Pre-Front Calm: South Coast Opens Tuesday

Northerlies, 1m seas: NSW south offshore today

A ridge of high pressure extending from a Tasman Sea high is giving NSW's south coast the most benign offshore conditions of the past week, with gentle northerlies and seas sitting at or below 1 metre from Sydney to the Victorian border.

The timing is ideal: yesterday's East Coast Low affecting the north coast has cleared east, and the next frontal system - a cold front approaching from the South Australian coast - is still 24 hours from crossing Bass Strait.

Between those two systems sits today's offshore window.

BOM's forecast for NSW Southern Coastal Districts has northerly winds at 10 to 15 knots, increasing to 20 knots south of Port Kembla in the afternoon as the pre-front pressure gradient tightens.

Seas are sitting at around 1 metre with a residual southerly swell of around 1 metre - a low, well-organised groundswell that makes for manageable offshore conditions across the exposed south coast region.

"Northerly winds becoming west to northwesterly as a ridge extending from a high over the Tasman Sea recedes ahead of a cold front approaching from the southwest."

Port Kembla, Ulladulla, Batemans Bay, and Narooma are all sitting in comfortable offshore territory through the morning, with the wind shift expected to arrive progressively south to north from Wednesday evening.

What is driving the window

Pre-front northerlies are a consistent feature of the Australian east coast weather cycle and one of the most predictable calm-water windows for offshore planning.

As a cold front approaches from the Southern Ocean, the pressure gradient ahead of it initially produces northerlies rather than the southwesterlies that follow the front's passage.

The warm-sector air mass between the frontal boundary and the preceding high creates light to moderate northerly flow - typically offshore on the NSW south coast.

Those northerlies knock down the residual southerly swell and produce a flatter sea state than the open ocean reading alone would suggest.

The duration of the window depends on how fast the front moves: this system is tracking at a moderate pace and is expected to clear Bass Strait Wednesday before the southwesterly change reaches the NSW south coast Thursday morning.

Fishers: your Tuesday plan

Offshore sportfishing from Port Kembla south is in good shape today for runs to the 200-metre ledge and the southern bluefin tuna grounds that have been holding in water 18 to 20 degrees Celsius across the south coast shelf.

Southern bluefin have been a feature of the July fishing picture along the NSW south coast and Victorian coast, with the cold Tasman water sitting close to the shelf edge bringing these pelagics within practical reach of trailer boats.

Bar crossings at Jervis Bay, Ulladulla, and Bermagui are in good shape this morning, with light northerlies removing the surge from yesterday's residual south swell and making entry and exit straightforward.

The afternoon wind increase south of Port Kembla is worth watching: northerlies building to 20 knots through the afternoon will generate a short-period wind chop on the return run that catches some skippers off guard on an otherwise calm day.

Plan departure times to be well inshore by 2pm on the south coast to avoid the worst of the afternoon build.

Sailors and coastal passages

Northerlies at 10 to 15 knots with 1m seas are as good as winter sailing conditions get on the NSW south coast.

Boats making southbound passages have a direct breeze from the NE to N through the morning, with Port Kembla, Eden, and the Victorian ports all accessible without significant headwinds until Wednesday evening.

Northbound passages will require motoring or motor-sailing through the morning, with the prospect of a useful SW breeze from Thursday once the front has passed and the conditions have settled behind the change.

Bass Strait transits should wait until after the front has cleared: northerlies switch to fresh SW during the crossing itself, with conditions deteriorating rapidly.

Strait conditions during any frontal crossing are inherently more severe than surrounding coastal waters due to the shallow depth and opposing sea states from different fetch directions.

After the front: what Thursday brings

The cold front crossing Wednesday night will deliver a moderate southwesterly wind change to the NSW south coast from Thursday morning - BOM forecasts 15 to 25 knots SW behind the front.

A high pressure system forming in the Great Australian Bight will extend a ridge east over Victoria and Bass Strait on Thursday behind the front, gradually easing conditions through Thursday afternoon and into Friday.

The useful by-product of the front is a fresh southerly swell on the back side - south coast surf is likely to build to 2 to 3 metres offshore Thursday and Friday.

That swell propagates north from the Southern Ocean storm track that generated this front, arriving with a clean SW direction on south-facing breaks.

Surfers and kiters on the south coast should note Thursday as a building surf day with clean SW-W winds offshore on east-facing beaches, shifting to better conditions Friday as the high pressure ridge establishes.

The next window

Friday and Saturday are currently shaping as the next settled period, with the high pressure system from the Bight extending coverage east and producing light to moderate SE winds along the coast.

Another frontal system in the Southern Ocean is expected to approach next week, though timing at this range is subject to revision - check the forecasts from Wednesday for updated timing on the next system.

Monitor NSW south coast conditions at NSW marine warnings on Seabreeze, and check Batemans or Ulladulla tides at NSW tides before your run today.