Why do fronts stall? They simply lose momentum. There are many ways in which this occurs. One of the most common is the front losing its definition, meaning the separation between the differing air masses becomes less defined. This commonly occurs during the summer months. For example, a cooler air mass associated with a cold front modifies or warms, making the frontal boundary weaker which leads to a stalled out front. High pressure is another external force that often slows down or leads to a stationary front. Another very common reason for frontal boundaries to stall is lack of upper-level support. Often times frontal boundaries are driven by upper-level systems which are typically stronger during the fall, winter and spring months, while generally weaker during the summer months.