Rain/Post-Frontal Fog

Pre-Warm Frontal Events

Fog and very low ceilings often form in the cold wedge of air beneath and adjacent to a warm front boundary as the warm air overruns the cold wedge. Fog and stratus are especially likely if precipitation is falling through the cold air. This allows the underlying cold air to become saturated through a combination of evaporational cooling and moisture advection into the layer. In these situations, precipitation is often observed in conjunction with very low ceilings or fog. Visibilities can be reduced to below airport and flight minima, especially if the underlying surface is very moist and cold, such as with the presence of snow cover.

Below are the three basic types of low-pressure systems that are prone to forming pre-warm frontal fog.

One type is caused by slow moving, non-intensifying lows or open waves with a flat orientation to the isobars. The fog forms in a broad zone poleward of the warm front and near the weak low center.

Conceptual graphic of slow moving, non-intensifying lows or open waves

The second type is caused by lows where there is a distinct surface flux of cold air toward the warm front. This frequently happens in cold air damming situations. Fog or low ceilings form in the cold wedge along and poleward of the front and can be extensive.

Conceptual graphic of lows with cold air damming

The final type is caused by filling lows that are still producing rainfall. Fog or low ceilings tend to form in extensive areas poleward of the occluded or warm front as gradients weaken and evaporational cooling allows saturation. These features also produce fog in and around the filling low center as winds weaken and convergence occurs.

Conceptual graphic of filling lows that are still producing rainfall

Note: Lows that are intensifying do not usually produce much pre-warm frontal fog. In fact when a low is intensifying, any existing fog tends to dissipate due to increased pressure gradients, vertical motions, and subsequent mixing.