12-Month Rainfall Forecast
Adelaide 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 5 Dec 2023
ENSO status: El Niño, reached a strong event. IOD status: Positive IOD, starting to weaken. SAM status: Neutral, trending positive. The El Niño Southern Oscillation (ENSO) is in an El Niño, with all international models indicating this event will continue until at least April 2024. Ocean warming is continuing to occur in the eastern and central Pacific and has reached the threshold for a 'strong event'. The Southern Oscillation Index (SOI) has reduced further and is comfortably below the El Niño threshold, especially over the last 90-days. El Niño only has a limited effect on rainfall during summer, drying Far North Qld, increasing rainfall over the Nullarbor, and having little effect elsewhere. El Niño also typically delays the first monsoon onset and reduces the number of tropical cyclones impacting Australia, but otherwise has little effect on Australia during January and February. The Indian Ocean Dipole (IOD) is in a positive phase but is weakening from its peak in late October. Winds areexpected to weaken with the current tropical activity, but generally SE wind near Jakarta should maintain the event The IOD typically takes 3-6 weeks to decay, so could persist into the start of 2024. However, the first monsoon onset over northern Australia will still sever its effect on Australia. A positive IOD greatly reduces the formation of northwest cloudbands, leading to reductions in spring rainfall over northern WA, central Aus, and the southeast. A positive IOD often has a compound effect with El Niño, reducing rainfall even further. The Southern Annular Mode (SAM) is currently neutral. It is expected to favour a positive event for the remainder of December. Longer-term, since negative SAM events become more likely during El Niño, models suggest it becoming strongly negative around February. A negative phase tends to increase cold front activity over southern Australia, increasing rainfall over southern Vic, western Tas, and southeast SA, while decreasing it for eastern NSW and Qld. The opposite occurs in a positive event, with easterly winds more likely, increasing rainfall and thunderstorm activity in the east, but reducing it in southern and Western Australia. Rainfall outlooks are predicting above average rainfall for eastern NSW & Qld, below average rainfall over the Tas and far northern Australia, and average elsewhere in December. Large rainfall deficits are expected over northern Australia from December, likely with a delayed initial monsoon onset. Rainfall is forecast to be above average in the NT and NSW during January, with average to slightly below average elsewhere, with drier than normal conditions forecast throughout in February.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.