12-Month Rainfall Forecast
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Melbourne 12-month Rainfall Forecast
Jul
25Aug
25Sep
25Oct
25Nov
25Dec
25Jan
26Feb
26Mar
26Apr
26May
26Jun
26
10
50
767676675537Rainfall deciles
10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normalIssue Notes
Issued 17 Jul 2025ENSO status: Neutral. IOD status: Neutral. Negative possible. SAM status: Neutral. Negative developing. The El Niño Southern Oscillation (ENSO) is in a neutral pattern. Neutral conditions are likely to persist through winter. A few climate models suggest an increased chance of a weak La Niña event developing this year, but remaining neutral is still the most likely outcome. The chance of an El Niño event is extremely unlikely. The Indian Ocean Dipole (IOD) is in a neutral phase, but the Indian Ocean overall is much warmer than average. The IOD is forecast to tend towards a negative phase, with clear building of ocean heat starting near Java, and greatly increased convection near Sumatra, and the index is starting to fall. Seven out of eight international models forecast a negative IOD to develop this year. A negative IOD typically increases the amount of northwest cloudbands that cross Australia, increasing rainfall from northwest WA through to southeastern Australia during winter and spring. TheSouthern Annular Mode (SAM) is in a neutral phase after a brief very strong positive phase (although this phase did not have the typical effect over Australia), meaning cold fronts are generally reaching as far north as you would typically expect at this time of year. Forecasts are indicating that the SAM is more likely to tip into a negative phase during August and possibly September, meaning more cold fronts and cut-off lows impacting Australia. A negative SAM during winter typically increases rainfall over southwest WA, SA, Vic, and Tas, and decreases rainfall over east of the Great Divide in NSW and Qld. Rainfall outlooks indicate higher than average rainfall is expected over southwest WA, SA, Vic, and Tas in August and September, transitioning to wetter over the eastern states and average elsewhere by October.
Forecast ExplanationNotes 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.