National rainfall outlook

Issued 25 September 2013

Drier conditions likely for much of eastern Australia

Text commentary of chance of increased rainfall

Summary

  • A drier than normal season is more likely for most of southern Queensland, central parts of the NT, and far northern NSW. Over Tasmania a wetter than normal season is more likely
  • The chances of a wetter or drier than normal season are roughly equal over WA, SA, most of the NT, northern Queensland, most of NSW and Victoria
  • Climate influences include locally warm sea surface temperatures around most of Australia while both the tropical Pacific and Indian oceans are in a neutral state
  • Outlook accuracy is moderate over most of Australia, except for parts of western WA and western and central parts of SA
Probability of exceeding median rainfall, large image

Details

The chance of exceeding the median rainfall during October to December is less than 40% over most of southern Queensland, central parts of the NT and northern NSW. Such odds indicate that for every ten years with similar climate patterns to those currently observed, about six to seven October to December periods would be expected to be drier than average over these areas, while about three to four would be wetter.

In contrast, odds suggest a greater than 60% chance of above average rainfall over Tasmania. In other words, the chance of below normal rainfall is less than 40%.

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

Climate influences

The negative Indian Ocean Dipole (IOD) event that was influencing Australian climate through late autumn into winter has weakened over the past few weeks, with IOD-neutral values prevailing since early August. The majority of climate models indicate that the IOD will remain neutral through to the end of the year, suggesting that the 2013 negative IOD is most likely at an end.

The tropical Pacific has remained ENSO-neutral since mid-2012. Dynamical models surveyed by the Bureau suggest ENSO-neutral conditions are likely for the remainder 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%.

With the main climate influences forecast to remain neutral, secondary influences may have higher impact. Warmer than normal sea surface temperatures currently surround much of southern and western Australia, potentially adding more moisture to the atmosphere and hence partially explaining the higher chance of wetter weather over Tasmania. They may also influence some broad scale atmospheric circulation patterns. POAMA suggests below normal pressures over the southeastern parts of the continent, particularly during October. This may result in drier continental air flowing into Queensland from the west.

How accurate is the outlook?

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