Question

What kind of front forms when a cold air mass catches up to a warm air mass?

Answer 1

It's not a front it is a TROWAL.

Explanation:

When the leading edge of a cold air mass (cold front) catches up to the retreating edge of another cold air mass (warm front) the warm air in between the 2 fronts is lifted. This occurs because the advancing cold air is denser than the warm air. When this occurs we have a TRough Of Warm Air Aloft or TROWAL form.

In a monochromatic display it looks like this. In a color display the shafts are blue and the barbs are red. Like this.