An occluded front consists of three fronts ganging up: a cool front followed by a warm front followed by a faster-moving cool front.
What happens depends on the temperatures of the three fronts. Warm air will always rise, usually causing precipitation as its water vapor cools and condenses into clouds.
If the first air mass is cool and the third air mass is cold, the front is called a cold front occlusion. The colder, fast-moving front scoops up the warm air and then raises the tail end of the cool air mass. Along the front three layers form: from top to bottom, warm air, cool air, cold air.
In the reverse—a warm front occlusion—when the rear front is cool and the first mass is cold, the incoming cool air will rise up over the tail end of the cold air mass and wedge in between the cold and warm air. The layers still come out to warm, cool, and cold, top to bottom.
Occluded fronts cause both widespread and abrupt precipitation. Abrupt storms form as the warm air rapidly shoots upward, cools, and condenses.
The more general precipitation occurs as the cool air is gradually lifted, cools further, and precipitates.