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Commentary on rainfall probabilities based on phases of the SOI

Previous commentary

Seasonal climate outlook message for November 2016 to December 2017

The monthly value of the Southern Oscillation Index (SOI) was +13.82 for September and -4.51 for October. According to the SOI Phase system, the SOI is in a ‘Rapidly Falling’ phase.

A map showing the probability of above-median rainfall for the next three-month period (November to January) is available. This map is based on previous years from 1900 to 1998 which, like 2016, had a ‘Rapidly Falling’ phase of the SOI for October (i.e. 1915, 1920, 1925, 1941, 1944, 1947, 1963, 1981 and 1992). While this map indicates a 30 to 50 per cent probability of above-median November to January rainfall for most of Queensland, it is difficult to draw meaningful statistics from a selection of only nine years. Furthermore, four of the nine years were influenced by an El Niño climate pattern, whereas conditions are currently described as a weak La Niña climate pattern. If the El Niño years are omitted, then the probability of exceeding median November to January rainfall would be closer to normal for most of Queensland.

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 November 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