12-Month Rainfall Forecast
Goldfields 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 5 Mar 2024
ENSO status: El Niño, breaking down. IOD status: Neutral. SAM status: Neutral, trending slightly positive. The El Niño Southern Oscillation (ENSO) is in an El Niño and is starting to break down. All international models indicate neutral conditions should return by late April or early May. The Southern Oscillation Index (SOI) is back below the threshold of -7, but is varying significantly with tropical activity over Australia and the Pacific. Waters in the Coral Sea are far warmer than normal, especially for an El Niño, and are offsetting the effects of this event. El Niño only has a small effect during autumn, typically increasing rainfall over the Nullarbor and SA, but having little effect elsewhere. The Indian Ocean Dipole (IOD) is in a neutral phase. Ocean temperatures are high across most of the Indian Ocean, with only cooler waters near the WA coast. The IOD has little to no effect during autumn. The Southern Annular Mode (SAM) is neutral and will slightly favour a positive phase during March and April and return closer to true neutral in May. A positive SAM increases easterly winds, increasing rainfall and thunderstorm activity in the east. As we go closer to winter, a positive SAM also reduces cold fronts and rainfall over southern Australia. Rainfall outlooks are predicting average rainfall for the vast majority of Australia throughout autumn. There are some weak signals that southwest WA could be slightly drier than normal in April, and southeast Qld and NSW could be slightly wetter during May, but otherwise near average.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.