National rainfall outlook

Issued 22 May 2013

Wetter season likely for most of Australia

Text commentary of chance of increased rainfall

Summary

  • A wetter than normal season is more likely for large parts of northern and eastern mainland Australia
  • Chances of above or below normal rainfall are roughly equal over southern WA, Tasmania and parts of the tropical north
  • 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 and the northeast half of Australia.
Probability of exceeding median rainfall, large image

Details

The chances of above-median winter rainfall are 60 to 70% over a broad area of Australia extending from the northwest across to the southeast (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 over the tropical north of Australia, it is seasonally dry at this time of year. The median rainfall at many tropical locations is between 0 and 1 mm for June to August, and even a small amount of rain would exceed the median.

Over the rest of the country, 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 sit 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. This is reflected in the rainfall outlook, with much of southern Australia expecting above normal rainfall.

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 northwest 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 winter, model accuracy shows the outlook to be moderately consistent over the southern half of WA, most of the NT, Queensland and northern NSW. Elsewhere, the effect is only weakly to very weakly consistent.