WEATHER

NIWA National Climate Centre Seasonal Climate Outlook June-August 2012

Wednesday 30 May 2012, 2:44PM
By NIWA
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Drier winter likely for some

Neutral conditions (neither La Niña nor El Niño) presently exist in the tropical Pacific and should persist through much of winter, but there is a likelihood of El Niño developing by spring.

Circulation patterns over New Zealand this winter (June – August) are likely to be close to the seasonal norm - winter is the time of year when weather features are typically fast-moving and relatively active.

Winter rainfall totals are likely to be below normal in the eastern South Island, normal or below normal for Nelson/Marlborough, as well as the west and north of the North Island. Near normal winter rainfall totals are predicted for the South Island West Coast, and the eastern North Island.

For winter (June – August), sea temperatures around New Zealand are likely to be slightly above normal. Seasonal temperatures are likely to be near average overall for all regions of the country. Frosts and snowfalls typical of winter will occur from time to time.

Overall picture

Temperature

Seasonal temperatures are likely to be near average overall for all regions of the country. Frosts and snowfalls typical of winter will still occur from time to time.

Rainfall, soil moisture and river flows

Winter rainfall totals and soil moisture levels are likely to be below normal in the eastern South Island, normal or below normal for Nelson/Marlborough, as well as the west and north of the North Island. Near normal winter rainfall totals and soil moisture levels are predicted for the South Island West Coast, and the eastern North Island. River flows are predicted to be below normal for the north and east of the South Island, near normal in the eastern North Island, and normal or below normal elsewhere.

 

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

Winter temperatures are likely to be near average overall. Seasonal rainfall totals, soil moisture levels and river flows are equally likely to be in the normal or 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             35                  20                       20                    20
Near average               50                  40                       40                    40
Below average            15                   40                       40                    40

 

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Seasonal temperatures are likely to be near average. Winter rainfall totals, soil moisture levels and river flows are equally likely to be in the normal or 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            35                    20                       20                    20
Near average              50                    40                       40                    40
Below average           15                     40                       40                    40

 

Gisborne, Hawke’s Bay, Wairarapa

Winter temperatures are likely to be near average overall. Seasonal rainfall totals, soil moisture levels and river flows are likely to be near normal for the 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                   30                     30                    25
Near average               50                   50                     45                    50
Below average             15                   20                     25                    25

 

Nelson, Marlborough, Buller

Winter temperatures are likely to be near average overall. Seasonal rainfall totals and soil moisture levels are equally likely to be in the normal or below normal range. River flows are likely to be below normal this winter.

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                   20
Near average                  50                  40                       40                   35
Below average               15                   40                       40                   45

 

 

West Coast, Alps and foothills, inland Otago, Southland

Winter temperatures are likely to be near average overall. Near normal seasonal rainfall totals and soil moisture levels are likely, with normal or below normal river flows.

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                 50                   50                     50                     45
Below average               15                   30                     30                     40

 

Coastal Canterbury, east Otago

Winter temperatures are likely to be near average overall. Winter rainfall totals and soil moisture levels are likely to be in the below normal range. Winter river flows are very likely to the 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                 35                  15                      15                     15
Near average                   50                  35                      30                     25
Below average                15                   50                      55                     60 

 

Background

Present conditions in the tropical Pacific are ENSO-neutral (neither La Niña nor El Niño).

The May Southern Oscillation Index (SOI) was close to zero. Sea surface temperatures over the central Pacific are close to the climatological average, but continue warmer than average for the second month running over the eastern Pacific. It is likely that seas in the tropical Pacific Ocean will warm further over the next six months. Half the global climate models that NIWA monitors indicate conditions are likely to approach or exceed El Niño thresholds by spring.

 

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.