Seasonal Climate Outlook Message for September to December 2011

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

Although the most recent monthly value of the SOI (+2.6 in August) was lower than the July value (+9.1), the SOI has nonetheless remained in positive territory for the last two months (average +5.9 for July and August). According to the SOI Phase system, this places the SOI in a ‘Consistently Positive’ phase.

Based on previous years when the SOI has been in a ‘Consistently Positive’ phase at the end of August, the probability of above-median rainfall for the next three-month period (September to November) is 60 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 ‘Consistently Positive’ SOI phase at the end of August. Since 1950, these include: 1950, 1955, 1956, 1958, 1960, 1973, 1974, 1975, 1981, 1988, 1996, 1998 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