Western Australian rainfall outlook
Wetter season likely for northern and eastern WA
Summary
- A wetter than normal season is more likely for much of northern and eastern WA
- Chances of above or below normal rainfall are roughly equal over western and southern WA
- Climate influences include a warmer than normal eastern Indian Ocean, a neutral tropical Pacific, and warm local sea surface temperatures
- Outlook skill is moderate over southern WA, with northern WA currently in its dry season.
Details
The chances of receiving above median rainfall during the June to August period are 60 to 70% in the Kimberley, adjacent east Pilbara, and Northern Interior (see map above). Such odds mean that for every ten years with similar climate patterns to those currently observed, about six or seven years would be expected to be wetter than average over these areas, while about three or four years would be drier. However, it should be noted that rainfall is commonly low over these areas at this time of year and even a small amount of rain would exceed the median.
Over southern and western parts of WA, the chances of a drier or wetter winter are roughly equal.
Climate influences
The tropical Pacific has remained ENSO-neutral since mid-2012. Climate models surveyed by the Bureau of Meteorology suggest sea surface temperatures in the tropical Pacific Ocean are likely to stay on the cool side of neutral during winter 2013.
Four of five international models surveyed by the Bureau favour the development of a negative Indian Ocean Dipole (IOD) event sometime during winter-spring 2013. A negative IOD during winter-spring increases the chances of above normal rainfall over southern Australia.
Warmer than normal ocean surface temperatures currently surround much of the continent. Warmer ocean temperatures can provide more moisture to the atmosphere, which in favourable weather conditions (e.g., interactions with fronts or north-west cloudbands) may result in increased rainfall.
How accurate is the outlook?
Outlook confidence is related to how consistently the oceans and broadscale climate affect Australian rainfall. During June to August, model accuracy shows the outlook to be moderately consistent over the southern half of WA. Elsewhere, the effect is only weakly to very weakly consistent (see further details below).
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 given 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:
