Seasonal Climate Outlook Message for August to November 2011

Commentary on rainfall probabilities based on 'phases' of the SOI

The Southern Oscillation Index (SOI) increased from +0.9 in June to +9.1 in July. According to the SOI Phase system, this places the SOI in a ‘Rapidly Rising’ phase.

Based on previous years when the SOI has been in a 'Rapidly Rising' phase at the end of July, the probability of above-median rainfall for the next three-month period (August to October) is 50 to 70 per cent for most of Queensland, which is higher than normal (50 percent).

In assessing this information, it is worth considering other years with a ‘Rapidly Rising’ SOI phase at the end of July. Since 1950, these include: 1954, 1960, 1963, 1974, 1979, 1984, 1985, 1988, 1995, 1999, 2003 and 2010.

When using a climate forecast it should be remembered that the probability, or percent chance, of something occurring is just that – a probability. For example, if there is a 70 per cent probability of above-median rainfall, then there is also a 30 per cent chance of below-median rainfall. It does not mean that rainfall will be 70 per cent more than the median.

Users are advised not to rely on a single climate outlook alone and should consider the range of climate risk assessment information that is available.

Further seasonal climate outlook information is available at www.longpaddock.qld.gov.au and www.bom.gov.au