12-Month Rainfall Forecast
South West 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 12 Jan 2022
ENSO status: La Niña. IOD status: Neutral. SAM status: Positive, it is forecast to be mostly positive during the remainder of January. The El Niño–Southern Oscillation (ENSO) is firmly in a La Niña phase. Both the atmosphere and the ocean are now showing established La Niña pattern. La Niña is likely to continue during February, while most models suggest it should return to neutral phase by 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, while a negative phase caused above average rainfall last winter and early spring. As such, average to above average rainfall is predicted across much of Australia this summer, particularly the north and the east. 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.