On the weekend, South Africa's coastline, particularly the Eastern Cape, was inundated by large waves that struck on Saturday, peaked later that day, and began abating on Sunday.

All weekend and continuing into today, the South African Weather Service has been warning small vessels "to seek shelter in harbours, bays or inlets", while the "public is advised to be aware of large waves along the beach and harbours/ports".

The waves caused damage to coastal properties and the loss of one life.

Though it was reported on some channels as a tsunami, the cause of the waves was a mere mid-latitude low that brushed the south-east of the country. Albeit, an exceptionally broad and deep mid-latitude storm - the hectopascals dipping down to approx. 970 hectopascals.

The storm reached maximum intensity just off South Africa's coast which meant both the waves were large and unruly, while the storm surge was significant. A storm surge is when the ocean surface is raised higher than surrounding areas owing to particularly low atmospheric pressure. They're usually associated with cyclones, however the same forces apply in any strong low pressure system.

If the cyclone or storm crosses land then the storm surge will follow, which is what happened in South Africa late on Saturday. With abnormally large waves breaking on top of the surge, the whitewater then reached areas far beyond the tide zone.

Cheeseball thumbnail on an otherwise good video

Today, the storm is dissapating, the weather service reducing the warnings issued, however the story of this storm will continue for a while yet. For one, the storm fired a direct hit up the 500 km wide Mozambique Channel which will light up the southern shores of the Comoro Islands, and is strong enough to penetrate deep into the Arabian Sea. Together with swell moving up Madagascar's east coast, the storm will arrive as long period swell to fringing coastlines that usually only see whitewater from short period monsoons.

Moving east into the Indian Ocean and the swell will similarly arrive deliver long period waves - think: 21+ seconds - from the French Indian Ocean islands through Lakshadweep and the Maldives, across Sri Lanka's southern shores, and then into the Bay of Bengal, including the Andaman and Nicobar Islands. I even saw Thailand-based Bert Burger readying himself for the swell which will lose little steam as it moves between the Nicobar Islands and Aceh, Indonesia's western-most province, towards Thailand.

Often dormant corners of the Arabian Sea will cop a long period strike from the swell

Fair to say that it's Indonesia where most surfers will be anticipating the coming swell. However, it's worth including a disclaimer that, being extremely long period, the swell may behave in unpredictable ways.

Waves that like a lot of period - Lances Right and Nemberala to name just two - will respond well, as will many quiet waves on less dynamic coasts that will benefit from increased refraction as the period stretches upwards of 20 seconds.

By the same token, reefs in smaller bays and compartments, or those that have outside bomboras, may suffer from the uncommon period. A more southerly-focused swell that's due to arrive at the same time may help the cause. If nothing else, it'll fill the gaps between infrequent sets of the long period swell.

As for numbers, the western tip of the archipelago will feel forerunners on midday Friday in the vicinity of 21 seconds (currently forecasting 21.9 seconds), with Bali expecting foreunners around 25 seconds on Friday, before settling into 19 - 20 seconds through Saturday. Meanwhile, further east, Roti will also experience foreunners around 25 seconds before also settling into a swell of 19-20 second period.

It's worth noting that, in Bali, the storm swell is being considered the secondary swell as the aforementioned southerly swell may be more dominant, yet in Roti the storm swell and its extreme period is the dominant swell - there'll be a very long wait between sets.

Although it's aimed towards Indonesia, side energy from the swell will also strike Australia coming in from the WSW with a reduced size and elongated period. Currently, Swellnet's Margaret River data point is forecasting 25 seconds swell at 0.2 m on Thursday, rising marginally in size, dropping marginally in period as the day goes on.

Elliston is currently producing similar numbers in period and size, albeit 24 hours later.

A Great Circle route links Baia Formosa in north-east Brazil with Nemberala in eastern Indonesia meaning that, despite being near 17,000 kms apart, a strong enough storm off the former would reach the latter.

A swell that diagonally crosses the Indian Ocean from the temperate latitudes of South Africa's Eastern Cape to the equatorial climes of Nusa Tengarra travels approximately 10,000 kms. Ostensibly, that'd appear the maximum swell range for Indonesia, however taking into account Great Circle routes, Indonesia's theoretical swell window widens even further.

Hypothetically, it's possible for a storm to track off Baia Formosa in Brazil's far north-east - also Italo Ferreira's hometown - travel south-east, and if it's strong enough move past Capetown and out into the Indian Ocean, with the Great Circle route then taking it in a north-east direction towards Nemberala where it will arrive 16,908 kms later.

Theoretical? Yes.

Possible? Not really.