A squall line is a line of active thunderstorms organized along or ahead of a boundary. The line often produces continuous or nearly continuous precipitation, a strong gust front on the leading edge, and swaths of damaging straight-line winds.
How it forms
The AMS Glossary defines a squall line as a line of active deep moist convection, characterized by a much larger length-to-width ratio than other mesoscale convective systems. NSSL describes it as a group of storms arranged in a line, often accompanied by squalls of high wind and heavy rain, that can extend for hundreds of miles while remaining only 10 to 20 miles wide.
Squall lines typically form along cold fronts, drylines, outflow boundaries, or ahead of an approaching cold front in the warm sector. Once established, the line generates its own cold pool. The leading edge of that cold pool lifts warm, moist inflow and continually forces new convection along the line's forward flank.
How forecasters and spotters read it
On radar the line shows an elongated band of high reflectivity, often with a trailing stratiform region behind it. The gust front is visible as a fine reflectivity line ahead of the precipitation and as a velocity discontinuity in Doppler data. Bowing segments within the line indicate local rear-inflow surges that can produce damaging winds.
Visually the classic squall line advances behind a long shelf cloud or roll cloud that marks the outflow. Rain and hail follow within minutes of the shelf's passage, and wind gusts near the leading edge are often the highest of the storm.
Where it misleads
A squall line can hide embedded supercells within its overall linear look, and brief tornadoes can spin up from mesovortices along the leading edge with little warning time. The linear appearance can mask fine-scale rotation that only shows up when the radar samples the correct height at the correct instant.
Not every long line of storms is a squall line in the operational sense. Loose alignment of discrete cells without a coherent shared gust front is a cluster, not a line, and the warning strategies differ. Watching the leading-edge structure over time is what separates the two.