WA Seasonal Rainfall Outlook: probabilities for July to September 2008, issued 26th June 2008

Drier conditions more likely from northwest to southeast WA

The Western Australian outlook for total rainfall during the September quarter shows the odds favouring drier than average conditions over a large area from northwest to southeast WA.

The pattern of seasonal rainfall odds across WA is a result of higher than average temperatures in the Indian Ocean surrounding the west coast of WA, and a warming trend in recent months over the equatorial Pacific.

probability of exceeding median rainfall - click on the map for a larger version of the map

The chances of exceeding the median rainfall over July to September are between 25 and 40% across a large area of WA, extending from the northwest to the southeast. In other words, there is a 60 to 75 % chance of below median rainfall in these parts (see map).

So for every ten years with ocean patterns like the current, about six or seven July to September periods are expected to be drier than average across this band, while about three or four are wetter. It should be noted, however, that this is a seasonally drier time of year in northwest WA.

Over the rest of the state, the chances of exceeding the seasonal median rainfall are 40 to 50%, meaning the odds of being wetter than normal are almost the same as the odds of being drier.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During July to September, history shows this effect to be generally weakly consistent across the state, reaching moderate in patches (see background information). This outlook needs to be used with caution in areas where skill is lower.

The equatorial Pacific Ocean has been gradually warming during autumn and early winter resulting in the decay of the 2007/08 La Niña event, with neutral conditions currently prevailing. Computer models indicate a continuation of the warming trend, with neutral conditions the most likely outcome during the forecast period. The 30-day value of the Southern Oscillation Index (SOI) was zero as at 23rd June. For routine updates and comprehensive discussion on any developments please see the ENSO Wrap-Up.

 

Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest forecast map is displayed.

 
More information on this outlook is available from 8.30am to 4.30pm (WST) Monday to Friday by contacting the Climate Services Centre in the Bureau's Perth Office: (08) 9263 2222.
 

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 25th July 2008

Corresponding temperature outlook

May 2008 rainfall in historical perspective

March to May 2008 rainfall in historical perspective

 

Background Information

  • The Bureau's seasonal outlooks are general statements about the probability or risk of wetter or drier than average weather over a three-month period. The outlooks are based on the statistics of chance (the odds) taken from Australian rainfall/temperatures and sea surface temperature records for the tropical Pacific and Indian Oceans. They are not, however, categorical predictions about future rainfall, and they are not about rainfall within individual months of the three-month outlook period. The temperature outlooks are for the average maximum and minimum temperatures for the entire three-month outlook period. Information about whether individual days or weeks may be unusually hot or cold, is unavailable.

  • This outlook is a summary. More detail is available from the contact people or from SILO (www.bom.gov.au/silo/products/SClimate.shtml).

  • Probability outlooks should not be used as if they were categorical forecasts. More on probabilities is contained in the booklet The Seasonal Climate Outlook - What it is and how to use it, available from the National Climate Centre. These outlooks should be used as a tool in risk management and decision making. The benefits accrue from long-term use, say over 10 years. At any given time, the probabilities may seem inaccurate, but taken over several years, the advantages of taking account of the risks should outweigh the disadvantages. For more information on the use of probabilities, farmers could contact their local departments of agriculture or primary industry.

  • Model Consistency and Outlook Confidence: Strong consistency means that tests of the model on historical data show a high correlation between the most likely outlook category (above/below median) and the verifying observation (above/below median). In this situation relatively high confidence can be placed in the outlook probabilities. Low consistency means the historical relationship, and therefore outlook confidence, is weak. 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 or guessing. The rainfall outlooks perform best in eastern and northern Australia between July and January, but are less useful in autumn and in the west of the continent. The skill at predicting seasonal maximum temperature peaks in early winter and drops off marginally during the second half of the year. The lowest point in skill occurs in early autumn. The skill at predicting seasonal minimum temperature peaks in late autumn and again in mid-spring. There are also two distinct periods when the skill is lowest - namely late summer and mid-winter. However, it must always be remembered that the outlooks are statements of chance or risk. For example, if you were told there was a 50:50 chance of a horse winning a race but it ran second, the original assessment of a 50:50 chance could still have been correct.

  • The Southern Oscillation Index (SOI) is calculated using the barometric pressure difference between Tahiti and Darwin. The SOI is one indicator of the stage of El Niño or La Niña events in the tropical Pacific Ocean. It is best considered in conjunction with sea-surface temperatures, which form the basis of the outlooks. A moderate to strongly negative SOI (persistently below –10) is usually characteristic of El Niño, which is often associated with below average rainfall over eastern Australia, and a weaker than normal monsoon in the north. A moderate to strongly positive SOI (persistently above +10) is usually characteristic of La Niña, which is often associated with above average rainfall over parts of tropical and eastern Australia, and an earlier than normal start to the northern monsoon season. The Australian impacts of 23 El Niño events since 1900 are summarized on the Bureau's web site (www.bom.gov.au/climate/enso/).