The Bureau of Meteorology’s latest three-month outlook for rain has lost some of the optimism present in the previous three-month outlook issued a month ago.

In that report, the Bureau said a wetter than normal August to October was likely for most of mainland Australia.

The latest outlook issued overnight, which covers the period from September to November, has wound back forecasts of a wetter than normal conditions to the Top End of the Northern Territory and southeast Australia.

It lists the chances of a wetter or drier season for Queensland, most of south Australia, most of Western Australia and the rest of the NT as being roughly equal.

Driving this latest forecast has been a weakening negative Indian Ocean Dipole, a neutral-to-cool tropical Pacific, and locally warm sea surface temperatures.

The outlooks lists the chance of exceeding the median rainfall for spring at more than 60pc over most of southeast Australia and the Top End of the NT, and over 70pc for central Victoria.

(Such odds mean that for every ten years with similar climate patterns to those currently observed, about six to seven spring periods would be expected to be wetter than average over these areas, while about two to three would be drier.)

Odds suggest a less than 40pc chance of above average rainfall over southern parts of the Kimberley in WA. In other words, the chance of below normal rainfall is greater than 60pc.

The chance of receiving a wetter or drier than normal spring is roughly equal (i.e., close to 50pc) over the remainder of the country.

Climate influences

The Bureau has provided the following explanation of the climate influences behind the latest outlook:

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 rainfall outlook, with most of southeast Australia expecting above normal rainfall.
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 50pc.

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.

Source: BOM