Western Australian rainfall outlook
Issued 19 June 2013
The chance of exceeding median rainfall for July to September is more than 60% over most of WA, except for parts of western WA. The probability of exceeding the median rises to more than 70% along parts of the south coast, in southeastern WA, and in the far north Kimberley. Such odds mean that for every ten years with similar climate patterns to those currently observed, about six or seven July to September periods would be expected to be wetter than average over these areas, while about three or 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.
Parts of western WA have no strong tendency towards being wetter or drier than normal over the July to September period.
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.
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 cold 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 the July to September period, historical accuracy shows the outlook to be moderately consistent over western and southern WA, as well as the far north Kimberley. For parts of central, eastern, and northern WA, the effect is only weakly to very weakly consistent.