Seasonal Outlooks are often interpreted as a clear indicator of expected conditions for the next 3 months when in fact the Seasonal Outlook uses probability to describe likely conditions during the outlook period.
The Seasonal Outlook is created by comparing current conditions across the Pacific and Indian Ocean against similar events in the past. We use these similar historical events as a guide to make broad predictions of temperature and rainfall in the next 3 months. In other words if the conditions we measure now are similar to 2 or 3 years in our historical record we can use those historical records as a basis for what may occur in the next 3 months.
This method can only provide broad probability forecasts. As a result the outlooks can never be used to definitively state conditions in the next 3 months. Rather the outlooks give guidance on the likelihood, or chance of certain conditions occurring.
You can see the full text of the seasonal outlooks for South Australia at www.bom.gov.au/climate/
Rainfall Outlook
The February to April 2010 rainfall outlook for South Australia indicates southern parts of the state have a 55 to 60% probability of exceeding the median rainfall. This does NOT indicate a wet 3 months ahead. It simply means wetter than normal conditions are slightly more likely over that period.
- 55 to 60% probability of above median falls means there is still a 40 to 45% chance of median falls or less.
- The seasonal outlook does not indicate whether rainfall will be a small amount or a great amount above median.
- The seasonal outlook spreads over a 3 month period and may comprise shorter bursts of wet or dry periods.
- February and March are generally dry months so relatively small falls in this period can determine whether above median rainfall occurs in the 3 month period.
- Prediction skill, or reliability, varies through the year and across the state. At this time of year the product shows moderate skill across parts of the southern agricultural areas of the state, but lower skill in other parts of the state. Use the product with caution in low skill areas.
One way of interpreting a 60% chance of exceeding median rainfall is locations can expect above median rainfall about 6 in every 10 years. That leaves 4 in every 10 years with a likelihood of median rainfall or less.
Temperature Outlook
The February to April 2010 temperature outlook for South Australia indicates a 30 to 40% chance of above median maximum temperatures for southern agricultural areas. Similarly, there is a 25 to 40% chance of above median minimum temperatures across a similar area for the same period. The outlook does NOT indicate a cool 3 months. It simply means that there is a higher chance of cooler than normal temperatures.
- The strong temperature signal is matched by relatively high skill in temperature prediction for the February to April period.
- There will still be periods of hotter weather and periods of cooler weather through the 3 months between February to April.
- Seasonally there will be a gradual cooling in conditions as we head into Autumn.
Alternative ways of looking at the data can be found in the temperature and rainfall sections of Water and the Land
Median is the midpoint of the dataset. In this case the median is similar to the average, mean or normal.
The word probability can be interchanged with likelihood, chance or odds.
Local contacts:
Paul Lainio, manager Media and Community Relations, Bureau of Meteorology, 0417 886 850
