National rainfall outlook
Issued 19 June 2013
The chance of exceeding the median rainfall for July to September is more than 60% over most of mainland Australia, except for parts of the tropical north and the western WA coastline. The chance rises to more than 80% over southern NSW and northern Victoria. Such odds mean that for every ten years with similar climate patterns to those currently observed, about six to eight July to September periods would be expected to be wetter than average over these areas, while about two to four would be drier. However, over tropical Australia it is seasonally dry at this time of year, with July-September median rainfall between 0 and 1 mm at many locations; even a small amount of rain would exceed the median in these areas.
The chance of receiving a wetter or drier than normal July to September is roughly equal (i.e., close to 50%) over the western coastline of WA, parts of the northern NT, northern Queensland, and Tasmania.
A negative Indian Ocean Dipole (IOD) event is favoured to develop during winter-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 has remained ENSO-neutral since mid-2012. The dynamical seasonal outlook model indicates an increased likelihood of La Niña forming during the next few months. This has increased the 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 showing stronger odds than most of a weak La Niña.
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 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 July to September, historical accuracy shows the outlook to be moderately consistent over most of eastern Australia, and western WA. Over eastern WA, the eastern NT, western and central SA, and the Cape York Peninsula in Queensland, the effect is only weakly to very weakly consistent.