There are different kinds of squall lines?

Most everyone is familiar with a squall line. It’s a long line of strong thunderstorms. Here’s a great example of one from the Chicago radar.

A trailing stratiform squall line

While the above is a more classic example of a squall line, there are actually 3 different types of squall lines. They are classified by how the stratiform precipitation and areas of lighter rain/thunderstorms, (seen as yellow, green or blue in the radar images) are organized.

  1. Trailing stratiform (like the one above)
  2. Leading stratiform
  3. Parallel stratiform
Squall line archetypes. Image taken from “Structures and Dynamics of Quasi-2D Mesoscale Convective Systems”, Parker and Johnson 2004

Below are examples from RadarScope of leading stratiform and parallel stratiform lines.

A leading stratiform squall line
A parallel stratiform squall line.

Differences in the wind shear (change of wind speed and direction with height), instability and storm interactions dictate how each system evolves. Trailing stratiform types are the most common, with leading and parallel being about equally uncommon. So the next time you’re looking at squall lines on radar, see if you are lucky enough to spot a leading or parallel stratiform type!

If you’d like to get dirty with the details of each squall line type, you can find a lot more information at the links below:

Organizational Modes of Midlatitude Mesoscale Convective Systems

Structures and Dynamics of Quasi-2D Mesoscale Convective Systems