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Seasonal Climate Outlook:May - July 2008


NATIONAL CLIMATE CENTRE 30 April 2008

Seasonal Climate Outlook: May - July 2008

Warm start to winter, but still drier than normal in the south

For the late autumn and early winter period (May- July), mild conditions are very likely in many areas according to NIWA’s National Climate Centre. Despite the overall temperature expectation, cold outbreaks typical of winter will nevertheless occur from time to time. There are signs that drier than normal conditions will likely continue in Westland, Fiordland, Alpine areas of the South Island and Southland.

For the three months as a whole, above average temperatures are the most likely outcome in many regions, with only a 10% chance of below average temperatures occurring. Soil temperatures are very likely to be above average as well. Above average temperatures are expected to continue in the seas around New Zealand.

Normal or below normal rainfall, and below normal river flows and soil moisture are likely in the west and south of the South Island. This outlook reflects the effects of a moderate but waning La Niña, with generally more north easterly winds over the country, bringing milder and wetter than normal conditions in the north and east of the North Island.

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

Overall Picture
Temperature:
Air temperatures are very likely to be above average in many areas. Despite the overall temperature expectation, cold outbreaks typical of winter will nevertheless occur from time to time. Sea surface temperatures around New Zealand are expected to remain above normal.

Rainfall, soil moisture, and stream flows:
Rainfall is expected to be normal or above normal in the north and east of the North Island and normal or below normal in the west and south of the South Island. Normal rainfall is likely in other places. Above normal soil moisture and river flows are expected in the north of the North Island, with normal or below normal soil moisture and river flows likely elsewhere.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Above average temperatures are very likely with above normal rainfall likely. Soil moisture and stream flows are likely to above normal for the season as a whole.

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Above average temperatures are very likely. Normal rainfall is likely, with normal or below normal soil moisture and stream flows.

Gisborne, Hawkes Bay, Wairarapa:
Above average seasonal temperatures are likely. Above normal rainfall is likely, with soil moisture and stream flows likely to be normal.

Nelson, Marlborough, Buller:
Above average temperatures are very likely. Normal rainfall, soil moisture and river flows are likely.

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

Coastal Canterbury, East Otago:
Above average temperatures are likely. Near normal rainfall, with normal or below normal soil moisture 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 the North Island, with more winds from the northeast than normal over the country.

La Niña is now weakening in the tropical Pacific, and is expected to ease to neutral conditions by July. At the ocean surface, below normal temperature anomalies have eased dramatically across much of the Equatorial Pacific. The Southern Oscillation Index is +1.0 and falling. Most climate forecasting models indicate conditions in the neutral range during May-July, or easing to neutral by the beginning of spring.



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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