Western Australian rainfall outlook

Issued 24 July 2013

No strong rainfall tendency for southwest Western Australia

Summary

  • Most of southwest WA and parts of the Kimberley have no strong tendency towards being wetter or drier than normal
  • A wetter than normal season is likely for the Pilbara, and eastern parts of WA
  • The main climate drivers for this outlook include a negative Indian Ocean Dipole, a neutral-to-cool tropical Pacific, and warm sea surface temperatures around the coast of Australia
  • Outlook accuracy is moderate over much of WA, except for inland central parts.
Probability of exceeding median rainfall, large image

Details

The chance of exceeding median rainfall for August to October is between 60% and 70% over the Pilbara, coastal parts of the Kimberley, and much of eastern WA. Such odds mean that for every ten years with similar climate patterns to those currently observed, about six or seven August to October periods would be expected to be wetter than average over these areas, while about three or four would be drier. However, it should be noted that rainfall is typically low at this time of year around northern WA, and contributes only a small part to the annual total. The chance of receiving a wetter or drier than normal August to October is roughly equal (i.e., close to 50%) over much of the Southwest Land Division and southern Gascoyne.

Climate influences

A negative Indian Ocean Dipole (IOD) event is in progress, and is expected to persist through spring 2013. A negative IOD during winter-spring generally increases the chances of above normal rainfall over southern Australia. This is reflected in the current rainfall outlook in eastern parts of WA, although negative IOD events have less of an influence in the southwest of the State.

The tropical Pacific is ENSO-neutral. The dynamical seasonal outlook model suggests a brief period of weak La Niña-like conditions forming during the next few months. This has contributed to an increased 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 being the coolest (most La Niña-like).

Warmer than normal sea surface temperatures currently surround much of Australia. Warmer sea surface temperatures can provide more moisture to the atmosphere, which in combination with 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 the August to October period, historical accuracy shows the outlook to be moderately consistent over much of WA, except for inland central parts where the effect is only weakly to very weakly consistent.