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Commentary on rainfall probabilities based on phases of the SOI

Previous commentary

Seasonal climate outlook message for September to November 2017

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

The monthly value of the Southern Oscillation Index (SOI) was +6.5 for July and +3.5 for August. According to the SOI Phase system, the SOI is in a ‘Consistently Positive’ phase.

A map showing the probability of exceeding-median rainfall for the next three-month period (September to November) is available. This map is based on previous years from 1900 to 1998 which, like 2017, had a ‘Consistently Positive’ phase of the SOI for August (i.e. 1900, 1909, 1910, 1915, 1916, 1917, 1920, 1924, 1938, 1947, 1950, 1955, 1956, 1958, 1960, 1973, 1974, 1975, 1981, 1988, 1996 and 1998). This map indicates a 60 to 70 per cent probability of exceeding median September to November rainfall for most of Queensland.

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 exceeding median rainfall, then there is also a 30 per cent probability 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.

The Department of Science, Information Technology and Innovation (DSITI) provides outlooks for the summer period (November to March). The outlooks for summer rainfall are based on conditions leading up to summer, including the state of the El Niño–Southern Oscillation (ENSO) phenomenon and 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 September 2017 is available.

Last updated 18 March 2011

Seasonal climate outlook