12-Month Rainfall Forecast

Issue Notes

ENSO 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.