Ask Josh Fitzpatrick: What's a Cutoff Low?

Cutoff lows bring days of dreary weather. Meteorologist Josh Fitzpatrick explains what they are and why they form.

You've probable heard us say from time to time, "this low pressure area is cut off."  What does that mean?  Here's what they are and how they impact the weather. 

A cutoff, upper-air low occurs when a counterclockwise wind circulation becomes separated from the main belt of west to east winds in the jet stream high above the earth's surface.  It's simply cut off from the jet stream so the low gets left behind.  Kind of like missing the bus.

With nothing to steer a cutoff low, it can simply sit and spin for days, often bringing damp, dreary weather.

Such lows usually form when a strong wind speed maximum riding along the main belt of westerly jet stream winds dives into a deep upper-level trough or dip in the jet. As the wind speed maximum dives into the bottom of the trough, it elongates the trough and also helps build a large downstream ridge of high pressure. 

The combination of the deepening trough and the strengthening downstream ridge allows the bottom portion of the trough to separate from the main belt of the westerlies. As a result, a large, circular, counterclockwise circulation forms high above the earth's surface.

Often, areas of low pressure at the earth's surface are associated with upper-level troughs and cutoff lows.

A cutoff low is usually slow moving and won't exit a region until it is picked up by the westerly steering flow. Sometimes they unravel as they spin, eventually weakening and then dissipating.

Either way, cutoff lows almost always bring several days of cloudy, dreary weather with periods of rain or snow. 

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