looks likely to be mild.

The NIWA National Climate Centre says that, on average over the next three months, above average temperatures are likely in several regions of the country [Reporters please note: Probabilities are assigned in three categories; above average, average, and below average. See end for more explanation.]. It is not likely that any major region will be cooler than normal over the three months as a whole, although this does not rule out occasional cold spells.

The latest NIWA Seasonal Climate Outlook, issued today by the National Climate Centre, indicates that the moderate La Niña in the tropical Pacific is expected to weaken to neutral conditions by early winter.

The latest outlook covers the three months April, May and June 2009, and indicates average or above average temperatures in all regions. While most regions are expected to experience near normal rainfalls for the three months as a whole, below normal rainfall is likely for the eastern South Island, and normal or above normal rainfall is likely over the north of the North Island.

Normal or above normal soil moisture and stream flows are likely in the northern North Island, while near normal conditions are likely over the rest of the North Island and northern South Island. Normal or below normal soil moisture and stream flows are expected in the rest of the South Island.

Mean sea-level pressures are likely to be higher than normal to the east and south of the country, and lower than normal to the north of New Zealand, resulting in a northeast flow anomaly, especially over the North Island.

For the remainder of the tropical cyclone season (through to May 2009), there is an average chance of an ex-tropical cyclone passing within 500 km of the country.

Overall Picture

Temperature:

Temperatures are expected to be average or above average in all regions. Sea surface temperatures around New Zealand are expected to be average or slightly above average by the start of winter.

Rainfall, soil moisture, and stream flows:

Rainfall is likely to be normal or above normal over the north of the North Island, below normal along the east coast of the South Island, and near normal elsewhere. Soil moisture and stream flows are likely to be normal or above normal in the northern North Island, while near normal conditions are likely over the rest of the North Island and northern South Island. Normal or below normal soil moisture and stream flows are expected in the rest of the South Island.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Above average temperatures are likely. Rainfall, soil moisture levels, and stream flows are likely to be normal or above normal for the season as a whole.

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

Average or above average temperatures are likely. Normal rainfall, stream flows, and soil moisture levels are likely for the three-month period.

Gisborne, Hawke’s Bay, Wairarapa:

Above average temperatures are likely for the three months. Normal rainfall, soil moisture levels and stream flows are likely overall.

Nelson, Marlborough, Buller:

Average or above average temperatures are likely. Rainfall is likely to be near normal, and soil moisture levels and stream flows are also likely to be near normal.

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

Above average temperatures are likely. Rainfall is likely to be near normal, while soil moisture levels and stream flows are likely to be normal or below normal.

Coastal Canterbury, East Otago:

Average or above average temperatures are likely for April–June. Rainfall is likely to be below normal, and soil moisture levels and stream flows are likely to be normal or below normal.

Background

In the New Zealand region, mean sea level pressures are likely to be higher than normal to the east and south of the country, and lower than normal to the north of New Zealand, resulting in a northeast flow anomaly, especially over the North Island. Sea surface temperatures around New Zealand are expected to be average or slightly above average through to the start of winter.

A moderate La Niña in the tropical Pacific is expected to weaken to neutral conditions by the end of the period.

The tropical cyclone season for the southwest Pacific runs from November to May. With current conditions in the Pacific, there is a near average chance of an ex-tropical cyclone passing within 500 km of the country during the last two months of the season (April-May). The districts at highest risk are Northland and Gisborne.

ENDS

© Copyright NIWA 2009. 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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