Long Paddock

Commentary on rainfall probabilities based on phases of the SOI

Previous commentary

Seasonal climate outlook message for October to December 2016

The monthly value of the Southern Oscillation Index (SOI) was +4.67 for August and +13.82 for September. According to the SOI Phase system, the SOI is in a ‘Rapidly Rising’ phase.

A map showing the probability of above-median rainfall for the next three-month period (October to December) is available. This map is based on previous years from 1900 to 1998 which, like 2016, had a ‘Rapidly Rising’ phase of the SOI for September (i.e. 1903, 1908, 1921, 1922, 1926, 1935, 1936, 1949, 1970, 1979, 1983 and 1989). While this map indicates a 40 to 60 per cent probability of above-median October to December rainfall for most of Queensland, it is difficult to draw meaningful statistics from a selection of only 12 years.

It should be noted that a further seven years in the longer term record (1890 to 2015) had a ‘Rapidly Rising’ phase of the SOI at the end of September (i.e.1890, 1899, 2001, 2005, 2009, 2011 and 2012). If these years are also taken into account, the probability of exceeding median rainfall would be close to normal for most of Queensland.

Rather than track the SOI, the Department of Science, Information Technology and Innovation (DSITI) provides outlooks for the summer period (November to March) based on conditions leading up to summer, including the state of the El Niño–Southern Oscillation (ENSO) phenomenon prior to summer, and on factors which alter the impact of ENSO on Queensland rainfall (i.e. the more slowly changing extra-tropical sea surface temperature (SST) pattern in the Pacific Ocean). The DSITI Monthly Climate Statement for October 2016 is available.

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.

Furthermore, while climate outlook schemes cannot provide outlooks with absolute certainty, users who follow a skilful scheme should benefit from doing so in the long-term. Thus, users should consider the historical track record of any scheme, and such information is becoming increasingly available.

Further seasonal climate outlook information for Queensland is available in the Monthly Climate Statement produced by the Department of Science, Information Technology and Innovation.

Last updated 18 March 2011

Seasonal climate outlook