Warmer nights favoured in the south and west; cooler in the northeast

WA Seasonal Temperature Outlook: probabilities for February to April 2011, issued 20th January 2011

Warmer nights favoured in the south and west; cooler in the northeast

The Western Australian temperature outlook, averaged over the February to April period, favours warmer than normal night-time temperatures in the west and south of WA; while cooler than normal night-time temperatures are more likely in the northeast. The outlook over the coming three months indicates an even chance of above or below normal day time temperatures for most of WA.

The pattern of seasonal temperature odds across WA is a result of recent warm conditions in the Indian Ocean as well as cool conditions in the central equatorial Pacific Ocean associated with the current La Niña event.

The chance that the average February to April maximum temperature will exceed the long-term median maximum temperature, is between 40 to 60% across WA, this means that the likelihood of a warmer or cooler than normal season is approximately even (see map).

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

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During the February to April period, history shows the effect on maximum temperatures to be moderately consistent over parts of northern, eastern and southwestern WA, but for the majority of the state the effect is only weakly or 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 February to April minimum temperature will exceed the long-term median minimum temperature is between 60 to 75% across much of western and southern WA. This means that for every ten years with ocean patterns like the current one, about six to seven years are expected to be warmer than average, while three to four years are expected to be cooler.

In contrast, overnight temperatures over the coming three months are more likely to be cooler than normal in northeast WA, with probabilities of exceeding the median between 20 and 40%. Thus the outlook suggests a 60 to 80% chance of lower than normal minimum temperatures.

History shows the oceans' effect on minimum temperatures during the February to April period to be moderately consistent over northeastern WA, as well as parts of the northwest and southwest of the state. The effect is only weakly or very weakly consistent elsewhere.

 

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 24th February 2011

Corresponding rainfall outlook

Maximum temperature departures from average for October to December 2010 - base period 1961-1990

Minimum temperature departures from average for October to December 2010 - 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. 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 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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