Observed Wet Season Onset
Wet season onset is defined to occur when a threshold rainfall accumulation of 50mm is reached from 1st September. This amount has been shown to be useful for many agricultural applications as this is about the amount of rainfall that is required to generate new plant growth after the usually dry winter.
The mean onset date (left panel) by this definition occurs first around Darwin in the north and Cairns in the east in late October. The mean onset is progressively later for locations further inland away from these locations, generally occurring before 1st December for locations north/east of 18°S/145°E and later for locations further south/west.
There is significant inter-annual variability (middle panel) in the onset, and skilful predictions of this can be very valuable. However there is also a significant trend (right panel) towards earlier onset over the interior of Western Australia and the southern parts of the Northern Territory which may confound statistical predictions of onset. In Central Australia near Alice Springs the trend has been -1.5 days per year, or 75 days earlier over the 1960 to 2009 period.