Western Australian rainfall outlook

Issued 28 August 2013

No strong rainfall tendency for most of Western Australia

Summary

  • Most of WA has no strong tendency towards being wetter or drier than normal
  • A wetter than normal season is more likely near the south coast of WA
  • A drier than normal season is more likely for parts of the Kimberley and Northern Interior
  • Climate influences include a weakening negative Indian Ocean Dipole, a neutral-to-cool tropical Pacific, and locally warm sea surface temperatures
  • Outlook accuracy is weak over much of the Southwest Land Division, central WA, and the west Kimberley, but moderate elsewhere
Probability of exceeding median rainfall, large image

Details

Along parts of the southwest and southern WA coast the chance of exceeding median rainfall for spring is above 60%. Such odds mean that for every ten years with similar climate patterns to those currently observed, about six or seven spring periods would be expected to be wetter than average over these areas, while about three or four would be drier. The chance of receiving a wetter or drier than normal spring is roughly equal for most other parts of WA, whilst the chance of exceeding the median rainfall for spring is less than 40% in parts of the Kimberley and Northern Interior. In other words, the chances of below median rainfall are 60 to 65% in these areas.

Climate influences

The negative Indian Ocean Dipole (IOD) event that has been influencing Australian climate since mid-May has weakened over the past four weeks. Despite this, sea surface temperature patterns continue to be consistent with a negative dipole event. The majority of climate models expect this negative IOD event to persist until mid-spring. A negative IOD during winter-spring increases the chances of above-average rainfall over southern Australia, while over parts of northern Australia it increases the chance of higher humidity. 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 has remained ENSO-neutral since mid-2012. The dynamical seasonal outlook model suggests ENSO-neutral conditions will remain for the rest of 2013. This means there is no strong shift in the odds from the tropical Pacific, and is reflected to some degree in the rainfall outlook, with much of the country having odds close to 50%.

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 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 September to November period, historical accuracy is weak over much of the Southwest Land Division, central WA, and the west Kimberley, but moderate elsewhere