Western Australian rainfall outlook

Issued 22 May 2013

Wetter season likely for northern and eastern WA

Summary

  • A wetter than normal season is more likely for much of northern and eastern WA
  • Chances of above or below normal rainfall are roughly equal over western and southern WA
  • Climate influences include a warmer than normal eastern Indian Ocean, a neutral tropical Pacific, and warm local sea surface temperatures
  • Outlook skill is moderate over southern WA, with northern WA currently in its dry season.
Probability of exceeding median rainfall, large image

Details

The chances of receiving above median rainfall during the June to August period are 60 to 70% in the Kimberley, adjacent east Pilbara, and Northern Interior (see map above). Such odds mean that for every ten years with similar climate patterns to those currently observed, about six or seven years would be expected to be wetter than average over these areas, while about three or four years would be drier. However, it should be noted that rainfall is commonly low over these areas at this time of year and even a small amount of rain would exceed the median.

Over southern and western parts of WA, the chances of a drier or wetter winter are roughly equal.

Climate influences

The tropical Pacific has remained ENSO-neutral since mid-2012. Climate models surveyed by the Bureau of Meteorology suggest sea surface temperatures in the tropical Pacific Ocean are likely to stay on the cool side of neutral during winter 2013.

Four of five international models surveyed by the Bureau favour the development of a negative Indian Ocean Dipole (IOD) event sometime during winter-spring 2013. A negative IOD during winter-spring increases the chances of above normal rainfall over southern Australia.

Warmer than normal ocean surface temperatures currently surround much of the continent. Warmer ocean temperatures can provide more moisture to the atmosphere, which in favourable weather conditions (e.g., interactions with fronts or north-west cloudbands) may result in increased rainfall.

How accurate is the outlook?

Outlook confidence is related to how consistently the oceans and broadscale climate affect Australian rainfall. During June to August, model accuracy shows the outlook to be moderately consistent over the southern half of WA. Elsewhere, the effect is only weakly to very weakly consistent.