Seasonal Climate Outlook: November 2012 - January 2013

Normal to dry early summer for North Island, Nelson and Marlborough.

Oceanic indicators in the west and central tropical Pacific remain close to El Niño thresholds, but the atmosphere has yet to show any significant response to the warmer than normal sea surface temperatures. The global forecast models indicate an approximately 50:50 chance of neutral versus weak El Niño conditions over the next three months. In the New Zealand region, lower than normal pressures are expected southeast of the Chatham Islands, with weakly enhanced south-westerly winds over New Zealand. 

For the early summer season, rainfall is likely to be near normal or below normal in all North Island regions and in Nelson-Marlborough, but near normal in other South Island regions. Soil moisture and river flows are forecast to follow the same regional pattern as rainfall, except that soil moisture levels are likely to be below normal in the north of the North Island, and both soil moisture levels and river flows are likely to be below normal in the east of the North Island. 

November-January air temperatures are likely to be average or above average in the north and east of the North Island, and near average in other regions.  Sea surface temperatures are expected to remain near average around New Zealand. 

Overall picture

Rainfall, soil moisture and river flows

For the November to January period, rainfall is likely to be near normal or below normal in all North Island regions and in Nelson-Marlborough, but near normal in other South Island regions. Soil moisture levels are likely to be below normal in the north and east of the North Island, near normal or below normal in southwest North Island and in Nelson-Marlborough, and near normal in remaining South Island regions. River flows are likely to be below normal in the east of the North Island, near normal or below normal in remaining North Island regions and in Nelson-Marlborough, and near normal in other South Island regions. 

Temperature

Air temperatures for early summer are likely to be in the average or above average range in the north and east of the North Island, but near average in other regions.  Sea surface temperatures are expected to remain near average around New Zealand. 

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

November to January temperatures are likely to be in the near average or above average range.  Seasonal rainfall totals and river flows are likely to be near normal or below normal, while soil moisture levels are likely to be below normal for this time of year.

Probabilities are assigned in three categories: above average, near average, and below average. The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

35

20

20

15

Near average

40

40

35

40

Below average

25

40

45

45

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

 Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Early summer temperatures are likely to be near average. Rainfall, soil moisture levels and river flows are all likely to be near normal or below normal for the season 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

River flows

Above average

30

20

25

25

Near average

50

40

35

40

Below average

20

40

40

35

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

Gisborne, Hawke’s Bay, Wairarapa

Seasonal temperatures are forecast to be in the near average or above average range. November to January rainfall totals are equally likely to be near normal or below normal, while soil moisture levels and river flows are likely to be below normal.

Probabilities are assigned in three categories: above average, near average, and below average. The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

40

20

20

20

Near average

40

40

35

35

Below average

20

40

45

45

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

Nelson, Marlborough, Buller

November to January temperatures are likely to be near average. Seasonal rainfall totals, soil moisture levels, and river flows are all expected 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

River flows

Above average

20

20

25

25

Near average

50

40

40

40

Below average

30

40

35

35

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

West Coast, Alps and foothills, inland Otago, Southland

Seasonal temperatures are likely to be in the near average range. Early summer rainfall, soil moisture and river flows are all likely to be in the near normal range for this time of the year.

Probabilities are assigned in three categories: above average, near average, and below average. The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

20

30

30

30

Near average

50

45

40

40

Below average

30

25

30

30

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

Coastal Canterbury, east Otago

November to January temperatures are likely to be in the near average range.  Near normal seasonal rainfall totals, soil moisture levels, and river flows are likely.

Probabilities are assigned in three categories: above average, near average, and below average. The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

20

20

25

25

Near average

50

45

45

45

Below average

30

35

30

30

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

Background

The equatorial Pacific Ocean remains warmer than normal, especially around the Dateline, but the tropical atmospheric conditions are close to neutral. The Southern Oscillation Index has returned to near zero or slightly positive during September and October, whereas values below -1 are expected with El Niño events. International guidance indicates that the tropical Pacific Ocean is likely to remain neutral over the next three months (November-January), although a transition to a weak El Niño could still occur. The consensus between forecast models for an El Niño over the summer period is much weaker now than two months ago.

For comment, please contact

Dr Brett Mullan, NIWA Principal Scientist, Climate Variability & Change

Tel  (04) 386 0508, Mobile (027) 294 1169

Georgina Griffiths, Climate Scientist

Tel (09) 375 4506, Mobile (027) 2936545

Notes to reporters and 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 river flows. For example, for winter (June–July–August) 2007, for all the North Island, we assigned the following probabilities for temperature:

  • Above average: 60 per cent
  • Near average: 30 per cent
  • Below average: 10 per cent

We therefore concluded that above average temperatures were very likely.

4. This three-way probability means that a random choice would be correct only 33 per cent (or one-third) of the time. It would be like randomly throwing a dart at a board divided into three 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 per cent 'hit rate' is substantially better than guesswork, 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 US 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 online and is sent to about 3500 recipients of NIWA's newsletters, including many farmers. See www.niwa.co.nz/our-science/climate/publications/all/cu

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. 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 for the long-term mean, whereas for rainfall the "near normal" category lies between approximately 80 per cent and 115 per cent 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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