SOI in Rapidly Falling Phase David McRae 31/10/06

The monthly value of the SOI dropped in value from minus 4.6 for September to minus 16.4 for October which places the SOI in a "Rapidly Falling" phase. Based on this phase and historical rainfall records there is a 10 to 40% chance of getting above median rainfall for November through to the end of December throughout most of the central and northern Queensland. For the southern third of the state there is a slightly higher 20 to 50% chance of getting above median rainfall.

For example Ayr has a low 18% chance of getting above its November to January median rainfall of 360 mm, Winton has a 25% chance of getting above its November to January median rainfall of 130 mm, Mackay has a 27% chance of getting above its November to January median rainfall of 500 mm, Wyandra has a 45% chance of getting above its November to January median rainfall of 100 mm and both Roma and Gatton have a 55% chance of getting above their November to January median rainfall of 190 mm and 255 mm respectively.

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 example based on historical rainfall figures and the current SOI phase Bowen has a 30% chance of getting its long term November to January median rainfall of 300 mm. Therefore Bowen has a 70% chance of NOT getting its November to January median rainfall of 300 mm. Another way of looking at this is that 3 times out of 10 historically with the current SOI phase Bowen has recorded above 300 mm during November to January. Therefore 7 times out of 10 historically Bowen has recorded below 300 mm during November to January.

When looking at the seasonal outlook for your area it may make it easier to think of rainfall probabilities in these terms. Probabilities above 80% equal a high chance, probabilities above 60% equal an above average chance, probabilities below 40% equal a below average chance and probabilities below 20% equal a very low chance of that occurring.

So currently Bowen has a below average chance of recording more than 300 mm during November to January. For more on rainfall probabilities for your location refer to Rainman StreamFlow. For those areas with a low or below average chance of getting above median rainfall it does not mean there will be no rainfall. What it does mean is that rainfall will more likely be below median to median rather than well above median. For the northern half of Australia there is still a reasonable chance of getting some useful relief rain during our spring/summer rainfall season.

The latest probability maps are at www.dpi.qld.gov.au/climate or www.longpaddock.qld.gov.au and daily updates on the SOI are available on (07) 46881439. For more on rainfall probabilities for your location refer to Rainman StreamFlow or contact 132523 or (07) 3404 6999.

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 November to January in the following years that have had a 'Rapidly Falling' SOI phase at the end of October; 1915, 1920, 1925, 1931, 1941, 1944, 1947, 1963, 1978, 1981 and 1992.

For example at Emerald, below average rainfall for November to January in those years was recorded 6 times, close to average rainfall was recorded 4 times with above average rainfall recorded once. Therefore rainfall during November to January at Emerald is more likely to be below average to average than well above average. Based on the current position of the MJO it would be reasonable to next expect it in early to mid November. Usually during summer stronger MJO signals can be expected. The MJO is a band of low air pressure originating off the east coast of central Africa travelling eastward across the Indian Ocean and northern Australia roughly every 30 to 60 days. Research has shown the MJO to be a useful indicator of the timing of potential rainfall events (but not amounts). For more information try www.apsru.gov.au/mjo/

Current rainfall probability maps are at www.dpi.qld.gov.au/climate or www.longpaddock.qld.gov.au and SOI updates are available on (07) 46881439. For more detailed information for your location refer to Rainman StreamFlow. For other enquires contact the DPI&F on 132523.