Mixed rainfall odds for late winter to mid spring

The national outlook for total August to October rainfall shows moderate swings in the odds towards below-normal rainfall in central WA and also in a band extending from northern Queensland through to central SA. In contrast, higher than average rainfall is favoured in the southwest of WA.

The pattern of seasonal rainfall odds across Australia is a result of a continued strong warming trend in the Indian Ocean off the west coast of WA, and a warming trend in recent months over the equatorial Pacific Ocean.

The chances of exceeding the median rainfall over the August to October period are between 30 and 40% over much of central WA. Below average rainfall is similarly favoured in a band stretching from central SA to the north of Queensland (see map). This means the chances of below average rainfall are typically between 60 and 70% in these areas

So in years with ocean patterns like the current, about six to seven August to October periods out of ten are expected to be drier than average in these parts of Australia, while three to four are wetter. In addition, confidence is not high through much of SA at this time of year so the outlook needs to be used with caution in this region. In the southwest of WA the chances of exceeding the median rainfall are between 60 and 70% for the August to October period.

Over the rest of the nation, late-winter to mid spring rainfall totals have a 40 to 60% chance of exceeding the three-month median. So the chances of being wetter than normal are about the same as the chances of being drier.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During August to October, history shows this effect to be moderately consistent across most of Queensland, the east and north of the NT and the northern inland of NSW. Elsewhere, it is generally weakly consistent, reaching moderate only in patches, including Northern Tasmania (see background information).

The 30-day value of the Southern Oscillation Index (SOI) was +3 as at 22 July. Computer models indicate a continuation of neutral conditions across the equatorial Pacific Ocean as the most likely outcome during the forecast period. 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.

Regional versions of this media release are available: | Northern Aust | Southeastern Aust | WA |

Regional commentary is available from the Climate Services Sections in the Bureau's Regional Offices:

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 27^th August 2008

Corresponding temperature outlook

June 2008 rainfall in historical perspective

April to June 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/).