Wet season favoured in the northeast and southwest

WA Seasonal Rainfall Outlook: probabilities for November 2011 to January 2012, issued 25th October 2011

Wet season favoured in the northeast and southwest

The Western Australian outlook for November to January shows the following:

  • a wetter season is more likely across much of the Kimberley
  • a wetter season is more likely in southwest WA, however rainfall during the November to January period is typically low.
  • A persistently warm Indian Ocean is the main driver behind this outlook although it is also consistent with the developing La Niña in the Pacific Ocean.

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

    The chances of receiving above median rainfall during the November to January period are between 60 and 80% over northeast WA (see map). Similar probabilities are evident in southwest WA, however rainfall totals during the coming three months are typically low, and confidence in the outlook model at this time of year is variable in this region. Such odds mean that for every ten years with similar ocean patterns to those currently observed, about six to eight November to January periods would be expected to be wetter than average over these areas, while about two to four would be drier.

    For the remainder of WA, in northwest, central and southeastern parts, the chances of a drier or wetter November to January are roughly equal.

    An expanded set of seasonal rainfall outlook maps and tables, including the probabilities of seasonal rainfall exceeding given totals (e.g. chance of receiving at least 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 November to January period, history shows the effect to be moderately consistent over northeast and western parts of WA, but only weakly to very weakly consistent in central and southwest parts of the state (see background information).

    Conditions in the tropical Pacific are consistent with the early stages of a late-forming La Niña event. The majority of leading climate models predict current patterns and trends will continue. For routine updates and comprehensive discussion on any developments regarding El Niño and La Niña, 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 22nd November 2011

    Corresponding temperature outlook

    September 2011 rainfall in historical perspective

    July to September 2011 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.

    • Probability outlooks should not be used as if they were categorical forecasts. 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 past El Niño events since 1900 are summarized on the Bureau's web site (El Niño - Detailed Australian Analysis), and past La Niña events (La Niña - Detailed Australian Analysis)

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