National rainfall outlook

Issued 28 August 2013

Wetter spring more likely for southeast Australia and the Top End

Text commentary of chance of increased rainfall

Summary

  • A wetter than normal season is more likely for southeast Australia and the Top End of the NT
  • The chances of a wetter or drier than normal season are roughly equal over Queensland, most of South Australia, most of Western Australia and the rest of the NT
  • Climate influences include a weakening negative Indian Ocean Dipole, a neutral-to-cool tropical Pacific, and locally warm sea surface temperatures
  • Outlook accuracy is moderate over most of Australia, except for parts of western WA and western and northern parts of SA
Probability of exceeding median rainfall - click on the map for a larger version
Probability of exceeding median rainfall, large image

Details

The chance of exceeding the median rainfall for spring is more than 60% over most of southeast Australia and the Top End of the NT. The chance rises to more than 70% over central Victoria. Such odds mean that for every ten years with similar climate patterns to those currently observed, about six to seven spring periods would be expected to be wetter than average over these areas, while about two to three would be drier.

Odds suggest a less than 40% chance of above average rainfall over southern parts of the Kimberley in WA. In other words, the chance of below normal rainfall is greater than 60%.

The chance of receiving a wetter or drier than normal spring is roughly equal (i.e., close to 50%) over the remainder of the country.

Climate influences

The negative Indian Ocean Dipole (IOD) event that has been influencing Australian climate since mid-May has weakened over the past four weeks. Despite this, sea surface temperature patterns continue to be consistent with a negative dipole event. The majority of climate models expect this negative IOD event to persist until mid-spring. A negative IOD during winter-spring increases the chances of above-average rainfall over southern Australia, while over parts of northern Australia it increases the chance of higher humidity. This is reflected in the rainfall outlook, with most of southeast Australia expecting above normal rainfall.

The tropical Pacific has remained ENSO-neutral since mid-2012. The dynamical seasonal outlook model suggests ENSO-neutral conditions will remain for the rest of 2013. This means there is no strong shift in the odds from the tropical Pacific, and is reflected to some degree in the rainfall outlook, with much of the country having odds close to 50%.

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.

How accurate is the outlook?

Outlook accuracy is related to how consistently the oceans and broadscale climate affect Australian rainfall. During spring, historical accuracy shows the outlook to be moderately consistent over most of eastern and northern Australia, and eastern WA. Over western and inland WA and western and northern parts of SA the effect is only weakly to very weakly consistent.

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