SOI in Near Zero Phase at end of June. Dave McRae, Qld Climate Change Centre of Excellence, 30/06/09.
The monthly value of the SOI for May was minus 4.9 and for June was minus 2.8. This places the SOI in a Consistently Near Zero SOI phase. Based on a Consistently Near Zero SOI phase at the end of June and historical rainfall records there is a 30 to 40% chance of getting above the long term median rainfall for July through to the end of September across most of Queensland (or depending on how you look at things a 60 to 70% chance of getting below median rainfall).
The exception is for the region to the south east of the Gulf of Carpentaria where there is a 50 to 70% chance of getting above median rainfall for July to September. However median rainfall levels at this time of year in that region are low. For example Normanton has a July to September median rainfall of 1mm, Julia Creek has a July to September median rainfall of 6mm and Richmond has a July to September median rainfall of 8mm.
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%.
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 July to September in the following years since 1950 that have had a 'Consistently Near Zero' SOI phase at the end of June; 1954, 1960, 1961, 1969, 1976, 1979, 1980, 1991, 1995 and 1999.
For example at Kingaroy, below average rainfall for July to September in those years was recorded 4 times, close to average rainfall was recorded 4 times and above average rainfall was recorded only 2 times. Therefore rainfall at Kingaroy during July to September 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
It will be interesting to see if strongly negative SOI values develop over coming months. If the 30 day average of the SOI where to fall into strongly negative values and remain there, it would increase the chance of a dry late winter/spring. You can receive a text message with the latest SOI values sent to your mobile phone. To subscribe to this free service, call me on (07) 4688 1459 or e-mail firstname.lastname@example.org