The cut-off low

How these weather features form and how to recognize them

- W. Frank Bohlen

Every sailor has experienced times when the weather forecast differs substantially from reality. To make matters worse, this condition might persist for several days, confounding efforts to determine the cause. Sometimes this is caused by a condition called a “cut-off” low, which voyagers should be able to recognize from weather charts.

On examination of the weather maps, one may observe that local conditions are dominated by a prominent low pressure system showing relatively little to no movement over a period of days, and even some retrograde motion with the system moving to the west, counter to the usual progressive easterly flow of weather systems in the Northern Hemisphere.Such aberrant behavior is characteristic of low-pressure credit right systems that have become separated from the upper-level steering winds, resulting in free-form movements that are difficult to predict.

These cut-off lows form a subset of the class of atmospheric low-pressure systems (other members include tropical and extratropical, as well as primary and secondary lows) that is often overlooked, despite their influence, since they are not prominently called out in the forecast. Resolution requires careful examination of the weather maps and some familiarity with the structure of the cut-off low and the processes responsible for its formation. Is this extra effort of value? To answer that, one must understand just what a cut-off low is and how its presence can affect surface conditions and forecast accuracy.

Conditions aloft are key