Squall lines generally form along or ahead of cold fronts and drylines and can produce severe weather in the form of heavy rainfall, strong winds, large hail, and frequent lightning. One type is a line of cumulonimbus clouds that grow and decay; the other is a line of steady supercells. A squall line can form from an individual storm that has split, which then helps to form a line of storms. Multi-cell lines, also called squall lines, consist of a more-or-less continuous line of storms that form at or ahead of a cold front with a gust front at the leading edge. The most extreme example of a squall line is what is known as a derecho, which is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms. One type is a line of cumulonimbus clouds that grow and decay; the other is a line of steady supercells. There are two types of squall lines. There are two types of squall lines. Multi-cell lines. As such, flying through a line of thunderstorms or a squall line … Squall lines can extend to hundreds of miles in length, simultaneously affecting several states at a time. A squall line can form from an individual storm that has split. I took this picture while flying behind the squall line. Another way a squall line forms is on the south side of an existing thunderstorm. Squall lines also typically have an area of stratiform rain that trails the main line of thunderstorms (called the "trailing stratiform" region), although depending on how the upper-level winds are blowing, the area of stratiform rain occasionally is ahead of the squall line or even parallel to it, so there's more than one flavor of squall line!! Squall lines are a serious hazard to pilots, as squall-type thunderstorms can extend well above the capability of many airplanes and can extend in a line stretching 300 to 500 miles, making them difficult to navigate around. This split storm helps to form the line of storms. d. Squall-line Thunderstorm: These are a wall of thunderstorms standing shoulder to shoulder, often along a cold front (the leading edge of a relatively cold air mass). Squall lines can be just as severe as a supercell thunderstorm. Top right. While these things can occur with any type of severe thunderstorm, they most commonly occur in squall line storms. The cold front is moving from left to right—I am flying in the cold air. Squall lines are generally like the name implies — a line of storms that could extend across several counties, the state or even hundreds of miles. These storms often have breathtaking shelf clouds and have the heaviest rain and hail just to the west of (behind) the updraft.