Seasonal Climate Outlook: July - September 2013

Mild conditions likely to continue over late winter.

Late winter (July–September) temperatures are very likely to be above average in North Island regions, and likely to be average or above average in all South Island regions. Nevertheless, cold snaps, frost and snow conditions will of course still occur in many areas from time to time, as is typical of this time of year. Sea surface temperatures around New Zealand’s coasts are likely to be above the climatological average for the coming three months near the coast and to the east and south of the country, with conditions near average further to the west in the Tasman Sea.

Rainfall for the July–September period as a whole is likely to be normal or above normal in the east of the North Island, normal or below normal in the west and south of the South Island, and in the near normal range for all other regions. Soil moisture levels are likely to be normal or above normal in the east of the South Island, and near normal in other regions. River flows are likely to be normal or above normal in the southwest of the North Island, normal or below normal in the west and south of the South Island, and near normal in other regions.

The equatorial Pacific Ocean remains in a neutral state (neither El Niño nor La Niña). International guidance indicates that these neutral conditions are likely to persist for at least the coming three months. For the New Zealand region, higher pressures than normal are forecast south of the country, with slightly lower pressures than normal in the north Tasman Sea. This circulation pattern is likely to produce a weak easterly flow anomaly over the country.

Overall picture

Rainfall, soil moisture and river flows

Rainfall, soil moisture and river flows for the July–September period as a whole are likely to be in the normal ranges for many regions of the country. Exceptions are as follows: in the southwest of the North Island, river flows are likely to be near normal or above normal; in the east of the North Island, rainfall is likely to be near normal or above normal; in the west and south of the South Island, both rainfall and river flows are likely to be near normal or below normal; and in the east of the South Island, soil moisture levels are likely to be near normal or above normal.  

Temperature

July to September temperatures as a whole are very likely to be above average in all North Island regions, and likely to be average or above average in all South Island regions, for this time of year.  Sea surface temperatures around New Zealand are also forecast to remain above average near to and southeast of the country, whereas sea temperatures west of New Zealand are likely to return to near average.

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

Temperatures for the July – September period as a whole are very likely to be above average. Rainfall totals, soil moisture levels and river flows during this period are all likely to be in their near normal ranges. 

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	60	35	35	30
Near average	30	45	45	45
Below average	10	20	20	25

 Click here to view the historic rainfall and temperature for this region 

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Late winter (July – September) temperatures are very likely to be in the above average range. Seasonal rainfall totals and soil moisture levels are likely to be in the near normal range, whereas river flows are 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	River flows
Above average	60	35	35	35
Near average	30	45	45	40
Below average	10	20	20	25

Click here to view the historic rainfall and temperature for this region 

Gisborne, Hawke’s Bay, Wairarapa

Late winter temperatures are very likely to be above average. July – September rainfall totals are likely to be near normal or above normal. Seasonal soil moisture levels and river flows are likely to be in the near normal range.

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	60	40	30	25
Near average	30	40	45	45
Below average	10	20	25	30

Click here to view the historic rainfall and temperature for this region 

Nelson, Marlborough, Buller

Late winter temperatures are likely to be above average or average.  Seasonal rainfall totals, soil moisture levels and river flows are all 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	River flows
Above average	45	25	25	25
Near average	40	50	50	50
Below average	15	25	25	25

Click here to view the historic rainfall and temperature for this region 

West Coast, Alps and foothills, inland Otago, Southland

July – September temperatures are likely to be average or above average. Seasonal rainfall totals and river flows are likely to be near normal or below normal. Soil moisture levels are likely to be at near normal levels, 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	40	20	25	20
Near average	40	40	45	40
Below average	20	40	30	40

Click here to view the historic rainfall and temperature for this region 

Coastal Canterbury, east Otago

July – September temperatures are likely to be average or above average, for the season as a whole. Late winter rainfall totals and river flows are likely to be in the near normal range. Soil moisture levels are likely to be at near normal or above normal levels.

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	35	40	35
Near average	40	45	45	45
Below average	20	20	15	20

Click here to view the historic rainfall and temperature for this region 

Background

Atmospheric indicators of ENSO, such as the Southern Oscillation Index (SOI), trade winds, and tropical cloud patterns are close to normal for this time of the year. The estimated NIWA SOI for June is 0.8, and the 3-month April–June estimate is 0.5. Ocean surface temperatures are slightly lower than normal in eastern tropical Pacific but still within the ENSO-neutral range. Climate models indicate that ENSO-neutral conditions are very likely to continue through the southern hemisphere late winter and spring seasons.

There is strong agreement between the dynamical and statistical models that NIWA uses to establish the seasonal forecast in predicting higher than normal temperatures for July-September in the North Island. In South Island regions, the global models consistently forecast above normal temperatures, whereas the local NIWA statistical models suggest either normal or above normal. As is common, the rainfall forecasts are less confident, and result in a flatter probability distribution, as shown in the tables above.

For comment, please contact

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

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

Dr Darren King, Climate Scientist

Tel (09) 375 2086, Mobile (021) 399 63

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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