Vigorous front to sweep the south Sunday

Big winds, snow and swell arrive this weekend

Southern Australia is enjoying a rare settled spell, with a high pressure ridge keeping winds light across the southeast on Thursday and Friday.

That calm is the pause before the punch, as a vigorous cold front is forecast to sweep across the south on Sunday.

The Bureau of Meteorology's coastal waters forecasts flag the front clearly, with a colder southwesterly airstream following it across Victoria and Tasmania.

On the western flank the same system is already at work, driving a big swell and gale-force gusts onto the Western Australian coast this weekend.

What is driving the change

The settled start comes from a broad high pressure ridge parked over the southeast, which blocks fronts and keeps the air stable and light.

That ridge slides east through the weekend, opening the door for a deep low and its trailing cold front to sweep in from the Southern Ocean.

Behind the front sits a pool of cold polar air, and it is that airmass, not the front itself, that delivers the alpine snow and the biting southwesterly.

The tighter the gap between the departing high and the incoming low, the stronger the wind, which is why the northerly ramps up so quickly on Saturday.

Calm now, but not for long

Right now a ridge of high pressure sits over the southeast, holding winds on Port Phillip to a light 10 to 15 knots on Friday.

Central Victorian coastal waters are much the same, with seas under a metre and only a low southwesterly swell running.

It is close to ideal for a fish, a sail or a paddle, and it is the best window you will get for days.

Make the most of Thursday and Friday, because the picture changes sharply as the weekend goes on.

The front arrives Sunday

Ahead of the front, northerly winds strengthen through Saturday and into Sunday as the pressure gradient tightens over Bass Strait.

The Bureau has Port Phillip winds reaching a northerly 20 to 30 knots on Sunday before the change swings them to the northwest in the evening.

Out on the central Victorian coast the northerly builds to 15 to 25 knots on Saturday, with the swell lifting to two to three metres in the far west.

When the front itself crosses on Sunday it drags a cold, gusty southwesterly change behind it, and that is when the wind turns nasty.

"The calm at the start of the week is the trap; the boats that get caught out are the ones still treating Sunday like a Thursday."

Snow and blizzard risk in the alps

The cold air behind the front is aimed squarely at the alps, where the Bureau's forecast has snow and very windy conditions from Sunday.

Mount Hotham is set for a top of just 2 degrees on Sunday, with snow showers continuing through Monday, Tuesday and into Wednesday.

Strong wind driving falling snow is the recipe for blizzard conditions on the exposed tops, so backcountry travellers should check the alpine forecast closely.

The snow is good news for a lean season, but the combination of cold, wind and low visibility is genuinely dangerous up high.

The west is already getting hit

Western Australia sees the front first, and the Perth coastal waters forecast shows just how much energy is in this system.

Winds there reach a west to southwesterly 25 to 35 knots on Saturday, with seas building to two to three metres beneath the swell.

The real story is the swell, which the Bureau has climbing from four metres to a heavy four to seven metres through Saturday morning.

A swell that size makes rock platforms and exposed reefs off the west coast extremely dangerous, well before the front reaches the eastern states.

What it means on the water

For boaters the message is simple: use the calm early window, and have everything home and secured before the northerly builds on Saturday.

Crossing a coastal bar into a rising wind and swell is a serious risk, and Sunday is not the day to test it.

Rock and beach fishers face the sharpest danger as the swell builds, because long-period sets surge far higher up the rocks than the average wave.

Surfers will find clean, building southwesterly swell, but the same energy that makes it fun doubles the power of every wave and hold-down.

Not just the coast

Marine wind warnings already stretch along the southern belt, covering Western Australian, Victorian, Tasmanian and New South Wales waters.

Inland, a frost warning covers northern and central Victoria on the clear nights before the front, a reminder of how cold the airmass is.

The water off the southern coast is near its coldest of the year, so any fall overboard carries the added shock of cold immersion.

Dress for the water temperature rather than the air, and wear a life jacket whenever you are near the edge or on a small boat.

How long it lasts

The front clears through Sunday night, but the cold, showery southwesterly airstream lingers behind it into the new week.

Alpine snow showers keep falling into Wednesday before the wind slowly eases, so the mountains stay wintry for days.

The leftover swell keeps running along the southern coast well after the wind drops, so the sea stays powerful into midweek.

Common questions

When does the front hit?

The Bureau has the vigorous cold front crossing southern Australia on Sunday, reaching Western Australia on Saturday and the southeast later in the day.

How strong will the wind get?

Port Phillip is forecast for a northerly 20 to 30 knots on Sunday, while Western Australian waters see a west to southwesterly 25 to 35 knots on Saturday.

How big is the swell?

The Perth coastal waters forecast has the southwesterly swell building to a heavy four to seven metres on Saturday morning.

Is it worth going out midweek?

Thursday and Friday offer the calmest window by far, so get your boating, fishing or paddling done before the northerly builds on Saturday.

Track the change with the Seabreeze Melbourne wind forecast , and check the latest Victorian warnings before you head out.