Long Paddock

Commentary on rainfall probabilities based on phases of the SOI

Previous commentary

Seasonal climate outlook message for May to July 2017

The monthly value of the Southern Oscillation Index (SOI) was +3.8 for March and -6.3 for April. According to the SOI Phase system, the SOI is in a ‘Rapidly Falling’ phase.

A map showing the probability of exceeding-median rainfall for the next three-month period (May to July) is available. This map is based on previous years from 1900 to 1998 which, like 2017, had a ‘Rapidly Falling’ phase of the SOI for April (i.e. 1909, 1912, 1914, 1922, 1924, 1944, 1945, 1947, 1952, 1965, 1967, 1969, 1976, 1993, 1994, 1995 and 1997). This map indicates a 30 to 50 per cent probability of exceeding median May to July rainfall for much 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 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 should note that the SOI in autumn has low reliability as an indicator of rainfall. 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) also 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 May 2017 is available.

Last updated 18 March 2011

Seasonal climate outlook