Today’s chart shows an impressive-looking low situated just west of the Azores, with a large area of storm-force winds on its south and southwest flanks. It is a cut-off low, associated with a breakaway area of low pressure in the upper airstream, while the main jet stream flow tracks across the far north of the north Atlantic.
Cut-off lows drift around on their own, not being driven by the strong westerly upper winds in the main jet stream. They tend to occur when you get a relatively weak or meandering jet stream, typically in summer, spring or autumn. A strong zonal (west-to-east) flow in the jet stream tends to get going in winter when temperature and pressure gradients in the atmosphere increase.
At first sight, you would think that the low on the charts at the moment is going to generate a massive west swell for Europe in the next few days. But the wave heights forecast for, say, Portugal, are not what you would expect for such an impressive-looking system with such strong winds around it.
The reason is the trajectory of the storm, and this is a classic example. Over the next few days, the system is forecast to drift westwards; in other words, in the opposite direction from the swell it is producing. As a result, the active fetch – the distance over which the swell-generating winds are blowing – is reduced. Those strong winds don’t persist for enough time over a long enough stretch of ocean to generate a really big swell.
Forecast and live cam: Nazare
Around the end of the week, another low pressure is due to form. But this time things are different. The upper airstream shifts southwards and the jet stream becomes strong and straight – more typical of a winter pattern. A moderate-looking low pressure develops near Cape Farewell, with an area of west-northwest winds on its southern flank. The system intensifies as it moves east towards Europe. This time it doesn’t drift around aimlessly like that cut-off low: it moves fast across the North Atlantic, being pushed and steered along by the jet stream.
In the case of this second system, even though the strength of the wind around it is not much different from the first one, the swell it is expected to generate is considerably bigger and longer lasting. The reason is that the windfield moves in the same direction as the swell it is producing, pumping energy into the ocean over a much longer active fetch, and for a longer duration.
The disadvantage, of course, is that these systems move towards the coast that will receive the swell, and sometimes end up right on top of the coast itself, with strong onshore winds accompanying the large swell.
To check it out for yourself, have a look at the wind and swell charts HERE.