UPDATE, Tuesday November 20 As I write this, the peak period off Portugal has just jumped from about 8 to 19 secs. The swell has arrived, and is expected to fill in overnight, peaking tomorrow and decreasing through Thursday and Friday. As is normal with any swell, the long periods arrive first, and then the size ramps up over the following few hours.
The swell was generated by an area of hurricane-force winds associated with a low pressure on the far side of the North Atlantic, that strengthened on Sunday and moved steadily eastwards, following its own swell. The system is currently about 1000 miles west of Ireland and is due to weaken tomorrow as it drifts slowly towards Biscay.
The fact that the swell was produced by such a powerful storm such a long way away means that is it arriving with unusually long periods. But also the lack of strong winds close to the coastline, and a lack of residual swell means that it will arrive clean and untainted at many spots.
The latest wind forecast is better than expected originally. In northern areas including Ireland and southwest England, tomorrow and Thursday see moderate south or south-easterlies, decreasing light by Thursday. In Biscay winds are light to moderate south or southwest, and down into Portugal expect windless conditions or light southerlies with a short interlude of moderate westerlies mid-afternoon.
Wave heights at Nazaré will probably be well over 20 feet, but the direction is just bordering on being too west to create really spectacular A-frame peaks. If you are not up for that kind of surfing, it also looks like many places in western Europe will get some good surf over the next few days.
EARLIER, Monday November 19: A super-long-period swell is forecast to hit the west coasts of Europe this week. Here, we dig a bit deeper into the workings of a swell like this – not something you find every day in the North Atlantic.
Over the last day or so, the combination of a fast-developing low pressure off Cape Farewell, and a band of high pressure east of Newfoundland has resulted in a broad area of hurricane-force winds in the western North Atlantic, pointing straight towards Europe. The windfield itself is moving in an easterly direction and is expected to persist for the next couple of days.
The result, as you might have guessed, is a large swell. Strong winds persisting for a long time over a large area, plus the fact that the windfield is following the swell it is creating (called dynamic or captured fetch) means that a massive swell is being generated. Open-ocean wave heights east of Cape Farewell are already hitting 16 to 18 metres.
Watch live: Nazare
Now, unlike most Atlantic storms, instead of continually gathering strength as it steams towards Britain and Ireland, this one is expected to reach its maximum power early on and then begin weakening as it passes north of the Azores. By the time it arrives somewhere west of Europe around late Wednesday, it will be a relatively weak system. The main long-period swell should mostly eclipse any locally-generated windseas, particularly on the northern flank of the system.
Current forecasts are showing a large, super-long-period west-northwest swell arriving at westerly exposures such as Ireland and Galicia sometime late Tuesday, and then hitting other areas on Wednesday. Wave heights are expected to exceed ten feet in most places, with peak periods in the 20-sec range.
It will be big almost everywhere, but whether the cleanest conditions will be found in northern, mid or southern areas depends on the final trajectory of that system as it approaches Europe. At the moment, Ireland is looking epic, with wave heights over ten feet at exposed spots, but much bigger at some of the big-wave reefs, and light or moderate southeast winds.
Down into Biscay things are also looking good, with light south or southeast winds. Wave heights will be biggest at places exposed to the west swell, and even bigger at spots with bathymetric focusing, that really come into their own with long periods.
The latest forecast for Portugal shows westerly winds on the southern flank of that system, but things could change before Wednesday. Wave heights will be huge at Nazaré, but the westerly direction might not produce such spectacular A-frame peaks. Further south into Morocco and the Canary Islands, conditions will almost certainly be much cleaner.
A swell like this is not the norm in the North Atlantic. Yes, you might see really long periods from time to time, but they are typically associated with those massive swells like the ones we had in early 2014, with wave heights of over ten metres right near the coast, and mostly huge, out-of-control surf.
The present storm is also really big and contains lots of energy in the long periods. The wave heights in the storm centre are as big as those 2014 swells, but this time it is on the opposite side of the ocean. As the swell propagates away from the storm centre, the wave heights will diminish due to circumferential spreading, but the long periods will still predominate.
Local reports: UK + Ireland
In order to get periods as long as 20 secs when a swell arrives, those periods have had to be present in the original storm centre. Such long periods are also present in smaller swells, but in much smaller proportions, with much less energy associated with them. Generally, the bigger the wave heights reach in a growing storm, the more energy ends up in the longer periods. As the waves grow, the energy is continually shifted from the shorter-period waves into the longer-period ones. As a result, a growing swell becomes progressively more dominated by longer-period waves.
This phenomenon – called non-linear wave-wave interactions – was discovered in the early 1960s by a group of scientists from the Max Planck Institute for Meteorology in Hamburg, led by the legendary Klaus Hasselmann. It was one of the fundamental principles upon which modern wave forecasting models were built.
Related content: For more surf science and in depth analysis see our series Making the Call