Seasonal Climate Outlook Message for May to August 2013

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

The monthly value of the Southern Oscillation Index (SOI) was +8.7 for March and +1.3 for April. 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 April, the probability of above-median rainfall for the next three-month period (May to July) is 40 to 60 per cent for much of Queensland.

Previous years with a ‘Consistently Positive’ SOI phase at the end of April include 1901, 1902, 1903, 1910, 1917, 1921, 1923, 1925, 1927, 1928, 1931, 1935, 1939, 1950, 1956, 1959, 1960, 1963, 1971, 1974, 1975, 2000, 2006, 2008 and 2011. The rainfall probabilities stated above are based on the 1900 to 1998 period and do not include 2000, 2006, 2008 and 2011. 

When using a climate outlook it should be remembered that the probability, or per cent 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.

As the SOI is less reliable as an indicator of rainfall for the autumn season, 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. In the first instance, users in Queensland may wish to consult the monthly climate statement produced by the Department of Science, Information Technology, Innovation and the Arts.

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