Mesoscale convective systems (MCS) often form at or ahead of a surface cold front, and they may persist for 6-12 hours, especially in the Great Plains area of the USA. The reason why the convection is sustained is shown above. Warm, moist air is forced to ascend over the gust front and produces deep convective towers, often along a more or less continuous line (often referred to as a squall line). At mid-levels, the frontal system often produces a current of cold, dry air, moving faster than the surface cold front, and this air penetrates into the line of thunderstorms. The snow/rain evaporates as it falls through this descending midlevel rear-to-front current, and as this air reaches the ground, it is cooler than the surrounding air. This cold pool maintains the gust front which sustains continued convection. The convective outflow often stretches to the rear, above the mid-level front-to-rear inflow. In this case, the squall line is accompanied by a trailing region of stratiform precipitation.