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What is a cut-off low and why do they matter?

Ben Domensino, Monday May 7, 2018 - 13:39 AEST

There are early indications that severe weather could affect southeastern Australia at the end of this week.

A number of forecast models suggest that a pool of cold air currently located about 2,000km to the southwest of Australia will traverse the nation's southeast at the end of this week.

While it's still many miles away, this is certainly worth taking note of. Rogue pools of cold air like this can cause cut-off low pressure systems, which rarely pass over southern Australia without causing severe weather.

So, what is a cut-off low and why do they cause intense weather?

Throughout the year, a pool of cold air sits over Antarctica and the Southern Ocean, surrounded by a band of very strong westerly winds. This swirling mass of air, which is constantly rotating in a clockwise direction, is called the 'polar vortex' and its one of two in our planet's atmosphere.

Every now and then, a smaller pool of cold air at the edge of the polar vortex breaks away and heads north. This vagrant pool often causes a low pressure system to form about five to six kilometres above the ground, which is called a cut-off low.

When upper level cut-off lows move over Australia, the cold air they bring up from the Southern Ocean interacts with warmer air sitting over the continent and its surrounding oceans. This interaction triggers an outbreak of intense weather as the atmosphere tries to restore equilibrium.

While it's too early to know exactly where severe weather will occur, computer models are in good agreement that a cut-off low will pass over southeastern Australia between Thursday and Sunday.

At this stage, anyone living in South Australia, Victoria, Tasmania, NSW and the ACT should closely monitor the latest forecasts and severe weather warnings as the week unfolds.

- Weatherzone

© Weatherzone 2018

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