Sea conditions are the product of the behaviour of the wind, not just in the immediate vicinity, but also at a distance - sometimes a great distance - from your location. Those wave trains generated by a storm maybe half an ocean away are known as swell.
The most extreme example of swell waves that I encountered during my stint on the marine forecasting bench occurred in mid-February 1979 in the English Channel. On the 13th of that month, a Tuesday, the weather pattern around the British Isles hardly set any alarm bells ringing. We were in the middle of a cold snap, high pressure lay near northern Scotland and low pressure over France, and a moderate easterly breeze blew down the Channel. Wind-waves were around 1.5m high in open waters of the Channel but on the eastern side of Lyme Bay - sheltered by the Isle of Portland and Chesil Beach - it should have been almost calm.
What actually happened was quite extraordinary. Huge waves raced in from the southwest, against the wind. Some of these waves overtopped the 12m high bank of pebbles which is Chesil Beach, and after an hour or so of relentless pounding the bank was breached in several places so that sea water came flooding through to add to the white water which had come flying over the top. At its southern end Chesil Beach lies hard against the Isle of Portland, and the urban area behind the bank was invaded, not only by the sea but also by millions of tons of pebbles. The Beach subsequently was rebuilt by bulldozers working day and night for weeks.
There was damage elsewhere along the English Channel coast. Vessels were torn from their moorings, sea walls damaged, and breaches of sea-defences were also reported in France, Spain and Portugal.
Subsequent analysis showed that this unprecedentedly powerful swell was generated by high winds in the circulation of an intense depression which was centred near Newfoundland three days earlier. The longer the wave period (that is, the time between each passing crest) the faster the waves travel; on this occasion the Atlantic depression travelled at about the same speed and in the same direction as the long-period swell, and therefore continued to add energy to the swell system. Although the depression was weakening by the morning of the 13th, a data buoy in the Southwest Approaches registered swell waves 7 to 8m high with a period of 18 seconds.
So, watching the Shipping Forecast does not tell the weekend sailor everything he needs to know. Powerful swells may seem to arrive from nowhere, but as we have seen they always come from somewhere.