Southeast Australian rainfall outlook

Issued 19 June 2013

Wetter conditions more likely for southeast Australia

Summary

  • A wetter than normal season is more likely for mainland southeast Australia
  • Tasmania has no strong tendency towards being wetter or drier than normal
  • Climate influences include a developing negative Indian Ocean Dipole, a neutral-to-cool tropical Pacific, and warm sea surface temperatures around the coast of Australia
  • Outlook accuracy is low through parts of central and western SA, and moderate elsewhere.
Probability of exceeding median rainfall, large image

Details

The chance of exceeding the median rainfall for July to September is greater than 60% over mainland southeast Australia. The chance rises to more than 80% over southern NSW and northern Victoria. Such odds mean that for every ten years with similar climate patterns to those currently observed, about six to eight July to September 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 July to September is roughly equal (i.e., close to 50%) across Tasmania.

Climate influences

A negative Indian Ocean Dipole (IOD) event is favoured to develop during winter-spring 2013. 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 has remained ENSO-neutral since mid-2012. The dynamical seasonal outlook model indicates an increased likelihood of La Niña forming during the next few months. This has increased the 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 showing stronger odds than most of a weak La Niña.

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 July to September, historical accuracy shows the outlook to be moderately consistent over most of southeast Australia, but only weakly to very weakly consistent over western and central SA.