21st October 2008
SOI remains in positive values
Based on the shift in monthly values of the SOI from plus 8.0 for August to plus 13.7 for September the SOI remains in a "Consistently Positive" phase.
Analysis of this SOI phase and historical rainfall data indicate a 50 to 70% chance of getting above the long term median rainfall for October to December throughout eastern Australia.
Generally for the rest of Australia there is a 40 to 60% chance of getting above the long term median rainfall for October to December. The exceptions are for some regions of southern Australia including western Tasmania where there is a lower 30 to 40% chance of getting above the long term median rainfall.
Another way of viewing this is that rainfall throughout most of Australia during October to December is more likely to be close to or above the long term average (or middle third to upper third) rather than well below average.
So while the chance of getting well above average rainfall is not as high as many would like there remains a quite reasonable chance of getting average summer rainfall. The current outlook is also an improvement when compared to the October to December outlooks over the last few years.
Interestingly if the SOI is in a Consistently Positive phase at this time of year the middle level atmosphere is generally warm and moist which reduces the opportunity for severe storm activity. These conditions favour more general widespread rain and tropical cyclones. This does not mean there will be no storms, as evident by the storm activity already experienced throughout southern, central and south-east Queensland over the past few weeks.
In contrast, a Near Zero SOI phase at this time of year is associated with increased severe hail and thunderstorm activity throughout northern NSW and southern Queensland. This is due to the increased instability of the middle atmosphere due to the cool air found at those levels.
A Consistently Negative SOI phase at this time of year is generally associated with a drier middle atmosphere reducing the potential for both severe storm activity and general rain.
The 30 day average of the SOI as of the 21st October was plus 13.5. As often stated it is beneficial to the overall general outlook if the SOI remains in 'Consistently Positive' values (say above plus 7.0) for a number of months.
For those interested daily updates on the SOI are available on (07) 46881439 or at www.longpaddock.qld.gov.au. You can also receive a text message with the latest SOI values sent to your mobile phone: to subscribe call (07) 4688 1588.