Final answer:
A squall line is a nonfrontal, narrow band of active thunderstorms that often develop ahead of a cold front.
Step-by-step explanation:
A nonfrontal, narrow band of active thunderstorms that often develop ahead of a cold front is known as a squall line. Squall lines are typically long and narrow, and they can bring strong winds, heavy rain, and sometimes tornadoes. They form in the warm, moist air ahead of a cold front, where the unstable conditions are favorable for thunderstorm development.