Southeast Australian rainfall outlook
Issued 25 September 2013
The chance of exceeding the median rainfall during October to December is greater than 60% across Tasmania. 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 wetter than average over these areas, while about three to four would be drier.
In contrast, odds suggest a greater than 60% chance of below average rainfall in parts of northern NSW and northeast SA. Across the remaining areas of mainland southeast Australia the chances of receiving a wetter or drier than normal October to December period are roughly equal (i.e. 50%).
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.
How accurate is the outlook?
Outlook confidence is related to how consistently the oceans and broadscale climate affect Australian rainfall. During October to December, historical accuracy shows the outlook to be moderately to strongly consistent across NSW, Victoria, Tasmania, and eastern SA, but only weakly consistent across parts of central and western SA.