The multicell line storm (or "squall line", as it is more commonly called) consists of a long line of storms with a continuous, well-developed gust front at the leading edge of the line. The line of storms can be solid, or there can be gaps and breaks in the line. Figure 10 shows a schematic diagram of a squall line. As the gust front moves forward, the cold outflow forces warm unstable air into the updraft. The main updraft is usually at the leading (eastern) edge of the storm, with the heaviest rain and largest hail just behind (to the west of) the updraft. Lighter rain, associated with older cells, often covers a large area behind the active leading edge of the squall line.

Figure 10: Schematic diagram of a squall line. Cloud outlines, radar intensities, and the area of greatest severe weather/tornado threat are shown.

Squall lines can produce hail up to about golfball size, heavy rainfall, and weak tornadoes, but they are best known as prolific downburst producers. Occasionally, an extremely strong downburst will accelerate a portion of the squall line ahead of the rest of the line. This produces what is called a bow echo (figure 11). As figure 11 illustrates, bow echoes can develop with isolated cells as well as squall lines. Bow echoes are easily detected on radar but are difficult (or impossible) to observe visually. It is not your job to detect bow echoes, but you do need to know what you will be up against should you encounter a bow echo complex: namely, very strong downburst winds.

Figure 11: Schematic diagram of a bow echo. Strong downburst winds accelerate a portion of the storm, producing the bow or comma echo configurations shown.

As with multicell cluster storms, squall lines usually produce severe weather near the UDI. Recall that this is near the leading (eastern) edge of the storm. If tornadoes are associated with a squall line, they will usually develop in cells that are just north of a break in the line or in the line's southernmost cell (sometimes called the "anchor cell"). Cells in these locations tend to behave more like supercells than typical squall line cells.
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