Equal chance of above or below average rainfall for WA

WA Seasonal Rainfall Outlook: probabilities for July to September 2010, issued 23rd June 2010

Equal chance of above or below average rainfall for WA

The Western Australian outlook for total rainfall over the September quarter (July to September), shows equal odds favouring above and below average rainfall.

The distribution of seasonal rainfall odds across Australia is due to recent patterns of higher than average temperatures in both the Pacific (decaying El Niño) and Indian Oceans, though only marginal influence is being exerted by these ocean basins on the rainfall outlook for WA.

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

For the July to September period, the chance of exceeding median rainfall is between 40 and 55% across most of WA, meaning above average falls are equally as likely as below average falls. This means that for every ten years with ocean patterns like the current, about five years would be expected to be wetter than average during the September quarter, with about five being drier.

A small part of the east Kimberley shows the probability of exceeding the median seasonal rainfall to be between 35 and 40%, however July-September is typically a dry time of year in this region.

An expanded set of seasonal rainfall outlook maps and tables, including the probabilities of seasonal rainfall exceeding given totals (e.g. 200 mm), is available on the "Water and the Land" (WATL) part of the Bureau's website.

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 a large part of WA, reaching moderate only in patches (see background information). Therefore, caution should be used when applying this outlook in southwest WA.

Most leading climate models indicate further cooling of the equatorial Pacific during the coming months, with the possible development of La Niña conditions by late winter or spring. The SOI is approximately +5 for the 30 days ending 20th June. For routine updates and comprehensive discussion on any developments regarding El Niño and La Niña, please see the 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 22nd July 2010

Corresponding temperature outlook

May 2010 rainfall in historical perspective

March to May 2010 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 (Seasonal Climate Outlook Products).

  • 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 25 El Niño events since 1900 are summarized on the Bureau's web site (El Niño - Detailed Australian Analysis).

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