Southern Oscillation Index
The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin.
A strongly and consistently positive SOI pattern (e.g. consistently above about +6 over a two month period) is related to a high probability of above the long-term average (median) rainfall for many areas of Australia, especially areas of eastern Australia (including northern Tasmania) - La Niņa.
Conversely, a 'deep' and consistently negative SOI pattern (less than about minus 6 over a two month period, with little change over that period) is related to a high probability of below median rainfall for many areas of Australia at certain times of the year - El Niņo.
However, it is important to remember that the pattern of relationship between SOI and rainfall (and temperature) can vary depending on the particular season and region. Additionally, the change in SOI over a specified period can be as important in understanding relationships between SOI and rainfall as is the absolute value in SOI.
The flood-hit area of Forbes in central-western NSW is readying itself for the worst of a slow-moving flood, with the peak expected to hit after midnight.
Farmers near Forbes in the New South Wales central west are bracing for extensive loses as their crops continue to be inundated with flood waters, which are expected to peak later today.
Southwestern parts of Western Australia have had rain of up to 24mm of rain this morning as another cold front swept over the region.