National Seasonal Temperature Outlook: probabilities for Autumn 2013, issued 20th February 2013

Cooler autumn for the southeast, warmer for the west and northern tropics

The national outlook averaged over March to May 2013 shows that:

• cooler days and nights are likely over much of southeast Australia, with cooler days also likely over central Australia
• a warmer autumn is likely over western and southern WA
• warmer nights are favoured over Queensland, parts of the NT and northern NSW, while warmer days are favoured for the northern tropics

This outlook is a result of warmer than normal waters in the Indian Ocean with near normal temperatures in the tropical Pacifc Ocean also contributing.

The chances that the autumn maximum temperature will exceed the long-term median maximum temperature are 25 to 40% over a large region on the WA-NT border stretching down through SA, Victoria, NSW and southern Queensland (see map above). Such odds means that for every ten years with similar ocean patterns to those currently observed, about three or four March to May periods would be expected to be warmer than average over these areas, while about six or seven years would be cooler.

In contrast, there is a 60 to 70% chance of warmer than normal days over western and southwestern WA, as well as northern parts of the NT and Queensland.

The chances that the average minimum temperature for autumn will exceed the long-term median minimum temperature are above 60% over western and southern WA, Queensland, northeastern NSW and parts of the northern NT. The highest odds occur over eastern Queensland (see map above).

In contrast, there is a 60 to 75% chance of cooler than normal nights over southern SA, southwestern NSW, Victoria and Tasmania.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During autumn, history shows the effect on maximum temperature to be moderately consistent over the tropics, most of Victoria and Tasmania. Elsewhere, the effect is only weakly to very weakly consistent (see background information).

The effect on minimum temperatures during this season is moderately consistent over most of Australia.

 

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

 

The following climate meteorologists in the National Climate Centre can be contacted about this outlook: Andrew Watkins on (03) 9669 4360, Catherine Ganter on (03) 9669 4679, Elise Chandler on (03) 9669 4748.

 

Regional versions of this media release are available: | Northern Aust | Southeastern Aust | WA |

Regional commentary is available from the Climate Services Sections in the Bureau's Regional Offices:

Queensland -	(07) 3239 8660
New South Wales -	(02) 9296 1555
Victoria -	(03) 9669 4949
Tasmania -	(03) 6221 2043
South Australia -	(08) 8366 2664
Western Australia -	(08) 9263 2222
The Northern Territory -	(08) 8920 3813

 

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 20^th March 2013

Corresponding rainfall outlook

Maximum temperature departures from average for November 2012 to January 2013 - base period 1961-1990

Minimum temperature departures from average for November 2012 to January 2013 - 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)