Seasonal Climate Outlook: April - June 2013

Mild conditions for late autumn.

The equatorial Pacific Ocean remains in a neutral state (neither El Niño nor La Niña). International guidance indicates that neutral conditions are very likely to continue over the next three months (April – June). In the New Zealand region, higher pressures are likely to prevail over and to the south of the South Island during the season. 

Temperatures for late autumn (April – June) are likely to be above average across the North Island, and are very likely to be above average across the South Island. Sea surface temperatures in late autumn are likely to remain above average around the South Island, and be close to normal around the North Island.

Rainfall for the April – June period as a whole is likely to be in the near normal range for all regions. Soil moisture levels are likely to be below normal for the north of the North Island for late autumn, and normal or below normal for the remainder of the country. River flows are likely to be below normal for the North Island and the north of the South Island, and normal to below normal elsewhere. Because of the existing soil moisture deficits across the North Island and in the eastern South Island, soil moisture levels and river flows are expected to take some time to recover in these areas. 

For this tropical cyclone season (November – April), the risk of an ex-Tropical Cyclone approaching New Zealand remains near normal. On average, one ex-Tropical Cyclone nears New Zealand during the season.

Overall picture

Rainfall, soil moisture and river flows

Rainfall totals over the April – June period as a whole are likely to be in the near normal range for all regions. Soil moisture levels are likely to be below normal for the north of the North Island for the April – June period, and normal or below normal for the remainder of the country. River flows are likely to be below normal for the North Island and the north of the South Island, and normal to below normal elsewhere. Because of the existing soil moisture deficits across the North Island and in the eastern South Island, soil moisture levels and river flows are expected to take some time to recover in these areas. 

For this tropical cyclone season (November – April), the risk of an ex-Tropical Cyclone approaching New Zealand remains near normal. On average, one ex-Tropical Cyclone nears New Zealand during the season.

Temperature

April to June temperatures are likely to be above average across the North Island, and are very likely to be above average across the South Island. Sea surface temperatures in late autumn are likely to remain above average around the South Island, and be close to normal around the North Island.

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

Temperatures for April – June are likely to be above average. Rainfall totals during this period are likely to be in the near normal range. Seasonal soil moisture levels and river flows are likely to be in the below 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	50	35	20	20
Near average	40	45	35	30
Below average	10	20	45	50

Click these links to view the historic rainfall and temperature. 

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Late autumn temperatures are likely to be in the above average range. Seasonal rainfall totals are likely to be in the near normal range. Seasonal soil moisture levels are likely to be in the near normal or below normal range, and river flows are likely to be below 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	50	30	20	20
Near average	40	40	40	35
Below average	10	30	40	45

Click these links to view the historic rainfall and temperature. 

Gisborne, Hawke’s Bay, Wairarapa

Seasonal temperatures are likely to be above average. April – June rainfall totals are likely to be near normal. Seasonal soil moisture levels are likely to be in the near normal or below normal range, and river flows are projected below 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	50	30	20	20
Near average	40	45	40	35
Below average	10	25	40	45

Click these links to view the historic rainfall and temperature. 

Nelson, Marlborough, Buller

April – June temperatures are very likely to be above average.  Seasonal rainfall totals are likely to be in the normal range. Soil moisture levels are likely to be near normal or below normal. River flows are likely to be below 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	60	30	25	20
Near average	30	45	40	35
Below average	10	25	35	45

Click these links to view the historic rainfall and temperature.

West Coast, Alps and foothills, inland Otago, Southland

April – June temperatures are very likely to be above average. Seasonal rainfall totals are likely to be in the near normal range. Late autumn soil moisture and river flows are likely to be at near normal or below 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	60	30	25	20
Near average	30	50	40	40
Below average	10	20	35	40

Click these links to view the historic rainfall and temperature.

Coastal Canterbury, east Otago

April – June temperatures are very likely to be above average. Seasonal rainfall totals are likely to be in the near normal range. Late autumn soil moisture and river flows are likely to be at near normal or below 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	60	25	25	20
Near average	30	50	40	40
Below average	10	25	35	40

Click these links to view the historic rainfall and temperature. 

Background

Atmospheric indicators of ENSO, such as the Southern Oscillation Index (SOI), trade winds, and tropical cloud patterns continue at neutral levels. The estimated NIWA SOI for March was 0.7, and the 3-month January-March estimate was 0.0. Tropical Pacific Ocean temperatures had small changes in March but remain in the ENSO-neutral condition. Climate models indicate this neutral state is likely to continue well beyond the southern hemisphere autumn. 

Intense anticyclones (‘highs’) dominated in the New Zealand region over the last three months, with associated widespread dryness in many regions. In the coming season, the higher pressures over and to the south of New Zealand are expected to be less dominant. That is why model guidance indicates near normal rainfall for the country. 

For comment, please contact

Dr Brett Mullan, Principal Scientist, NIWA National Climate Centre Tel (04) 386 0508, Mobile 027 294 1169

Dr Andrew Tait, Principal Scientist, NIWA National Climate Centre Tel (04) 386 0562, Mobile 027 327 7948

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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