12-Month Rainfall Forecast

Issue Notes

ENSO status: Neutral. IOD status: Negative developing. SAM status: Weakly positive. 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 actively developing into a negative phase. There is clear evidence of ocean downwelling off Indonesia and upwelling near Africa, coupled with strong atmospheric linking. The index is now below the threshold for 3 of the required 8 weeks. If this continues, which is expected, a declaration will be made in late September. If this negative IOD develops it will likely continue impacting Australia till December until the monsoon first arrives, breaking the impact. A negative IOD typically increases the amount of northwest cloudbands that cross Australia, increasingrainfall from northwest WA through to southeastern Australia during winter and spring. The Southern Annular Mode (SAM) is in a weak positive phase, and is expected to generally mainly alternate between neutral and positive phases over the coming weeks. A positive SAM during spring typically increases rainfall rainfall over NSW and southern Qld, and decreases rainfall over southwest WA, SA, Vic, and Tas (weak effect at first, strong by late spring). A positive SAM also increases the chance of cut-off low pressure systems. Rainfall outlooks indicate wetter than average conditions across the eastern two-thirds of Australia over the next several months, except for parts of southern and western Tas that are forecast to be drier than average. WA is forecast to be slightly wetter than average during August, then average during spring.