Understanding probabilities Dave McRae, Qld Climate Change Centre of Excellence, 21/04/09.
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 a Rapidly Falling SOI phase, Pittsworth currently has a 70% chance of getting at least 90 mm during April to June. Therefore Pittsworth has a 30% chance of NOT getting at least 90 mm during April to June.
Another way of looking at this is that 7 times out of 10 historically with the current SOI phase Pittsworth has recorded above 90 mm during April to June. Therefore 3 times out of 10 historically Pittsworth has recorded below 70 mm during April to June.
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.
For more on rainfall probabilities for your location refer to Rainman StreamFlow or www.longpaddock.qld.gov.au
The monthly value of the SOI has fallen from plus 15.2 for February to minus 1.5 for March. This is the lowest monthly value of the SOI since May 2008.
Based on historical rainfall records and a Rapidly Falling SOI phase at the end of March, there is a lower 30 to 50% chance of getting above median rainfall during April through to the end of June across most of Queensland.
In terms of the seasonal outlook for winter and spring it would be worthwhile to watch SOI trends over coming months. For example, if the current fall in value of the SOI where to continue into strongly negative values and remain there through to the end of autumn, it would be a warning sign for a likely dry winter/spring.
As autumn is a key time for the establishment of climate phenomena such as El Nino and La Nina QCCCE climate staff will continue to closely monitor what happens over the next few months. 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.