Reduced winter rainfall more likely in southwest WA

There is a moderate shift in the odds towards below average winter rainfall across much of southwest WA, the Bureau of Meteorology announced today. However, on a cautionary note, the skill of the outlook method is generally low (see below) in this part of WA, so this outlook should be used with caution.

The pattern of winter rainfall odds is a result of above average temperatures in both the Pacific and Indian Oceans. Across southwest WA and southeastern Australia, the warm oceans have a reinforcing effect with the odds tilted in favour of a drier than average winter. However, across southern Queensland and northern NSW, the effects from the two oceans tend to cancel each other, with drier conditions promoted by the warm Pacific and wetter conditions by the warm Indian.

So for the June to August period, the chances of above median rainfall are between 35 and 40% over much of the southwest corner of WA (see map). This means that below average falls have a 60 to 65% chance of occurring. So in years with ocean patterns like the current, about six winters out of ten are expected to be drier than average in this part of the State, with about four out of ten being wetter.

For the rest of WA, the chances of accumulating at least average rain during winter are between 40 and 50%. It should be remembered though, that parts of far northern WA are seasonally dry in winter, and significant falls of rain are uncommon.

Outlook confidence is related to the influence of Pacific and Indian Ocean temperatures on seasonal rainfall. During winter, history shows this influence to be mainly weakly or very weakly consistent over the southern two-thirds of WA, and moderate in patches across the Kimberley (see background information).

The Southern Oscillation Index (SOI) has shown a high level of volatility recently with values of –11, zero and –29 for April, March and February respectively. The approximate SOI for the 30 days ending 14th May was –10.

Low SOI values and warming in the Pacific Ocean over the past three months have increased the chances of an El Niño developing in 2005. However, it is still too early to predict what might happen in the Pacific, with any confidence. For routine updates and comprehensive discussion on the latest data relating to El Niño, together with details on what the phenomenon is and how it has affected Australia in the past, 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.

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 17^th JUNE 2005

Corresponding temperature outlook

April 2005 rainfall in historical perspective

February to April 2005 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/).