Spring time in Australia.

Not a season that's noted for surf, yet the East Coast will soon see a sustained run of large easterly swell. Scroll to the first map if you're in a hurry for details or enjoy Craig's sprawling opening gambit.

As the days grow and the sun shines longer, conditions become less favourable for the afternoon and evening surf sessions.

Land temperatures start increasing, yet ocean temps are lagging behind, the downflow effect being a shorter period of offshore land breeze in the morning.

You see, the land warms quite quickly in spring, easily surpassing the temperature of the adjacent ocean, leading to rising inland air. This is replaced by the cooler air sitting above the ocean, known as the sea breeze.

As the day continues to warm, so does the temperature differential, driving stronger sea breezes which only start to abate into the late evening and early the following morning when the land cools, lowering the temperature differential from the ocean. Sea breezes generally start straight onshore but tend anti-clockwise during the day thanks to the Coriolis Force and Earth's rotation.

What's left the following day is generally lumpy, morning sick conditions that iron themselves out in the few hours of slack wind before the thermal gradient picks up again and the sea breeze process repeats.

On the East Coast, this pattern usually becomes entrenched and strengthened thanks to the shift of the sub-tropical high pressure system southward, into a semi-stationary slot over the Tasman Sea.

North to north-easterly gradient winds strengthen across the eastern seaboard as weakening cold fronts lose their sting after pushing across the southern states, resulting in week-long periods of northerly winds that aren't quite strong enough to generate worthwhile north-east swell energy, yet are great for upwelling colder water off the shelf.

We see a classic positive feedback loop developing, with the cooler upwelled water feeding the thermal difference from the warm land resulting in even stronger north-east winds.

La Niña helps to break that cycle.

Under La Niña, the land is generally saturated and cooler. Wth the increased soil moisture it takes a lot more solar heating to get the land above the temperature of the adjacent ocean. This leads to longer periods of slack morning conditions and land breezes, while also preventing major upwelling events due to the delayed and weaker development of the sea breeze.

There's also one more important factor at play in Niña years and that's the shift of the sub-tropical ridge further south than normal, which shifts the north-east gradient winds more towards Tasmanian latitudes, releasing its grip from Sydney, north.

With the third La Niña signal developing through our winter and strengthening further over the past week or two, one look at the coming long-range charts along the East Coast show a setup more often seen deep into summer or early autumn.

That being a strong stubborn high drifting slowly east across New Zealand, squeezed by tropical depressions in the New Caledonia and Fiji regions. The stubborn nature of the high and the deepening of depressions into a tropical low is set to deliver a large, prolonged and super-charged easterly trade-swell event for most of the East Coast, building in the north from the weekend and across southern NSW into early next week, lasting for over a week.

Forecast wind field and Mean Sea Level Pressure charts for next week

Now to the exciting part.

With the activity occurring at some distance to the Australian mainland we're set to see troughy weather bringing generally favourable winds and conditions throughout the swell event, sometimes from the north and at others out of the south. 

The first pulse of punchy trade-swell energy should fill in across the Gold Coast and Northern NSW on Monday, reaching 5-6ft ahead of further bump in size during Tuesday/Wednesday. 6ft to occasionally 8ft surf is likely across open beaches, with a possible stronger pulse of E'ly groundswell from the deepening low due Thursday/Friday.

For southern NSW, there'll be some sizey, localised NE windswell for the weekend with flukey winds, but the better and stronger easterly swell will be delayed by half a day or so and come in a little smaller (compared to further north), with surf to 4-6ft expected from Tuesday afternoon/Wednesday, becoming a little larger into the end of the week.

It's a dynamic period to say the least and one which shows up on the medium-range, weekly Mean Sea Level Pressure anomaly charts. That being the difference in pressure for the week 10th of October through the 17th of October compared to the long-term climatic 30 year average.

Mean Sea Level Pressure anomaly (difference from normal) for the week Oct 10-17. (ECMWF)

What we can see is the higher than normal pressure signal extending under Tasmania and over to the east of New Zealand, along with the lower than normal pressure in the Tasman Sea, Coral Sea and south-east of Fiji. A classic La Niña setup though deep in the middle of spring.

Longer-range forecasts have a similar pattern continuing into November thanks to the robust La Niña signal throughout the Pacific Ocean but keep an eye on the local Forecaster Notes for the finer intricacies surrounding the expected size and local winds, along with the running commentary below.

Gold Coast 16-day Surf Forecast
Northern Beaches 16-day Surf Forecast