What is an easterly trough?
An easterly trough is a long area of low pressure that forms inland of the Great Dividing Range.
It's a dominant feature of Australia's synoptic pattern in summer.
These troughs bring rainfall to central and inland parts of eastern Australia. They are also called inland troughs.
An easterly trough is partly formed by:
- intense heating of the land during summer
- topography of the region.
Where easterly troughs form
The Great Dividing Range runs 3,500 km along Australia's east coast.
The easterly troughs are on the sheltered west side of this mountain range – the lee side. They extend through central Queensland and central New South Wales. Sometimes they stretch into northern Victoria.
When this trough forms, a ridge of higher pressure forms along the coast on the east side of the range. It's called a windward ridge.
This pattern of a lee-side trough and a windward ridge also happens in other parts of the world. You may see it where wind patterns and topography are similar. For example, on New Zealand's South Island.
A trough system is also present near the west coast of Western Australia during summer. The west coast trough does not have as big an impact on rainfall as the easterly trough.

Easterly troughs form west of the Great Dividing Range. They are most common during summer.
Timing and duration of easterly troughs
Easterly troughs begin to form as the land warms up.
They can start appearing over Australia in September and may still happen through to May. They are most common during the summer months (December to February).
Easterly troughs generally last from a few days to a week.
How easterly troughs affect Australia
An easterly trough creates a boundary between moist air near the coast and dry air inland. As the temperature rises during the day, the trough deepens. As it moves towards the coast, showers and thunderstorms form in the unstable air.
The easterly trough is a major contributor to rainfall in eastern Australia. It also enhances low pressure troughs or cold fronts moving through southern Australia.
Rainfall can be particularly heavy when the easterly trough interacts with other features. For example, when:
- an upper level trough approaches from the west, or
- there's a north-easterly onshore flow and no ridge of higher pressure east of the Great Dividing Range.
Climate drivers related to easterly troughs
Frontal systems
Frontal systems bring rainfall to southern Australia. Cold fronts are the most common in Australia but we can experience warm fronts too. Learn more about frontal systems.
Subtropical ridge
The subtropical ridge is a belt of high pressure around the globe in the middle latitudes. It brings dry and stable conditions to large parts of Australia. Learn about the subtropical ridge.