Mixed seasonal temperature odds in SE Australia

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

Mixed seasonal temperature odds in SE Australia

The outlook for mean maximum temperatures over the late summer to mid-autumn period (February-April), favours warmer conditions in northeast NSW but cooler conditions for Victoria, Tasmania and a band from northwest to southeast SA.

The pattern of seasonal temperature odds across 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 75% for the northeastern half of NSW (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 two to four would be cooler.

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

In contrast, there is a 60 to 70% chance of cooler than normal days averaged across the season over Tasmania, Victoria, southern and western SA and the southern fringes of NSW. It is important to recognise that even though seasonal mean maximum temperatures in these areas are likely to be cooler than average, this does not rule out the possibility of individual hot days occurring during the late summer to mid-autumn period.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During the period February to April, history shows this effect on maximum temperatures to be moderately consistent over the north and central coasts of NSW, parts of southern SA, southern Victoria and northeast Tasmania. However, over the remainder of NSW, SA, northern Victoria and southern Tasmania the effect is only weakly to very weakly consistent (see background information). In these areas where confidence is not high, caution should be used when interpreting these outlooks.

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 below the long-term median minimum temperature over northern Tasmania, Victoria, and southern areas of both SA and NSW (see map). The chances of increased overnight warmth (averaged over Feb-Apr) are between 25 and 40% in these areas, which means that a cooler than average season has a 60 to 75% chance of occurring. However, this should be viewed with some caution given the varied skill levels of the outlook model in the southeast at this time of year.

History shows the oceans' effect on minimum temperatures during the February to April period to be moderately consistent over the southern half of SA, the northeast and the southern inland of NSW and over small patches in Victoria. However, in other parts of southeastern Australia the effect is generally only weakly or very weakly consistent.

 

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 Monday to Friday from 9.00am to 5.00pm local time by contacting the Bureau's Climate Services sections in Queensland, NSW, SA, Victoria and Tasmania at the following numbers:

Sydney -(02) 9296 1555
Adelaide -(08) 8366 2664
Melbourne -(03) 9669 4949
Hobart -(03) 6221 2043

 

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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