NSW rainfall odds neutral for spring

The Bureau's spring rainfall outlook shows a moderate swing towards wetter conditions in southwest WA. However odds are near 50:50 for the remainder of the country with no big swings towards a wetter or drier season across any State or Territory.

For the September to November period in NSW, the chances of above median rainfall are mostly between 45 and 55% (see map). So with climate patterns like the current, about 5 seasons out of 10 are expected to be wetter than average across the State, with about 5 out of 10 being drier.

Outlook confidence is related to the influence of Pacific and Indian Ocean temperatures on seasonal rainfall. During spring, history shows this influence to be moderate across the state with slightly stronger influences in the south and weaker influences to the north. Users are urged to exercise caution when applying probabilites in areas where the signal is weak (see background information).

The tropical Indian Ocean is currently warmer than average, whilst the Pacific is showing near normal temperatures. For more detail see the El Niño Wrap-Up. The excess warmth in the Indian Ocean has had the greater influence on the overall pattern of probabilities.

July's value of the Southern Oscillation Index (SOI) was +3, fifteen points above the June value of –12. The approximate SOI for the 30 days ending 16th August was +1.

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 16^th SEPTEMBER 2003.

July 2003 rainfall in historical perspective

May to July 2003 rainfall in historical perspective

• 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/).