Warmer nights favoured over western and central WA

WA Seasonal Temperature Outlook: probabilities for February to April 2012, issued 19th January 2012

Warmer nights favoured over western and central WA

The Western Australian temperature outlook for February to April 2012 shows the following:

  • warmer than normal nights more likely over western and central WA
  • cooler nights more likely over parts of the Kimberley and Interior
  • The drivers of this outlook are a warmer than normal Indian Ocean and cool conditions in the tropical Pacific associated with La Niña.

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

    The chance that the average February to April maximum temperature will exceed the long-term median maximum temperature is about 50% over almost all of WA (see map above). This means that the chances of a warmer or cooler February to April are roughly equal. However, there is a 35% to 40% chance of warmer than normal days in a small area in the northeast Interior of WA. In other words, there is a 60% to 65% chance of cooler days over this region.

    Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During the February to April period, history shows this effect on maximum temperatures to be moderately consistent over northern WA, and large parts of eastern WA. Elsewhere, the effect is only weakly to very weakly consistent (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 February to April will exceed the long-term median minimum temperature is between 60% and 80% in western and central WA, with probabilities over 80% in parts of the inland Gascoyne. Conversely, there is a 25 to 40% chance of warmer than normal nights over large parts of the Kimberley and the adjacent northern Interior. In other words, there is a 60% to 75% chance of cooler nights over this region.

    History shows the oceans' effect on minimum temperatures during the February to April period to be moderately consistent over much of WA, apart from areas in central and eastern WA where skill is only weakly to very weakly consistent. Users should exercise caution when using this outlook in these areas of low skill.

     

    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.

     

    THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 23rd February 2012

    Corresponding rainfall outlook

    Maximum temperature departures from average for October to December 2011 - base period 1961-1990

    Minimum temperature departures from average for October to December 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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