Chris Tibbs offers a few different ways to get past a cut-off low pressure system.
How many times do skippers – racers and cruisers – report that some of the worst Atlantic conditions are to be found between mainland Europe and the Canary Islands? Many of these reports are from those caught out by a cut-off low – often an insignificant-looking weather system that can pack quite a punch.
Once past Finisterre, race skippers in the Vendée Globe, or those attempting the Jules Verne Trophy, should soon pick up the Portuguese Trades and start their long, fast run to the Doldrums, usually passing to the west of the Canary and Cape Verde Islands. Cruisers heading south to the Canaries prior to an Atlantic crossing also enjoy these typically strong, favourable winds as they leave Spain, Portugal or join the route having passed through the Straits of Gibraltar.
Routeing charts show the wind as being predominantly from the north-east, but it does vary as depressions pass close to the north, temporarily displacing the Azores High. However, when the Azores High is north of its ‘normal’ position it is possible to get a small low trapped, or cut off, south of the high, somewhere between the latitude of Lisbon and the Canary Islands. These cut-off lows, detached from the steering force of the jet stream, can then linger for days following a track that is erratic and difficult to forecast. Indeed they sometimes even track to the west.
It is easy to underestimate cut-off lows due to the fact that, for a given pressure gradient, the further south we are (in the northern hemisphere) the stronger the wind. For example, isobar spacing that would indicate 20 knots at 50° latitude, will, for the same spacing, give 30 knots at 30°.
Whether racing or cruising, these lows create a dilemma: the shortest route will most likely be between the low and Africa, but this will give strong headwinds and an uncomfortable sea state. However, to pass north of the low by heading out into the Atlantic will add a considerable number of miles and there is still the uncertainty as to where the low will track. It may stay in place or track east, but if it does go west, skippers could find themselves heading in the same direction as the weather system they are trying to avoid.
For racing skippers with a long-term strategy, a fast westerly route may look more appealing: get north of the low in favourable winds then head south. However apart from the fact that the low may drift west, if it does drift east, there is often a hole in the wind as the low moves away before the Trades are re-established, delaying the moment when the skipper can get back on track. Passing to the east, the beat may not be very appealing and with such a long way to go, any damage or breakage could be expensive later in the race. Sometimes the Trades will stay as a band of light wind close to the African coast, and if the low fills or drifts west, the Trades will fill in on the east side first. However should the low track east, the door shuts, leaving the skipper facing an increasing then veering south-westerly along with a lee shore.
There are no easy answers. For the racing yachts the decision may well determine the overall results months down the line, and regularly provides excitement for the audience following along at home. For yachts heading to the Canaries ahead of a transatlantic passage, passing to the west of a cut-off low is usually not an option as it adds too many miles and may make landfall in the Canaries difficult. The key advantage for cruisers, of course, is that they can choose their departure time. A five-day forecast will indicate a cut-off low so the best option is to delay and avoid it.