Commentary on rainfall probabilities based on 'phases' of the SOI
The monthly value of the Southern Oscillation Index (SOI) was +1.8 for July and -6.7 for August. 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 (September to November) is now available. This map is based on previous years from 1889 to 2015 which, like 2018, had a ‘Rapidly Falling’ phase of the SOI for August (i.e. 1902, 1921, 1934, 1936, 1953, 1957, 1979, 1986, 1989, 1991, 1997, 2002, 2005 and 2014). This map indicates a 30 to 50 per cent probability of above-median rainfall for much of Queensland, with lower probabilities of exceeding median rainfall for parts of central 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 Environment and Science (DES) 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 pattern in the Pacific Ocean). The DES Monthly Climate Statement for September 2018 is now available.