Know Your Lows
Cut-off lows occur when a mid-latitude low is separated from the jet stream. They are critical for sailors to understand so study this well and memorize it. They have unpredictable movement as they are “cut off” from both the easterly tradewinds and the prevailing westerlies. Most of these lows are weak and also have a weak wind field. Their movement can be erratic; some move quickly, while others are stationary for days.
Cut-off lows can be extremely dangerous and should be avoided. They originate in the lower latitudes – between 30N and 30S – and can have some tropical low pressure characteristics. They can even transition into a tropical low if they remain over warm water and the jet stream is nonexistent.
Examples of deadly cut-offs include the Halloween Storm of 1991 (made famous as The Perfect Storm), which caused eight mariner fatalities; the 1979 Fastnet Race storm (18 fatalities); the 1998 Sydney-Hobart Race (six fatalities); and Hurricane Sandy in 2012. All were strong cut-off lows that mixed tropical characteristics with a mid-latitude weather system.
When two extreme weather systems merge, the combination can result in extraordinary weather.
Here are some danger signs to watch out for: a tight core at the center of the low; rapid pressure drop; significant temperature gradient on the polar side of the low; a comma shape; and fast-moving jet stream over the top, which can be seen in the 500mb charts.
Note: This is an excerpt from the Safety at Sea Guide