SOI continues to fluctuate. Dave McRae, Queensland Climate Change Centre of Excellence, 02/06/09.
The monthly value of the SOI dropped in value from plus 8.0 for April to minus 5.0 for May placing the SOI in a Rapidly Falling phase. An analysis of historical rainfall records indicates that a Rapidly Falling SOI phase at the end of May increases the risk of drier than normal conditions along the central and northern coastal strip of Queensland and throughout much of the southern third of the state.
Generally throughout these regions there is a 20 to 40% chance of getting above median rainfall for June to August (or depending on how you like to look at things a 60 to 80% chance of getting below median rainfall for June to August). Throughout the rest of the state, the chance of getting above the long term June to August median rainfall is generally between 40 to 60%. For more information on rainfall probabilities for your specific location refer to Rainman Streamflow.
While fluctuations in the value of the SOI are not unusual at this time of year (during the autumn predictability barrier) it will be interesting to see if this downward trend of the SOI is maintained. If the 30 day average of the SOI where to remain in strongly negative values, it would be a warning sign for a likely dry winter/spring.
For those who like to follow the relationship between the SOI and rainfall patterns in more detail have a look at what happened in your area during June to August in the following years since 1900 that have had a 'Rapidly Falling' SOI phase at the end of May; 1911, 1925, 1928, 1929, 1934, 1935, 1947, 1953, 1957, 1958, 1972, 1999, 2001, 2002, 2006 and 2008.
For example at Toowoomba, below average rainfall for June to August in those years was recorded 6 times, close to average rainfall was recorded 8 times and above average rainfall was recorded only 2 times. Therefore rainfall at Toowoomba during June to August is more likely to be below average to average than well above average. For more information on historical rainfall data for your region try Rainman Streamflow or www.longpaddock.qld.gov.au
When using a climate forecast you should remember that the probability or percent chance of something occurring is just that - a probability. For example if there is a 70% chance of recording more than 100 mm there is also a 30% chance of recording less than 100 mm i.e. 70-30; 30-70. It does not mean that you will get 70% more than 100 mm or 100 mm plus another 70%.
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