Commentary on rainfall probabilities based on phases of the SOI
Seasonal climate outlook message for June 2015 to August 2015
Commentary on rainfall probabilities based on 'phases' of the SOI
The monthly value of the Southern Oscillation Index (SOI) was -3.1 in April and -13.1 in May. According to the SOI Phase system, this places the SOI in a ‘Rapidly Falling’ phase.
Based on previous years when the SOI has been in a ‘Rapidly Falling’ phase at the end of May, the probability of above-median rainfall for the next three-month period (June to August) is 40 to 50 per cent for much of Queensland. For some inland southern areas of the state, the probability of above-median June to August rainfall is less than 40 per cent.
Previous years with a ‘Rapidly Falling’ SOI phase at the end of May include 1911, 1925, 1928, 1929, 1934, 1935, 1947, 1953, 1957, 1958, 1972, 1999, 2001, 2002, 2006, 2008, 2009 and 2011. The rainfall probabilities stated above are based on the 1900 to 1998 period (that does not include 1999, 2001, 2002, 2006, 2008, 2009 and 2011).
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