Commentary on rainfall probabilities based on 'phases' of the SOI
The monthly value of the Southern Oscillation Index (SOI) was +2.7 for May and -6.3 for June. 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 (July to September) 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 June (i.e. 1903, 1904, 1906, 1918, 1930, 1933, 1941, 1943, 1948, 1949, 1959, 1963, 1965, 1982, 1983, 1984, 1985, 1988, 1992, 2000, 2003, 2004, 2012 and 2017). This map indicates a 40 to 60 per cent probability of above-median rainfall for much of Queensland. However, the probability of exceeding median July to September rainfall is lower than 40 percent for some northern and central parts of the state.
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 July 2018 is now available.