A weather front marks the boundary between two air masses with different densities and temperatures. The occluded front represents the final stage in the lifecycle of a mid-latitude low-pressure system. It forms when a faster-moving cold front overtakes a slower warm front, completely lifting the intervening warm air mass off the ground. This process creates a single, continuous boundary that merges the weather characteristics of both cold and warm fronts. The weather associated with its passage follows a timeline, moving from a gentle approach to an intense storm and final clearing.
Formation and Structure of Occluded Fronts
An occluded front develops when a denser, faster-moving cold air mass catches up to a preceding warm front. As the cold air mass pushes under the warm air mass, it forces the lighter warm air entirely aloft. The resulting frontal boundary on the ground separates two different cold air masses, with the warm air existing only in the upper atmosphere.
The specific weather depends on which of the two cold air masses is colder, leading to two distinct types of occlusion. In a cold occlusion, the air mass behind the cold front is colder than the air mass ahead of the warm front, forcing all air masses to rise. A warm occlusion forms when the air mass behind the cold front is milder than the air mass ahead of the warm front, causing the cold front to ride up and over the colder air mass.
Cloud Systems and Initial Precipitation
The initial signs of an approaching occluded front mimic those of a warm front. High-level clouds are the first indication, beginning with thin cirrus clouds that gradually thicken into cirrostratus. This sequence demonstrates the gradual, large-scale lifting of the warm air in the upper atmosphere ahead of the surface front.
As the system moves closer, the clouds become lower and thicker, transitioning to altostratus and then nimbostratus. This deep, layered cloud structure produces light, continuous precipitation, such as rain, drizzle, or snow. During this pre-frontal phase, winds typically blow from the southeast or south, and temperatures remain relatively steady and mild.
Intense Weather During Frontal Passage
The most significant change occurs precisely as the occluded front passes, shifting from steady, stratiform precipitation to intense, showery conditions. The rapid, forceful uplift of the warm air mass creates atmospheric instability, leading to the development of towering cumulus and cumulonimbus clouds. This instability produces heavy downpours, strong thunderstorms, hail, or blizzards in colder environments.
A sharp and noticeable wind shift accompanies the passage of the surface front, veering abruptly from a southerly direction to the west or northwest. Simultaneously, the temperature drops significantly, especially in a cold occlusion where the coldest air mass replaces the cool air at the surface. This rapid shift in wind, temperature, and precipitation intensity is the hallmark of the occluded frontal passage.
Post-Frontal Conditions and Atmospheric Clearing
Once the occluded front has moved past, the atmosphere quickly transitions into a colder and more stable state. The region is dominated by the cold air mass that swept in behind the system. The heavy, continuous precipitation ceases, and the deep, layered clouds break up.
Skies begin to clear, often showing broken patches of cumulus or stratocumulus clouds. Although the severe weather has passed, the air mass may remain unstable enough for scattered, short-lived rain or snow flurries to develop. The barometric pressure, which dropped to its lowest point during the frontal passage, begins a steady rise.