Commentary on rainfall probabilities for September to November 2024: based on the phase of the SOI

This commentary is based solely on output from the SOI Phase system, which uses a statistical analysis of phases of the Southern Oscillation Index (SOI) and historical rainfall, to indicate whether rainfall over the coming three-month period is likely to be above or below the long-term median. There are five phases of the SOI, which were defined by Dr Roger Stone using a statistical technique (cluster analysis) to group all sequential two-month pairs of the SOI (from 1882 to 1991) into five clusters (click for more detail).

Based on values of the SOI* over the most recent two months (-5.8 for July and +5.9 for August), the SOI Phase system has classified the current phase as ‘Rapidly Rising’. A map showing the probability of exceeding median rainfall for the next three-month period (September to November), based on a 'Rapidly Rising' phase of the SOI in August, is available. The map is based on 22 previous years from 1889 to 2015 which, like 2024, had a ‘Rapidly Rising’ phase of the SOI in August (i.e., 1901, 1904, 1906, 1908, 1928, 1937, 1942, 1943, 1944, 1945, 1954, 1962, 1964, 1966, 1967, 1970, 1971, 1983, 1985, 2000, 2007 and 2008). The map indicates a higher than normal (60 to 80 per cent) probability of exceeding median September to November rainfall for much central and north-eastern Queensland, with a closer to normal (40 to 60 per cent) probability of exceeding median September to November rainfall for many southern and western parts of the State.

Readers should also note that seasonal outlooks are stated in terms of probabilities. For example, if an outlook were to be stated as having ‘a 20 to 30 per cent probability of exceeding median rainfall’, such a statement should be interpreted as also meaning a 70 to 80 per cent probability of below-median rainfall. In cases where outcomes with a high probability may be more likely, this does not mean that less probable events will not occur in any given year.

Rainfall probabilities produced by the SOI Phase system may differ from those produced for the same period by other climate outlook schemes, due to the different methodologies used. 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 Science Division of the Department of Environment, Science and Innovation (DESI) 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 DESI Monthly Climate Statement for September 2024 is now available.

^{* www.longpaddock.qld.gov.au/soi/soi-data-files (monthly SOI 1887-1989 base period)}