What is a blocking high?
Blocking highs are strong high pressure systems that form further south than usual. They stay nearly stationary for days to weeks.
These highs block west to east movement of weather systems across southern Australia. This means affected areas can have the same kind of weather for an extended time.
Blocking highs sometimes occur with a cut-off low to the north. This is a 'blocking pair' and strengthens the blocking pattern. For more about blocking pairs, see Cut-off lows.
As frontal systems approach a blocking high, they slow down. The fronts weaken and tend to slip to south of the high pressure system.
How blocking highs affect Australia
Blocking highs can affect all of southern Australia. They can happen at any time of year, lasting from several days to several weeks.
These high pressure systems have a wide range of impacts, depending on their location and strength. A blocking high can bring:
- a hot spell
- a cold spell
- dry conditions
- wet conditions.
Fog and frost may be more likely when a blocking high is present.
Generally:
- areas north-west of a blocking high may have warmer than average conditions, with winds blowing from the north-west
- south-east of a blocking high, there may be cooler than average conditions as cold air is drawn from the far south.
Impact of nearby weather systems
The impact of a blocking high also depends on nearby weather systems. For example:
An area influenced by a blocking high could have dry and stable conditions. There might be wet conditions west of the high, as approaching frontal systems slow. If the high and a cut-off low form a blocking pattern, these wet conditions may include sustained heavy rainfall.
Climate drivers related to blocking highs
Cut-off lows
Cut-off lows are low pressure systems that break away from a main belt of low pressure south of Australia. They enhance rainfall in southern Australia. Learn about cut-off lows.
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 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.