How Squall Lines Form

The term “squall” comes from the world and the time of the sailing ship: It’s a sudden and fierce wind, and the storm that comes with it.

Squall lines are lines of intense thunderstorms. Storms form in this way because a zone of rising air develops in an elongated area. Rising motion may be triggered in such a zone by a cold front, a warm front, a dry line, an upper level trough, or an old outflow boundary from earlier storms.

Squall lines will typically develop during times of peak heating in the mid to late afternoon. The hotter the air, the more potential energy available for updraft formation: a dry line is a good example of a surface boundary (in this case, a division between hot, dry air to the west and warm, moist air to the east) and daytime heating working together to generate a sharp demarcation between calm and stormy.

The more vigorous the lift, the more rapid formation will be. The transition from a few built-up clouds to an intense band of storms on radar may take only 20-30 minutes if conditions are extremely unstable.

Squall lines will occasionally form along a mountain range as wind blows upslope. The uplift provided by the mountain gives the air a strong push. The steering winds will then blow these storms downwind from the mountains in a long line.

Sometimes, a huge squall line complex known as an MCS (Mesoscale Convective System) will form even at night. Warm, moisture-laden air is forced upward as it blows from eastern portions of Kansas, Oklahoma and Texas onto the higher terrain traveling north and west.

If other factors are in play, a squall line and later an MCS may form here and move hundreds of miles downwind over many hours. They are known to contain extremely heavy rainfall and destructive straight-line winds sometimes exceeding 90mph.

Squall lines produce only a small percentage of all tornadoes. The mechanism necessary for tornado production is rotation. The term Supercell refers to a discrete thunderstorm (not part of a line) that is rotating. A Supercell has plenty of warm moist environmental air on which to draw in further energy to keep the updraft going and increasing; in contrast, thunderstorms within a squall line must compete with each other for warm air and moisture.

The air currents for line-embedded storms are also not conducive to individual updraft rotations necessary for tornadoes. If a tornado producing storm forms on a squall line, it is usually the southernmost storm, since this one will have maximum access to the inflow of moisture and warm air, and less wind interference.

Infrequently, small tornadoes known as gustnadoes can develop along the leading edge of the line as it approaches; these are most always a weak tornado.

Squall lines are a fine example where nature can occasionally play it straight, but in this case it also plays fairly rough.