Commentary on rainfall probabilities based on phases of the SOI
Seasonal climate outlook message for May 2015 to July 2015
Commentary on rainfall probabilities based on 'phases' of the SOI
The monthly value of the Southern Oscillation Index (SOI) was -10.7 in March and -3.1 in April. According to the SOI Phase system, this places the SOI in a ‘Consistently Negative’ phase.
Based on previous years when the SOI has been in a ‘Consistently Negative’ phase at the end of April, the probability of above-median rainfall for the next three-month period (May to July) is 40 to 60 per cent for most of Queensland.
Previous years with a ‘Consistently Negative’ SOI phase at the end of April include 1900, 1905, 1915, 1926, 1940, 1941, 1966, 1977, 1980, 1981, 1983, 1987, 1991, 1992 and 1998. The rainfall probabilities stated above are based on the 1900 to 1998 period.
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 SOI values in autumn are least reliable as indicators of rainfall for the coming season. 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 and at www.bom.gov.au.
Last updated 18 March 2011