Port Phillip Weekend: Westerlies After the Front

The cold front has crossed.

After westerly and northwesterly winds shifted to fresh southwesterly airflow as the front crossed Bass Strait coastal waters during Thursday, the weather pattern over southeastern Australia has flipped back to a high-pressure regime.

BOM's Port Phillip Local Waters forecast, issued Thursday 23 July, describes the post-frontal pattern as a firm high pressure ridge persisting over Bass Strait through the weekend, directing westerly flow across the region.

What the westerlies mean for Port Phillip

Port Phillip in a westerly regime is a bay of two sides.

The eastern shore from Frankston down to Mornington sits sheltered from the prevailing wind, with the Westernport-facing headlands flattening the chop to near-mirror conditions through the morning.

The western shore - from Geelong north toward Altona - takes the full fetch of the westerly and builds short, steep chop that becomes uncomfortable for smaller vessels by midday in any wind above 12 knots.

At The Rip, the narrow entrance to Port Phillip at Point Nepean, a westerly above 10 knots on the morning ebb creates a rough, confused sea that is hazardous for vessels under 6 metres.

Experienced local skippers plan any Rip crossing for the two hours following low water, when the outflow has eased and the cross-sea settles.

"A firm ridge persisting over Bass Strait from Friday, directing a westerly flow across the region."

That was BOM's Port Phillip Local Waters assessment for the post-frontal period - and it remains the active call as the high edges east out of South Australia.

Bass Strait passage this weekend

For passage-makers heading south from Port Phillip toward King Island or the Tasmanian coast, westerly conditions in Bass Strait require careful timing.

The common trap is departing Queenscliff or Geelong with a moderate westerly and finding it has freshened to 20-25 knots by the time you clear The Rip, with Bass Strait chop building from the west.

A firm ridge directing 15-20 knots of westerly across open Bass Strait means a 1.5 to 2.5 metre beam sea for vessels on a southeasterly course toward King Island or Flinders Island.

The rule most experienced passage-makers follow is to anchor in the Heads or at Queenscliff Harbour and wait for the next weather window if the ridge is sitting tight with winds above 15 knots at departure time.

A firmer southerly window, which typically follows a ridge tightening past Bass Strait, offers a better angle for south-bound passages - but that window is not evident in the current three-day outlook.

Fishers: where to be this weekend

Westerly conditions in Port Phillip push bait fish schools into the eastern shore's calmer water, and that generally concentrates snapper and squid along the Mornington Peninsula side.

Frankston and Mornington are worth targeting Saturday morning before the westerly fills in, with squid sitting in the cleaner water close to structure on the post-frontal flush.

The Barwon Heads bar, on the Bellarine Peninsula, can be uncomfortable with a westerly above 12 knots on a flooding tide - reef fishers should plan to be out before 9am on Saturday or wait for a better window.

The post-frontal period often concentrates snapper on the shallow reefs of the southern bay as cold, oxygenated water pushes in behind the change - the next 48 hours are worth targeting if you can work around the wind timing.

The bigger pattern: why winter fronts keep coming

The current system is part of the regular procession of Antarctic-driven cold fronts that sweep across southeastern Australia from May through August each year.

The Southern Annular Mode (SAM) is the large-scale driver - an index that tracks the latitude of the westerly wind belt encircling Antarctica.

When the SAM is in a negative phase, as it tends to be in mid-winter, the westerly belt shifts northward and cold fronts track more directly across the southern states rather than passing below the continent.

This creates the pattern most Victorians know well: a burst of northwest wind ahead of the front, the cold change sweeping through, then a high moving up from the Bight with a period of westerlies before the next system arrives.

Planning your weekend session

For Victorian sailors and fishers, the short answer is to work the eastern shore in the morning, before the westerly builds, and avoid The Rip and open Bass Strait until the next confirmed southerly window.

Saturday morning is the best option in Port Phillip, with the ridge not yet fully consolidated and winds likely lighter on the eastern side before midday.

Sunday may see the westerly freshen again as the ridge tightens further, with afternoon gusts possibly touching 18-22 knots across the open bay.

Common questions

Is it safe to cross The Rip this weekend?

In a westerly above 10 knots on an ebb tide, The Rip is not suitable for vessels under 6 metres or crews without Heads experience.

Where is the calmest water in Port Phillip in a westerly?

The eastern shore from Frankston to Sorrento has the most shelter from a westerly, with the Mornington Peninsula blocking the fetch across the bay.

Will the snapper bite after the front?

Post-frontal flushes regularly activate the southern bay reefs - the first 24-48 hours after a front clears is worth targeting on the eastern side.

How long will the westerlies last?

BOM's Port Phillip forecast shows the firm ridge persisting at least through Saturday, with the pattern reassessed for next week.

Track the latest conditions at the Seabreeze Victoria warnings page and the Melbourne wind forecast before you head out.