Southeast Australian rainfall outlook

Issued 24 July 2013

A wetter season likely for southeast Australia

Summary

  • A wetter than normal season is likely for mainland southeast Australia
  • Tasmania has no strong tendency towards being wetter or drier than normal
  • The main climate drivers for this outlook include a negative Indian Ocean Dipole, a neutral-to-cool tropical Pacific, and warm sea surface temperatures around the most of Australia
  • Outlook accuracy is moderately consistent across most of southeast Australia, excluding the lower southeast of NSW and the far west of SA.
Probability of exceeding median rainfall, large image

Details

The chance of exceeding the median rainfall for August to October is greater than 60% over mainland southeast Australia. The chance rises to more than 80% over southern NSW, most of Victoria, and southeast SA. Such odds mean that for every ten years with similar climate patterns to those currently observed, about six to eight August to October periods would be expected to be wetter than average over these areas, while about two to four would be drier.

The chance of receiving a wetter or drier than normal August to October is roughly equal (i.e., close to 50%) across Tasmania.

Climate influences

A negative Indian Ocean Dipole (IOD) event is in progress. A negative IOD during winter-spring increases the chances of above normal rainfall over southeast Australia, which is reflected in the rainfall outlook. See http://www.bom.gov.au/climate/IOD/negative/ for more information on typical rainfall patterns during negative IOD events.

The tropical Pacific is ENSO-neutral. The dynamical seasonal outlook model suggests a brief period of weak La Niña-like conditions forming during the next few months. This has contributed to an increased chance of above normal rainfall for southeast Australia. However, international climate models surveyed by the Bureau of Meteorology suggest ENSO-neutral is the most likely outcome over the coming season, with the Bureau model being the coolest (most La Niña-like).

Warmer than normal sea surface temperatures currently surround much of western and southern Australia. Warmer sea surface temperatures can provide more moisture to the atmosphere, which in combination with the right weather systems (e.g., interactions with fronts or northwest cloudbands) may result in increased rainfall across the southeast.

How accurate is the outlook?

Outlook accuracy is related to how consistently the oceans and broadscale climate affect Australian rainfall. During August to October, historical accuracy shows the outlook to be moderately consistent over most of southeast Australia, except for the lower southeast of NSW and the far west of SA, where the outlook is only very weakly consistent.