National rainfall outlook

Issued 24 July 2013

Wetter conditions likely for most of mainland Australia

Text commentary of chance of increased rainfall

Summary

  • A wetter than normal season is likely for most of mainland Australia
  • The Kimberley, parts of western WA, and Tasmania have 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 most of Australia
  • Outlook accuracy is moderate over most of Australia except the interior of WA.
Probability of exceeding median rainfall, large image

Details

The chance of exceeding the median rainfall for August to October is more than 60% over most of mainland Australia. The chance rises to more than 80% over southeast SA, southern NSW and most of Victoria. 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. However, it should be noted that rainfall is typically low at this time of year over tropical Australia, and contributes to only a small part of the annual total.

The chance of receiving a wetter or drier than normal August to October is roughly equal (i.e., close to 50%) over the Kimberley, western WA, parts of the Cape York Peninsula, and Tasmania.

Climate influences

A negative Indian Ocean Dipole (IOD) event is in progress, and is expected to persist through spring 2013. A negative IOD during winter-spring increases the chances of above normal rainfall over southern Australia. This is reflected in the current rainfall outlook, with most of southern Australia expecting above normal rainfall.

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 northern and eastern 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 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 August to October, historical accuracy shows the outlook to be moderately consistent over most of Australia, except for the interior of WA, where the outlook is only very weakly consistent.