Western Australian rainfall outlook

Issued 25 September 2013

No strong rainfall tendency for most of Western Australia

Summary

  • The chances of a wetter or drier than normal season are roughly equal over WA
  • 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 much of WA except for northeastern and southern parts of the State
Probability of exceeding median rainfall, large image

Details

The chance of receiving a wetter or drier than normal October to December is roughly equal (i.e. close to 50%) across most of WA. Such odds mean that for every ten years with similar climate patterns to those currently observed, about five October to December periods would be expected to be wetter than average over these areas, while five would be drier.

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.

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, while potentially adding more moisture to the atmosphere, they may also influence some broad scale atmospheric circulation patterns. However, POAMA suggests there will be minimal influence on the rainfall outlook from these waters, with most of WA having odds close to 50%.

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 is weak over much of the Southwest Land Division, and the east Kimberley, but moderate elsewhere