Increased chance of storm activity.
Dave McRae, Qld Climate Change Centre of Excellence, 9th October 2007.
Based on a shift in monthly SOI values from plus 1.8 for August to plus 2.2 for September the SOI is in a "Near Zero" phase. An analysis of historical rainfall data and this SOI phase indicate the chances of getting the long term median rainfall for October to December are close to 40 to 50% for most of Australia.
Further analysis indicates rainfall for Queensland is more likely to be close to the long term average (or middle third) rather than well above or below average. So while the chance of getting above median rainfall is not high there remains a quite reasonable chance of getting some rainfall. The current outlook is also an improvement on this time last year when much of the state had as low as a 10% chance of getting median rainfall.
Interestingly a Near Zero SOI phase at this time of year is also associated with an increased chance of hail and thunderstorm activity throughout northern NSW and southern Queensland. This is due to the increased instability of the middle atmosphere (600mb to 500mb) combined with cool air at those levels.
When the SOI is in a Consistently Positive phase at this time of year the middle atmosphere is generally warm and moist reducing the opportunity for severe storm activity but favouring more general widespread rain and tropical cyclones. If the SOI is in a Consistently Negative phase this is generally associated with a drier middle atmosphere reducing the potential for severe storm activity.
The 30 day average of the SOI as of the 8th October is plus 4.2. Again it would help if the SOI went 'Consistently Positive' (say above plus 7.0) and remained there for a number of months. The last time the SOI was in a "Consistently Positive" phase for more than one month was from September 2000 to March 2001.
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)468814