Seasonal Climate Outlook: July - September 2010

La Niña on the way - mild conditions likely.

The NIWA National Climate Centre outlook for the rest of winter and into early spring 2010 says temperatures are likely to be above average for the time of year across much of the country. However, short-term cold snaps and frosty periods typical of winter will still occur.

The equatorial Pacific is now in a neutral state, but is bordering on a La Niña. La Niña conditions are expected to develop fully by early spring.
Normal or below normal rainfalls are likely in the North Island, with near-normal rainfalls likely over much of the South Island. But, the Centre says that the west and south of the South Island is likely to experience normal or above normal rainfalls.

Soil moisture levels and stream flows are likely to be normal or below normal in the north and east of the North Island, normal or above normal in the western South Island, and near normal in other regions.

The National Climate Centre’s latest outlook states that mean sea level pressures are likely to be above normal over the North Island and to the north of the country, with stronger than average westerly wind flow over the South Island and south of the country, for July-September as a whole.


Overall Picture

Temperature:

For the rest of the winter and into early spring, temperatures are likely to be above average in the North Island and northern South Island, and average or above average elsewhere. Nevertheless, cold spells and seasonal frosts are still expected, especially in the South Island and central North Island.  Sea surface temperatures are expected to be above average around New Zealand through the July-September period.

Rainfall, soil moisture, and stream flows:

Rainfall is likely to be normal or below normal over the North Island, normal or above normal over the western South Island, and near normal elsewhere. Soil moisture levels and stream flows are likely to be normal or below normal in the north and east of the North Island, normal or above normal in the western South Island, and near normal in other regions.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Temperatures are very likely to be above average.  Seasonal rainfall totals, soil moisture levels, and stream flows are equally likely to be near 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 60% 20% 20% 20%
Near average 30% 40% 40% 40%
Below average 10% 40% 40% 40%

Click these links to view the historic rainfall and temperature ranges for this region

 

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

Above average seasonal temperatures are likely. Rainfall totals are equally likely to be near normal or below normal, while stream flows and soil moisture levels are likely to be in the near normal range, for the three months as a whole.
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 50% 20% 20% 20%
Near average 30% 40% 45% 45%
Below average 20% 40% 35% 35%

Click these links to view the historic rainfall and temperature ranges for this region

 

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures averaged over the three months are very likely to be in the above average category. Seasonal rainfall totals, stream flows and soil moisture levels, are all equally likely to be in the normal or below normal range.

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 60% 20% 20% 20%
Near average 30% 40% 40% 40%
Below average 10% 40% 40% 40%

Click these links to view the historic rainfall and temperature ranges for this region

 

Nelson, Marlborough, Buller:

Seasonal temperatures are likely to be in the above average range. Rainfalls, stream flows and soil moisture levels are all likely to be near normal overall.

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 45% 25% 25% 25%
Near average 35% 50% 50% 50%
Below average 20% 25% 25% 25%

Click these links to view the historic rainfall and temperature ranges for this region

 

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

Temperatures are equally likely to be in the average or above average category. Seasonal rainfall, stream flows and soil moisture levels are all equally likely to be near normal or above 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% 40% 40% 40%
Near average 40% 40% 40% 40%
Below average 20% 20% 20% 20%

Click these links to view the historic rainfall and temperature ranges for this region

 

Coastal Canterbury, East Otago:

Temperatures are equally likely to be in the average or the above average category, on the whole during July-September. Seasonal rainfall totals, soil moisture levels and stream flows are likely to be in the normal range.

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% 30%
Near average 40% 50% 50% 50%
Below average 20% 20% 20% 20%

Click these links to view the historic rainfall and temperature ranges for this region

Background

The equatorial Pacific is now in a neutral state but is bordering on a La Niña. La Niña conditions are expected to develop fully by early spring.

For comment, please contact:

Dr James Renwick, NIWA Principal Scientist, Climate Variability & Change

Tel (04) 386 0343 (office DDI), mobile (021) 178 5550

Georgina Griffiths, NIWA Climate Scientist

Tel (09) 375 4506 (office DDI), mobile (027) 293 6545

© Copyright NIWA 2010. All rights reserved. Acknowledgement of NIWA as the source is required.

Notes to reporters & editors

  1.    
  2. 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.   
  3. 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.   
  4. 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.
  5. 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.   
  6. 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).   
  7. 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   
  8. 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.   
  9. 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.