A blast of Antarctic air later this week is set to end the balmy start to May in south-east Australia.

Forecasters say a pool of cold air currently sitting about 2000kms from our shores will cross the southeast of the continent from Thursday.

Temperatures are predicted to plunge in NSW, Victoria and the ACT, says Weatherzone.

Canberra residents should rug up on Friday, with the coldest May day in 18 years forecast as the mercury struggles to reach double figures.

Back in May 2000, an autumn cold snap famously caused snow to fall on a Canberra Raiders NRL match in the national capital.  

Melbourne will be the first mainland capital city to feel the plunge in temperatures from Wednesday, as they drop by five degrees to a maximum of 13°C the following day.

There will also be heavy rainfall and the chance of thunderstorms.

But then Sydneysiders - who will enjoy balmy conditions for the early part of this week - will get a rude awakening.

The mercury is forecast to crash from a balmy 26°C on Thursday to 17°C on Friday.

But wind chill effects are likely to shave a couple of degrees off that, setting up the NSW capital for a plunge of 10 degrees.

While cold Autumn snaps are not uncommon, the warm starts to this month enjoyed in Sydney and Melbourne will make it a rude awakening, according to a Weatherzone spokesman.

“The temperatures in Sydney and Melbourne are five degrees above the average for early May. Sydney especially has had a very warm Autumn so far.”

“So when this cold air comes through from Thursday, it will be a severe jolt.”

The low pressure system heading our way is from a polar vortex – a swirling mass of air that sits over Antarctica and the Southern Ocean.

At times a smaller pool of chilly air at the vortex edges breaks away and heads north – towards Australia.

When it mixes with warmer air over the continent it triggers an outbreak of intense conditions.

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