Warmer days for west and far north over late summer to mid-autumn

WA Seasonal Temperature Outlook: probabilities for February to April 2010, issued 19th January 2010

Warmer days for west and far north over late summer to mid-autumn

The WA outlook for mean maximum temperatures over the late summer to mid-autumn period (February-April), shows a moderate tendency in the odds favouring above average values in the west of WA and in the north Kimberley.

The pattern of seasonal temperature odds across Western Australia is due to higher than average temperatures in both the Pacific (El Niño) and Indian Oceans, with the Pacific influence being dominant.

The chance that the average February-April maximum temperature will exceed the long-term median maximum temperature, is between 60 and 70% in the north Kimberley and between 60 and 65% in western WA (see map). This means that for every ten years with ocean patterns like the current, about six or seven February to April periods would be expected to be warmer than average over these areas, while about three or four would be cooler.

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

In remaining areas, the chances are between 40 and 60% which indicates that a warmer than average season is about as likely as a cooler than average one.

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 only weakly or very weakly consistent for western parts of the state and in much of the Kimberley (see background information). Caution is therefore advised when using this outlook over these areas. Through central WA, the effect is moderately consistent.

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

The average minimum temperature for February to April is favoured to be above the long-term median minimum temperature over much of WA, apart from in the southeast (see map). The chances of increased overnight warmth (averaged over Feb-Apr) are between 70 and 80% in the northeast, grading to 60 to 65% for the remainder of the state, apart from in the south and along the west coast where chances are about even for above or below normal minimum temperature for the coming three months.

History shows the oceans' effect on minimum temperatures during the February to April period to be moderately consistent in the north and southwest, tending weakly or very weakly consistent over some central and southeastern parts.

 

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 2010

Corresponding rainfall outlook

Maximum temperature departures from average for October to December 2009 - base period 1961-1990

Minimum temperature departures from average for October to December 2009 - 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 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).

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