National rainfall outlook


Wetter season more likely for much of WA

Text commentary of chance of increased rainfall

Summary

  • A wetter than normal season is more likely for the southern two-thirds of WA and the far southeast of Australia
  • A drier than normal season is more likely for some parts of the northern tropics
  • The chances of a wetter or drier than normal season are roughly equal over the majority of Australia
  • Climate influences include a warming tropical Pacific, and a warm Indian Ocean
  • Outlook accuracy is moderate over large parts of Australia, with low accuracy generally over northwest Australia and parts of outback SA.
Probability of exceeding median rainfall, large image

Details

The chances of exceeding the median rainfall during May to July are greater than 60% over southern and western WA. Chances rise to 65 to 70% over the southwest and Pilbara regions of WA (see map above). For every ten May to July outlooks with similar odds to these, about six or seven of them would result in above-average rainfall over these areas, while about three or four would be below average.

Conversely, the chance of exceeding the median rainfall is less than 40% over the Top End of the NT, and the northern tip of Queensland. In other words, the chances of below average rainfall are greater than 60% over these areas. However, it should be noted that average rainfall at this time of year is far less than is received during the summer months for most of northern Australia (see Rainfall averages tab).

Over the rest of the country, there is no significant shift in the odds towards either a wetter or drier than normal season.

Climate influences

The El Niño-Southern Oscillation (ENSO) currently remains neutral, but is in a state of transition towards El Niño. Dynamical models surveyed by the Bureau indicate that further warming of the tropical Pacific is likely in the coming months, with models approaching or exceeding El Niño thresholds during the southern winter.

Sea surface temperatures in the eastern Indian Ocean, to the south of Australia, and in parts of the Tasman Sea are currently warmer than normal, and are generally expected to remain warm through the forecast period. The warmer waters to the west of the continent may lead to increased evaporation and cloudiness, and hence a wetter than normal outlook for parts of the west. These warmer waters surrounding Australia are not typical of El Niño, and may be somewhat negating the effect of the developing El Niño on the eastern regions.

The Indian Ocean Dipole (IOD) is expected to remain neutral for the next three months, and is therefore unlikely to have a significant influence upon this outlook.

How accurate is the outlook?

Outlook accuracy for the May to July period is:

  • Moderate over western and southern WA, the southern NT and eastern Australia
  • Low to very low over most of eastern WA, the northern half of the NT, and outback SA

Bureau climatologists continually monitor the climate for any significant developments, with information on the likelihood of El Niño/La Niña and IOD events available fortnightly at the ENSO Wrap-Up. For a summary of Pacific and Indian Ocean outlooks, please see the Climate Model Summary.

Further information

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Model accuracy

Model accuracy (also known as model confidence or model skill) is a measure of how well a model has performed at that time of year in the past. One way that the Bureau measures the accuracy of its climate models is by comparing how often the outlook favoured a particular category (for example, when above median rainfall was more likely to occur than below median rainfall), and that favoured category was then subsequently observed. This measurement of accuracy is known as "Percent Consistent", and has been tested for the official seasonal outlook model over the period from 1981 to 2010.

The accuracy levels on the maps give an indication of how well the outlooks match the observed outcomes. High accuracy means that tests of the model on historical data show a strong relationship between the most likely outlook category (above or below median) and the subsequent observation (above or below median). In areas with high accuracy, historically the model has performed very well, and hence a high degree of confidence can be placed in future outlooks. On the other hand, low accuracy means the model has not performed well in these regions and therefore the outlook should be used with caution. 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 random chance (equivalent to the "flip of a coin").

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 climate influence (for example, no El Niño or La Niña is present).

The Rainfall outlook has highest accuracy during autumn and spring, while in summer and winter there is lower skill, particularly over central Australia.

As a guide, the Bureau uses the following terminology when referring to the accuracy of the outlooks:

Map keyTerminology
Legend of the model accuracy map 75% and above - very high
65 to 75% - high
55 to 65% moderate
50 to 55% - low
45 to 50% - low
Below 45% - very low

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.

Median rainfall for January to March

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

Indian Ocean Dipole

Statistical model outlooks

The official dynamical outlooks supercede the statistical outlooks. Statistical outlook maps will continue to be available for a review period: