Western Australian rainfall outlook

Issued 18 December 2013

Wet January to March most likely for western WA

Summary

  • A wetter than normal season is most likely for parts of western WA
  • The chances of a wetter or drier than normal season are roughly equal over eastern WA and the Kimberley
  • Climate influences include a neutral tropical Pacific, and a warm Indian Ocean
  • Outlook accuracy is moderate over western WA, much of the Kimberley, and near the WA-NT border. Elsewhere, outlook accuracy is low. See outlook accuracy tab for more information.
Probability of exceeding median rainfall, large image

Details

The chances of exceeding median rainfall during the January to March period are 60 to 65% over parts of western WA. In other words, the chances of below average rainfall are 35 to 40% over these areas. So, for every ten summer outlooks with similar odds to these, about six of them would result in above-average rainfall over these areas, while about four would be below average.

The chance of receiving a wetter or drier than normal January to March period is roughly equal (i.e., close to 50%) over the remainder of the State.

Climate influences

The El Niño-Southern Oscillation (ENSO) remains neutral, with the majority of atmospheric and oceanic indicators close to their long-term average. Dynamical models surveyed by the Bureau suggest ENSO-neutral conditions are likely to persist at least for the austral summer and early autumn. This means there is no strong shift in the odds from the tropical Pacific in this outlook.

Although the Indian Ocean Dipole has limited impact upon Australia during the months from December to April, the broad area of warmer than average sea surface temperatures across the central Indian Ocean has been shown to influence the climate over large parts of Australia at this time of year.

Lower surface pressures are likely to bring cooler and potentially wetter conditions to parts of southern Australia (such as Tasmania and parts of southwest WA) early in the outlook period. For northern regions, tropical Australian sea surface temperatures are currently average to slightly cooler than average, and are expected to remain that way over the forecast period. Weak to moderate tropical activity (cloud and rainfall associated with the Madden-Julian Oscillation) is expected in the first month.

How accurate is the outlook?

Outlook accuracy for the January to March period is:

  • Moderate over much of western WA and the Kimberly and near the WA - NT border
  • Weak throughout eastern and southern parts of WA