Friction slows this inrush of air so that upper divergence is generally not balanced by lower convergence. If you were to study the movement of the isobars on an 500mb chart over time, you would see that the waves or ridges and troughs of the lines do not stay in one spot but move from left to right. The lines themselves will evolve over time, but the overall movement is to the east. Where a cold front and a warm front meet an occluded front is created. This is the space between the warm and cold front that is shutting or closing.
Divergence normally occurs to the east of a trough in the upper troposphere.
Directly underneath there will be a dynamic low pressure area. Air moving around the surface low will move counterclockwise and inward to the dynamic low, just as it would around any surface low pressure area. In the case examined in class, warm, moist air moved north from the Gulf of Mexico to a low pressure are over the Dakotas. A cold front to the southeast gradually moved east towards the warm air. When warm air moves over cold air, it rises, nimbostratus clouds form, and precipitation often occurs.