Cut-Off Low

A cut-off low is a closed upper-level low that has become completely separated from the main westerly current, as defined by the US National Weather Service, typically found in the mid-latitudes between about 20 and 45 degrees and capable of sitting nearly stationary for days. It forms when a trough in the upper-level flow pinches off into its own closed circulation, creating a cold-core low with concentric rings of equal temperature around its center, cut off from the jet stream westerly winds five to nine kilometres up. Because it no longer moves with the main flow, a cut-off low can linger unpredictably in a blocking pattern for days or even weeks, and it appears most often in autumn, winter and spring when the jet stream carries polar air toward lower latitudes.