12-Month Rainfall Forecast
Sydney 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 23 Nov 2021
ENSO status: La Niña. IOD status: Neutral, returned from negative phase this month. SAM status: Positive, it is forecast to be mostly positive until at least December. The El Niño–Southern Oscillation (ENSO) is currently neutral. However, Over the past few months, sea surface temperatures in the central and eastern equatorial Pacific Ocean have continued to cool. This oceanic cooling has also been accompanied by a response in the atmosphere, which has included trade winds strengthening over the central and western Pacific Ocean in the last few weeks. Both the atmosphere and the ocean are now showing clear signs of La Niña pattern and all forecast models suggest that this trend will continue during December and January. Four of seven models continue the event until February and only one through until March. La Niña typically brings above average rainfall to much of Australia, particularly the north and east. The Indian Ocean Dipole (IOD) is currently neutral, it returned from a negative phasein winter and early spring. While the IOD has returned to neutral, the abnormally warm oceans to the northwest of Australia, will take a month or two to cool, bringing cooler daytime temperatures to southern Australia for another month or so. As such, average to above average rainfall is predicted across much of the country for the remainder of spring and summer. Heavy rainfall events are also more likely during La Niña years, due the abundant moisture it feeds into Australia.Forecast Explanation
Notes on the concept of deciles
If all the data in a record are ranked from lowest to highest they can then be divided into 100 equal blocks. These blocks are known as percentiles. The values that fall into the lowest 10% range (from 0 to 10%) are said to be in the first decile, those in the group 10+% to 20% are in the second decile, and so on. Those in the group 90+% to the maximum value recorded are in the 10th decile. The 50% value is a special one known as the 'median'. It is noteworthy since there is the same number of records above and below its value.
Deciles have been found to be very useful for analysing rainfall in particular as its distribution is not the normal bell-shape distribution but is skewed towards many low values with only a few high values. The deciles can be described in qualitative terms. A table is provided in the accompanying results.