A cutoff low is a closed upper-level low that has been fully displaced from the main westerly jet stream flow, so it drifts, stalls, or occasionally retrogrades against the prevailing pattern. On a 500 mb chart it appears as a bull's-eye of closed height contours south of the belt of west-to-east flow that dominates the mid-latitudes.
How a cutoff low forms
A cutoff low typically starts as a shortwave trough digging south along the west side of a larger upper-level trough. NOAA JetStream describes the process as a strong shortwave dropping equatorward until it closes off a circulation of its own, at which point the low is no longer steered by the surrounding westerlies.
Once separated, the low sits in slack flow and moves at the whim of what surrounds it. It can loop, drift westward against the pattern in a motion forecasters call retrogression, or stall for days before another wave rounds the base and reconnects it to the jet.
How forecasters use it
Forecasters treat a cutoff low as a persistence problem first and an ingredient problem second. Because the low is slow, the same corridor sits under the same broad ascent, the same cold air aloft, and often the same moisture tap for the entire life of the system. That is what produces the multi-day flash flood episodes cutoff lows are known for in the Southwest and along the Southeast coast.
In the cool season, a cutoff over the Great Basin or Baja pushes cold midlevel air over a warmer surface for many days, steepening midlevel lapse rates and repeatedly firing showers and thunderstorms over the same terrain.
Where it misleads
Cutoff lows are notoriously hard for global models to move. Different runs often drift the low back into the westerlies too soon, dry out the wrong region, or shift heavy rain axes by hundreds of miles between cycles. A forecast leaning on a single deterministic model position is usually wrong by day three.
A closed low on the chart is also not proof of an active pattern. Cutoffs that lose their moisture supply and their coupling with a jet streak can sit almost benignly, producing only high clouds and cool days until a new shortwave rounds the base and puts them back to work.