Definition
A type of occluded front that forms when a fast-moving cold front catches up to a slow-moving warm front, and the air behind the cold front is warmer (less dense) than the cool air ahead of the warm front. The advancing air rides up and over the cooler air ahead, lifting the original warm front aloft. The result is a frontal system with cool air at the surface, cold air aloft ahead, and warm air aloft behind.
Plain English
When a cold front catches up to a warm front but the air behind the cold front is actually warmer than the cool air sitting ahead of the warm front. The warmer pushing air slides up over the colder air on the ground, and the two fronts merge into one combined front.
Context Anchor
Seen in aviation weather study, surface weather analysis, and discussions of clouds and precipitation near occluded fronts.
Derivation
Called a warm front occlusion because the air mass overtaking the system is warmer than the air ahead of it — the opposite of a cold front occlusion. The word occlusion comes from the Latin occludere, meaning to shut or close off; here, the warm air gets closed off from the surface as the two fronts merge.
Why Pilots Care
This front produces widespread low ceilings, reduced visibility, and possible icing or turbulence that can affect flight planning and safety.
Grounding Statement
Picture very cold air lying on the ground ahead of the system while less-cold air from behind glides up over it, forcing the warmer air upward.
Intuition Check
“Warm” does not mean the weather will feel warm or pleasant. Here it means the air behind the catching cold front is warmer than the colder air already ahead of the warm front. “Front” does not mean the front side of a storm. In weather, it means a boundary between different air masses.
Example Sentence 1
The briefer pointed out a warm front occlusion across the route, warning of widespread low ceilings and steady rain along the entire leg.
Example Sentence 2
During the weather briefing the instructor noted that a warm front occlusion often brings prolonged instrument conditions compared with a standard warm front.