Apologies in advance for content that will only be interesting to die-hard weather nerds. Fair warning….
The typical strategy for sailing from Bermuda to the Azores is to take advantage of the series of low pressure systems that pulse off the east coast of the US. The Azores High – a large area of high pressure generally located directly between the start and finish of the passage – generally keeps these lows to the north. Since low pressure causes counter-clockwise wind circulation (in the Northern Hemisphere), the result is wind that starts in the northwest, becomes west, and then turns to southwest as the low passes by – all favorable directions for sailing generally east.
So when a large low pressure system started to deviate from its typical westward track to circle back and put strong headwinds more or less directly in our path, we weren’t sure quite what to think. Once we arrived in the Azores, we did a bit more research and learned something new about weather – the cut-off low.
A cut-off low is a low pressure system that becomes detached, or cut-off, from the westward-flowing jet stream. Once no longer pushed by the jet stream, the low can move in any direction, or even sit in one location for several days or up to a week. Since the low is no longer driven by the jet stream, forecasting its movement becomes significantly more difficult.
This matches up exactly with what we experienced on our passage. The low was located to the north of us, moving steadily from east to west and giving us the favorable winds that we expected from the forecast. Once it became cut-off, it drifted back westward and then to the south, directly into our path.
We also got direct experience with the difficulty in forecasting these lows. We sailed south until both of the forecast models we use showed that we should have reasonably fair winds for the sail back to the northeast. When we arrived at our turning point, however, the wind that filled in was 30-40 degrees off from the forecast, unfortunately not in our favor.
In our experience, that kind of forecast error in the near term (1-3 days) is quite rare, and now we know why it happened in this case. Forecasting cut-off lows is hard!
The center of the cut-off low was forecast to be a bit north of us, but the actual conditions put it a bit south of us. The resulting wind was from the southeast instead of the forecasted south or southwest, and that meant that Madrone got to pound upwind for four days straight.
In retrospect, we’re not sure that we would have done anything differently, even if we knew then exactly what a cut-off low was. When dealing with a weather system many hundred of miles in diameter from the deck of a sailboat that moves at 7 knots or so, options are limited. But at least now we know what happened…
