Southeastern Australian rainfall outlook
Wetter conditions more likely for southeast Australia
Summary
- A wetter than normal season is more likely for mainland southeast Australia
- Tasmania has no strong tendency towards being wetter or drier than normal
- Climate influences include a developing negative Indian Ocean Dipole, a neutral-to-cool tropical Pacific, and warm sea surface temperatures around the coast of Australia
- Outlook accuracy is low through parts of central and western SA, and moderate elsewhere.
Details
The chance of exceeding the median rainfall for July to September is greater than 60% over mainland southeast Australia. The chance rises to more than 80% over southern NSW and northern Victoria. Such odds mean that for every ten years with similar climate patterns to those currently observed, about six to eight July to September periods would be expected to be wetter than average over these areas, while about two to four would be drier.
The chance of receiving a wetter or drier than normal July to September is roughly equal (i.e., close to 50%) across Tasmania.
Climate influences
A negative Indian Ocean Dipole (IOD) event is favoured to develop during winter-spring 2013. A negative IOD during winter-spring increases the chances of above normal rainfall over southeast Australia, which is reflected in the rainfall outlook. See http://www.bom.gov.au/climate/IOD/negative/ for more information on typical rainfall patterns during negative IOD events.
The tropical Pacific has remained ENSO-neutral since mid-2012. The dynamical seasonal outlook model indicates an increased likelihood of La Niña forming during the next few months. This has increased the chance of above normal rainfall for southeast Australia. However, international climate models surveyed by the Bureau of Meteorology suggest ENSO-neutral is the most likely outcome over the coming season, with the Bureau model showing stronger odds than most of a weak La Niña.
Warmer than normal sea surface temperatures currently surround much of western and southern Australia. Warmer sea surface temperatures can provide more moisture to the atmosphere, which in combination with the right weather systems (e.g., interactions with fronts or northwest cloudbands) may result in increased rainfall across the southeast.
How accurate is the outlook?
Outlook accuracy is related to how consistently the oceans and broadscale climate affect Australian rainfall. During July to September, historical accuracy shows the outlook to be moderately consistent over most of southeast Australia, but only weakly to very weakly consistent over western and central SA.
Climatologists will continue to monitor conditions and outlooks closely for any further developments over the coming months, with information on the likelihood of El Niño available fortnightly at the ENSO Wrap-Up. For a summary of Pacific and Indian Ocean outlooks, please see the Climate Model Summary.
- Regional versions:
- Australia
- Northern Australia
- Southeastern Australia
- Western Australia
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More rainfall outlook maps, tables and graphs
An expanded set of seasonal rainfall outlook maps and tables, including the probabilities of seasonal rainfall exceeding given totals (e.g. chance of receiving at least 200 mm), is available from Water and the Land.
Outlook confidence
Outlook confidence (or accuracy) is measured by comparing how often the outlook favoured a particular category (for instance, when above median rainfall was more likely to occur than below median rainfall in a particular season), and that the more likely category was then subsequently observed. This measurement of skill is known as "Percent Consistent", and has been tested over the period from 1981 to 2010.
Strong consistency means that tests of the model on historical data show a strong relationship between the most likely outlook category (above/below median) and the verifying observation (above/below median). In areas with strong consistency, relatively high confidence can be placed in future outlook probabilities. Very weak consistency means the historical relationship, and therefore outlook confidence, is low. In the places and seasons where the outlooks are most skilful, the category of the eventual outcome (above or below median) is consistent with the category favoured in the outlook about 75% of the time. In the least skilful areas, the outlooks perform no better than chance.
A random forecast of above median rainfall will be correct about 50% of the time. For this reason, the green shading on the map shows areas where the model has greater than 50% accuracy only. In areas which are not coloured in green on the map, some caution should be taken when using the forecast, notably at times when there is not a strong driver of our climate (e.g., no El Niño or La Niña is present; for commentary on the state of the main climate drivers, please see our ENSO Wrap Up).
The Rainfall outlook has highest accuracy during autumn and spring, while in summer and winter there is lower skill, particularly over central Australia.
What is normal for this period?
This map shows the median (or 50th percentile) rainfall for the given three months. The median rainfall is calculated from the 1981-2010 period.
The maps will differ from other median maps on the Bureau's website. This is because the dynamical model forecasts use an averaging period of 1981-2010. The quality of the dynamical model forecasts is in-part determined by the coverage and accuracy of the observations fed into it. Therefore, to be consistent from one year to the next, the Bureau has only run the model during the modern satellite era.
About the outlook
Using the outlook
The Bureau's rainfall seasonal climate outlooks are general statements about the likelihood of wetter or drier than average weather over a three-month period. The probabilities are generated from the Predictive Ocean Atmosphere Model for Australia (POAMA), the Bureau of Meteorology's dynamical climate model. It is important to note that they are not categorical predictions about future rainfall, and hence the success or failure of one individual outlook does not infer that the model has low skill. Skill is assessed over multiple runs of the model. Likewise, temperature outlooks give the likelihood or chance of exceeding the average maximum and minimum temperatures over the entire three-month outlook period. Information about whether individual weeks or months may be unusually hot or cold, is presently unavailable.
Probability outlooks should not be used as if they were categorical (yes/no) forecasts. These outlooks should be used as a tool in risk management and decision making. Greatest benefits accrue from long-term use, say over 10 years.
About the model
The seasonal climate outlooks are generated by the Predictive Climate Ocean Atmosphere Model for Australia (POAMA), a dynamical (physics based) climate model developed by the Bureau of Meteorology and CSIRO Marine and Atmospheric Research. This coupled atmosphere-ocean model is a state of the art seasonal forecast system. Read more about POAMA.
The POAMA model is undergoing continuous research and development. Advances in the science of seasonal prediction, improvements in the observations and how they are fed into the model, as well as increases in supercomputing power are just some of the ways the model's accuracy will increase over time.
El Niño and La Niña
- About ENSO
- Australian rainfall patterns in:
Indian Ocean Dipole
- Indian Ocean Dipole (IOD)
- Australian rainfall patterns in:
Statistical model outlooks
The official dynamical outlooks supercede the statistical outlooks. Statistical outlook maps will continue to be available for a review period:
