WA Seasonal Rainfall Outlook: probabilities for February to April 2010, issued 19th January 2010
The outlook for total rainfall over late summer to mid-autumn (February to April) indicates a moderate tendency in the odds towards a wetter than normal three months in northwest and central WA, in contrast to a moderate tendency towards a drier three months in parts of the southwest, and the far northeast Kimberley.
The pattern of seasonal rainfall odds across Australia has been produced using recent Pacific and Indian Ocean temperature patterns, with the warm Pacific (El Niño) having the greater influence on rainfall probabilities in the southwest and far northeast Kimberley.
The chances of exceeding the median rainfall for February to April are between 60 and 70% in a band stretching from the northwest to the interior of WA (see map). This means that for every ten years with ocean patterns like the current, about six or seven February to April periods are expected to be wetter than normal, while three or four are expected to be drier.
In contrast, there is a 35 to 40% chance of above average rainfall for the coming three months in parts of southwest WA and the far northeast Kimberley indicating that drier than normal conditions are more likely. However, totals are normally rather low for this period in southwest WA in any case.
Across the rest of the state, the chances of exceeding the median February to April rainfall are between 40 and 60%, meaning that above average falls are about as equally likely as below average falls.
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 the February to April period, history shows the effect to be moderately consistent over most of WA apart from the far southwest and southern Kimberley where the effect is only weakly or very weakly consistent (see background information).
An El Niño event persists across the Pacific Basin, with most leading climate models suggesting the tropical Pacific will gradually cool during the next three to six months. The influence of El Niño on Australia's rainfall patterns often weakens in the second half of summer - we've already seen heavy rain across parts of inland eastern Australia during December. The SOI is approximately −5 for the 30 days ending 16 January. 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 24th February 2010
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).
© Australian Government, Bureau of Meteorology