Warmer nights favoured in WA, but cooler days likely in western parts

WA Seasonal Temperature Outlook: probabilities for January to March 2012, issued 20th December 2011

Warmer nights favoured in WA, but cooler days likely in western parts

The Western Australian temperature outlook for January to March 2012 shows the following:

  • cooler days are more likely over western WA
  • warmer nights are more likely over much of WA
  • A persistently warm Indian Ocean and cool conditions in the tropical Pacific Ocean associated with the La Niña are driving this outlook.

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

    The chance that the average January to March maximum temperature will exceed the long-term median maximum temperature is between 30 to 40% over western WA (see map). In other words, there is a 60 to 70% chance of cooler days over this region. This means that for every ten years with ocean patterns like the ones currently observed, about three or four January to March periods would be expected to be warmer than average in these areas, with about six or seven being cooler.

    Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During the January to March period, history shows this effect on maximum temperatures to be moderately consistent over parts of western, central and southern WA. The effect is only weakly to very weakly consistent near the WA west coast, and in eastern parts of the state (see background information).

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

    The chance that the average minimum temperature for January to March will exceed the long-term median minimum temperature is 60 to 70% across most of WA, grading to 70 to 80% in the Kimberley and northern Interior (see map above).

    History shows the oceans' effect on minimum temperatures during the January to March period to be moderately consistent over most of northern and eastern WA, but only weakly to very weakly consistent in central and southwest parts of the state.


    Click on the maps above for larger versions of the maps. Use the reload/refresh button to ensure the latest forecast maps are 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.



    Corresponding rainfall outlook

    Maximum temperature departures from average for September to November 2011 - base period 1961-1990

    Minimum temperature departures from average for September to November 2011 - base period 1961-1990


    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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